IGCSE Chemistry CIE PDF

Head to savemyexams.co.uk for more awesome resources YOUR NOTES IGCSE Chemistry CIE  2. Atoms, Elements...

Head to savemyexams.co.uk for more awesome resources YOUR NOTES IGCSE Chemistry CIE  2. Atoms, Elements & Compounds CONTENTS 2.1 Atomic Structure & the Periodic Table 2.1.1 Elements, Compounds & Mixtures 2.1.2 Atomic Structure 2.1.3 Electronic Configuration 2.1.4 Isotopes 2.2 Ions & Ionic Bonds 2.2.1 Ions & Ionic Bonds 2.2.2 Ionic Bonds & Lattice Structure 2.2.3 Properties of Ionic Compounds 2.3 Simple Molecules & Covalent Bonds 2.3.1 Covalent Bonds 2.3.2 Molecules & Compounds 2.3.3 Properties of Simple Molecular Compounds 2.4 Giant Structures 2.4.1 Diamond & Graphite 2.4.2 Silicon(IV) Oxide 2.4.3 Metallic Bonding Page 1 of 40 © 2015-2021 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to savemyexams.co.uk for more awesome resources 2.1 Atomic Structure & the Periodic Table YOUR NOTES  2.1.1 Elements, Compounds & Mixtures Elements, Compounds & Mixtures Elements, compounds and mixtures All substances can be classified into one of these three types Element A substance made of atoms that all contain the same number of protons and cannot be split into anything simpler There are 118 elements found in the Periodic Table Compound A pure substance made up of two or more elements chemically combined There is an unlimited number of compounds Compounds cannot be separated into their elements by physical means E.g. copper(II) sulfate (CuSO4), calcium carbonate (CaCO3), carbon dioxide (CO2) Mixture A combination of two or more substances (elements and/or compounds) that are not chemically combined Mixtures can be separated by physical methods such as filtration or evaporation E.g. sand and water, oil and water, sulfur powder and iron filings Particle diagram showing elements, compounds and mixtures Page 2 of 40 © 2015-2021 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to savemyexams.co.uk for more awesome resources 2.1.2 Atomic Structure YOUR NOTES  Atomic Structure All substances are made of tiny particles of matter called atoms which are the building blocks of all matter Each atom is made of subatomic particles called protons, neutrons, and electrons The protons and neutrons are located at the centre of the atom, which is called the nucleus The electrons move very fast around the nucleus in orbital paths called shells The mass of the electron is negligible, hence the mass of an atom is contained within the nucleus where the protons and neutrons are located The structure of the carbon atom Page 3 of 40 © 2015-2021 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to savemyexams.co.uk for more awesome resources Protons, Neutrons & Electrons YOUR NOTES The size of atoms is so tiny that we can't really compare their masses in  conventional units such as kilograms or grams, so a unit called the relative atomic mass is used One relative atomic mass unit is equal to 1/12th the mass of a carbon-12 atom. All other elements are measured relative to the mass of a carbon-12 atom, so relative atomic mass has no units Hydrogen for example has a relative atomic mass of 1, meaning that 12 atoms of hydrogen would have exactly the same mass as 1 atom of carbon The relative mass and charge of the sub-atomic particles are shown below: Table of Subatomic Particles  Exam Tip Knowing the exact mass of an electron is not in the specification and saying it is almost nothing or negligible will be sufficient. It does, however, sometimes appear in particle identification questions, but you can usually deduce that it is the electrons from other information in the question. Defining Proton Number The atomic number (or proton number) is the number of protons in the nucleus of an atom The symbol for atomic number is Z It is also the number of electrons present in a neutral atom and determines the position of the element on the Periodic Table Page 4 of 40 © 2015-2021 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to savemyexams.co.uk for more awesome resources Defining Mass Number YOUR NOTES The Nucleon number (or mass number) is the total number of protons and  neutrons in the nucleus of an atom The symbol for nucleon number is A The nucleon number minus the proton number gives you the number of neutrons of an atom Note that protons and neutrons can collectively be called nucleons. The atomic number and mass number of an element can be shown using atomic notation The Periodic Table shows the elements together with their atomic (proton) number at the top and relative atomic mass at the bottom - there is a difference between relative atomic mass and mass number, but for your exam, you can use the relative atomic mass as the mass number (with the exception of chlorine) Diagram showing atomic notation Atomic notation for carbon  Exam Tip Both the atomic number and the relative atomic number (which you can use as the mass number) are given on the Periodic Table but it can be easy to confuse them. Think MASS = MASSIVE, as the mass number is always the bigger of the two numbers, the other smaller one is thus the atomic / proton number. Beware that some Periodic Tables show the numbers the other way round with the atomic number at the bottom! Deducing protons, neutrons & electrons Page 5 of 40 © 2015-2021 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to savemyexams.co.uk for more awesome resources Finding the protons YOUR NOTES The atomic number of an atom and ion determines which element it is  Therefore, all atoms and ions of the same element have the same number of protons (atomic number) in the nucleus E.g. lithium has an atomic number of 3 (three protons) whereas beryllium has atomic number of 4 (4 protons) The number of protons equals the atomic (proton) number The number of protons of an unknown element can be calculated by using its mass number and number of neutrons: Mass number = number of protons + number of neutrons Number of protons = mass number – number of neutrons Finding the electrons An atom is neutral and therefore has the same number of protons and electrons Finding the neutrons The mass and atomic numbers can be used to find the number of neutrons in ions and atoms: Number of neutrons = mass number – number of protons  Worked Example Determine the number of protons, electrons and neutrons in an atom of element X with atomic number 29 and mass number 63 Answer: The number of protons of element X is the same as the atomic number Number of protons = 29 The neutral atom of element X therefore also has 29 electrons The atomic number of an element X atom is 29 and its mass number is 63 Number of neutrons = mass number – number of protons Number of neutrons = 63 – 29 Number of neutrons = 34 Page 6 of 40 © 2015-2021 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to savemyexams.co.uk for more awesome resources 2.1.3 Electronic Configuration YOUR NOTES  Electronic Configuration Electronic configuration We can represent the structure of the atom in two ways: using diagrams called electron shell diagrams or by writing out a special notation called the electronic configuration (or electronic structure) Electron shell diagrams Electrons orbit the nucleus in shells (or energy levels) and each shell has a different amount of energy associated with it The further away from the nucleus, the more energy a shell has Electrons fill the shell closest to the nucleus When a shell becomes full of electrons, additional electrons have to be added to the next shell The first shell can hold 2 electrons The second shell can hold 8 electrons For this course, a simplified model is used that suggests that the third shell can hold 8 electrons For the first 20 elements, once the third shell has 8 electrons, the fourth shell begins to fill The outermost shell of an atom is called the valence shell and an atom is much more stable if it can manage to completely fill this shell with electrons A simplified model showing the electron shells The arrangement of electrons in shells can also be explained using numbers Instead of drawing electron shell diagrams, the number of electrons in each electron shell can be written down, separated by commas This notation is called the electronic configuration (or electronic structure) E.g. Carbon has 6 electrons, 2 in the 1st shell and 4 in the 2nd shell Its electronic configuration is 2,4 Page 7 of 40 © 2015-2021 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to savemyexams.co.uk for more awesome resources Electronic configurations can also be written for ions YOUR NOTES E.g. A sodium atom has 11 electrons, a sodium ion has lost one electron,  therefore has 10 electrons; 2 in the first shell and 8 in the 2nd shell Its electronic configuration is 2,8 The Electronic Configuration of the First Twenty Elements Note: although the third shell can hold up to 18 electrons, the filling of the shells follows a more complicated pattern after potassium and calcium. For these two elements, the third shell holds 8 and the remaining electrons (for reasons of stability) occupy the fourth shell first before filling the third shell. Page 8 of 40 © 2015-2021 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to savemyexams.co.uk for more awesome resources YOUR NOTES  Exam Tip You need to be able to write the electronic configuration of the first twenty  elements and their ions. You may see electronic configurations using full stops instead of commas. You would not be penalised for using full stops. Page 9 of 40 © 2015-2021 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to savemyexams.co.uk for more awesome resources Electron Shells & The Periodic Table YOUR NOTES There is a clear relationship between the electronic configuration and how the  Periodic Table is designed The number of notations in the electronic configuration will show the number of occupied shells of electrons the atom has, showing the period in which that element is in The last notation shows the number of outer electrons the atom has, showing the group that element is in (for elements in Groups I to VII) Elements in the same group have the same number of outer shell electrons The electronic configuration for chlorine Period:The red numbers at the bottom show the number of notations which is 3, showing that a chlorine atom has 3 occupied shells of electrons and is in Period 3 Group: The final notation, which is 7 in the example, shows that a chlorine atom has 7 outer electrons and is in Group VII The position of chlorine on the Periodic Table Page 10 of 40 © 2015-2021 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to savemyexams.co.uk for more awesome resources In most atoms, the outermost shell is not full and therefore these atoms react with YOUR NOTES other atoms in order to achieve a full outer shell of electrons (which would make  them more stable) In some cases, atoms lose electrons to entirely empty this shell so that the next shell below becomes a (full) outer shell All elements wish to fill their outer shells with electrons as this is a much more stable configuration The noble gases The atoms of the Group VIII elements (the noble gases) all have a full outer shell of electrons All of the noble gases are unreactive as they have full outer shells and are thus very stable The noble gases are on the Periodic Table in Group 8/0  Exam Tip The electrons in the outer shell are also known as valency electrons. Page 11 of 40 © 2015-2021 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to savemyexams.co.uk for more awesome resources 2.1.4 Isotopes YOUR NOTES  Defining Isotopes Isotopes are different atoms of the same element that contain the same number of protons but a different number of neutrons The symbol for an isotope is the chemical symbol (or word) followed by a dash and then the mass number So C-14 ( or carbon-14) is the isotope of carbon which contains 6 protons, 6 electrons and 14 - 6 = 8 neutrons It can also be written as 14C or 146 C The Atomic Structure and Symbols of the Three Isotopes of Hydrogen Page 12 of 40 © 2015-2021 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to savemyexams.co.uk for more awesome resources Why Isotopes Share Properties YOUR NOTES EXTENDED  Isotopes of the same element display the same chemical characteristics This is because they have the same number of electrons in their outer shells and, therefore, the same electronic configuration and this is what determines an atom's chemistry The difference between isotopes is the number of neutrons which are neutral particles within the nucleus and add mass only The difference in mass affects the physical properties, such as density, boiling point and melting point Isotopes are identical in appearance, so a sample of C-14 would look no different from C-12 Water made from deuterium oxide is known as 'heavy' water, and has a relative formula of mass 20, compared to 18 for water, so it is 20% heavier, but it would look, taste and feel just like normal water However, it wouldn't be a good idea to drink it because it is toxic as it interferes with biochemical reactions in your cells! Page 13 of 40 © 2015-2021 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to savemyexams.co.uk for more awesome resources Calculating Relative Atomic Mass YOUR NOTES EXTENDED  Relative Atomic Mass The symbol for the relative atomic mass is Ar The relative atomic mass for each element can be found in the Periodic Table along with the atomic number The atomic number is shown above the atomic symbol and the relative atomic mass is shown below the atomic symbol Atoms are too small to accurately weigh but scientists needed a way to compare the masses of atoms The carbon-12 is used as the standard atom and has a fixed mass of 12 units It is against this atom which the masses of all other atoms are compared Relative atomic mass (Ar) can therefore be defined as: the average mass of the isotopes of an element compared to 1/12th of the mass of an atom of 12C The relative atomic mass of carbon is 12 The relative atomic mass of magnesium is 24 which means that magnesium is twice as heavy as carbon The relative atomic mass of hydrogen is 1 which means it has one-twelfth the mass of one carbon-12 atom The relative atomic mass of an element can be calculated from the mass number and relative abundances of all the isotopes of a particular element using the following equation: ( % of isotope 1 x mass number of isotope 1) + ( % of isotope 2 x mass number of isotope 2) Ar = 100 The top line of the equation can be extended to include the number of different isotopes of a particular element present. Example The table shows information about the isotopes in a sample of rubidium (72 x 85) + (28 x 87) Ar = = 85. 6 100 Page 14 of 40 © 2015-2021 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to savemyexams.co.uk for more awesome resources Is mass number and relative atomic mass the same thing? YOUR NOTES On the Periodic Table provided in your exam you will see that lithium has a relative  atomic mass of 7 Although it seems that this is the same as the mass number, they are not the same thing because the relative atomic mass is a rounded number Relative atomic mass takes into account the existence of isotopes when calculating the mass Relative atomic mass is an average mass of all the isotopes of that element For simplicity relative atomic masses are often shown to the nearest whole number The relative atomic mass of lithium to two decimal places is 6.94 when rounded to the nearest whole number, the RAM is 7, which is the same as the mass number shown on this isotope of lithium Page 15 of 40 © 2015-2021 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to savemyexams.co.uk for more awesome resources 2.2 Ions & Ionic Bonds YOUR NOTES  2.2.1 Ions & Ionic Bonds The Formation of Ions An ion is an electrically charged atom or group of atoms formed by the loss or gain of electrons An atom will lose or gain electrons to become more stable The loss or gain of electrons takes place to gain a full outer shell of electrons which is a more stable arrangement of electrons The electronic configuration of an ion will be the same as that of a noble gas – such as helium, neon and argon Formation of positively charged sodium ion Page 16 of 40 © 2015-2021 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to savemyexams.co.uk for more awesome resources YOUR NOTES  Formation of negatively charged chloride ion Ionisation of metals and non-metals Metals: all metals can lose electrons to other atoms to become positively charged ions, known as cations Non-metals: all non-metals can gain electrons from other atoms to become negatively charged ions, known as anions Page 17 of 40 © 2015-2021 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to savemyexams.co.uk for more awesome resources The Formation of Ionic Bonds YOUR NOTES Ionic compounds are formed when metal atoms react with non-metal atoms  Metal atoms lose their outer electrons which the non-metal atoms gain to form positive and negative ions The positive and negative ions are held together by strong electrostatic forces of attraction between opposite charges This force of attraction is known as an ionic bond and they hold ionic compounds together Dot-and-cross diagrams Dot and cross diagrams are diagrams that show the arrangement of the outer- shell electrons in an ionic or covalent compound or element The electrons are shown as dots and crosses In a dot and cross diagram: Only the outer electrons are shown The charge of the ion is spread evenly which is shown by using brackets The charge on each ion is written at the top right-hand corner Electrostatic forces between the positive Na ion and negative Cl ion Page 18 of 40 © 2015-2021 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to savemyexams.co.uk for more awesome resources Ionic Bonds between Group I & Group VII Elements YOUR NOTES Example: Sodium Chloride, NaCl  Sodium chloride ionic bonding Explanation Sodium is a Group I metal so will lose one outer electron to another atom to gain a full outer shell of electrons A positive sodium ion with the charge 1+ is formed Chlorine is a Group VII non-metal so will need to gain an electron to have a full outer shell of electrons One electron will be transferred from the outer shell of the sodium atom to the outer shell of the chlorine atom A chlorine atom will gain an electron to form a negatively charged chloride ion with a charge of 1- Page 19 of 40 © 2015-2021 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to savemyexams.co.uk for more awesome resources The oppositely charged ions are held together by strong electrostatic forces of YOUR NOTES attraction  The ionic compound has no overall charge Formula of ionic compound: NaCl Page 20 of 40 © 2015-2021 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to savemyexams.co.uk for more awesome resources 2.2.2 Ionic Bonds & Lattice Structure YOUR NOTES  The Lattice Structure of Ionic Compounds EXTENDED Lattice structure Ionic compounds have a giant lattice structure Lattice structure refers to the arrangement of the atoms of a substance in 3D space In lattice structures, the atoms are arranged in an ordered and repeating fashion The lattices formed by ionic compounds consist of a regular arrangement of alternating positive and negative ions The lattice structure of NaCl Page 21 of 40 © 2015-2021 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to savemyexams.co.uk for more awesome resources Ionic Bonds between Metallic & Non-Metallic Elements YOUR NOTES EXTENDED  Ionic compounds Ionic compounds are formed when metal atoms and non-metal atoms react The ionic compound has no overall charge Example: Magnesium Oxide, MgO Diagram showing the dot-and-cross diagram of magnesium oxide Explanation Magnesium is a Group II metal so will lose two outer electrons to another atom to have a full outer shell of electrons A positive ion with the charge 2+ is formed Oxygen is a Group VI non-metal so will need to gain two electrons to have a full outer shell of electrons Two electrons will be transferred from the outer shell of the magnesium atom to the outer shell of the oxygen atom Oxygen atom will gain two electrons to form a negative ion with charge 2- Magnesium oxide has no overall charge Formula of ionic compound: MgO  Exam Tip When drawing dot and cross diagrams, you only need to show the outer shell of electrons. Remember to draw square brackets and include a charge for each ion. Make sure the overall charge is 0; you may need to include more than one positive or negative ion to ensure the positive and negative charges cancel each other out. Page 22 of 40 © 2015-2021 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to savemyexams.co.uk for more awesome resources 2.2.3 Properties of Ionic Compounds YOUR NOTES  Properties of Ionic Compounds Ionic compounds are usually solid at room temperature They have high melting and boiling points Ionic compounds are good conductors of electricity in the molten state or in solution They are poor conductors in the solid state Explaining the Properties of Ionic Compounds EXTENDED Ionic substances have high melting and boiling points due to the presence of strong electrostatic forces acting between the oppositely charged ions These forces act in all directions and a lot of energy is required to overcome them The greater the charge on the ions, the stronger the electrostatic forces and the higher the melting point will be For example, magnesium oxide consists of Mg2+ and O2- so will have a higher melting point than sodium chloride which contains the ions, Na+ and Cl- For electrical current to flow there must be freely moving charged particles such as electrons or ions present Ionic compounds are good conductors of electricity in the molten state or in solution as they have ions that can move and carry a charge They are poor conductors in the solid state as the ions are in fixed positions within the lattice and are unable to move Molten or aqueous ions move freely but cannot in solid form Page 23 of 40 © 2015-2021 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to savemyexams.co.uk for more awesome resources 2.3 Simple Molecules & Covalent Bonds YOUR NOTES  2.3.1 Covalent Bonds The Formation of Covalent Bonds Covalent compounds Covalent compounds are formed when pairs of electrons are shared between atoms Only non-metal elements participate in covalent bonding As in ionic bonding, each atom gains a full outer shell of electrons, giving them a noble gas electronic configuration When two or more atoms are covalently bonded together, we describe them as ‘molecules’ Dot-and-cross diagrams can be used to show the electric configurations in simple molecules Electrons from one atom are represented by a dot, and the electrons of the other atom are represented by a cross The electron shells of each atom in the molecule overlap and the shared electrons are shown in the area of overlap The dot-and-cross diagram of the molecule shows clearly which atom each electron originated from Page 24 of 40 © 2015-2021 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to savemyexams.co.uk for more awesome resources YOUR NOTES  Diagram showing how a covalent bond forms between two chlorine atoms  Exam Tip When drawing dot-and-cross diagrams for covalent compounds, make sure that the electron shell for each atom is full (remember that the 1st shell can only hold 2 electrons). Page 25 of 40 © 2015-2021 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to savemyexams.co.uk for more awesome resources Single Covalent Bonds YOUR NOTES Many simple molecules exist in which two adjacent atoms share one pair of  electrons, also known as a single covalent bond (or single bond) Common Examples of Simple Molecules Hydrogen: Chlorine: Water: Methane: Page 26 of 40 © 2015-2021 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to savemyexams.co.uk for more awesome resources YOUR NOTES  Ammonia: Hydrogen chloride: Page 27 of 40 © 2015-2021 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to savemyexams.co.uk for more awesome resources 2.3.2 Molecules & Compounds YOUR NOTES  Covalent Bonds in Complex Covalent Molecules EXTENDED Some atoms need to share more than one pair of electrons to gain a full outer shell of electrons If two adjacent atoms share two pairs of electrons, two covalent bonds are formed, also known as a double bond If two adjacent atoms share three pairs of electrons, three covalent bonds are formed, also known as a triple bond Nitrogen: When 2 nitrogen atoms react they share 3 pairs of electrons to form a triple bond Ethene: In ethene, the 2 carbon atoms share 2 pairs of electrons This is known as a double bond Methanol: Page 28 of 40 © 2015-2021 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to savemyexams.co.uk for more awesome resources YOUR NOTES  Carbon Dioxide:  Exam Tip Be careful when drawing dot-and-cross diagrams, it is a common mistake for students to draw the wrong type of diagram. Remember, if the compound contains metal and non-metal, it is an ionic compound and you need to draw the ions separated, with square brackets around each ion, together with a charge. If the compound contains non-metal atoms only, it is a covalent compound, the shells should overlap and contain one or more pairs of electrons. Page 29 of 40 © 2015-2021 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to savemyexams.co.uk for more awesome resources 2.3.3 Properties of Simple Molecular Compounds YOUR NOTES  Properties of Simple Molecular Compounds Small molecules are compounds made up of molecules that contain just a few atoms covalently bonded together They have low melting and boiling points so covalent compounds are usually liquids or gases at room temperature As the molecules increase in size, the melting and boiling points generally increase Small molecules have poor electrical conductivity Page 30 of 40 © 2015-2021 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to savemyexams.co.uk for more awesome resources Explaining the Properties of Simple Molecular Compounds YOUR NOTES EXTENDED  Small molecules have covalent bonds joining the atoms together, but intermolecular forces that act between neighbouring molecules They have low melting and boiling points as there are only weak intermolecular forces acting between the molecules These forces are very weak when compared to the covalent bonds and so most small molecules are either gases or liquids at room temperature As the molecules increase in size the intermolecular forces also increase as there are more electrons available This causes the melting and boiling points to increase The bonds between hydrogen and oxygen in water are COVALENT, and the attractions between the molecules are INTERMOLECULAR FORCES which are about one tenth as strong as covalent bonds  Exam Tip The atoms within covalent molecules are held together by covalent bonds while the molecules in a covalent substance are attracted to each other by intermolecular forces. Electrical Conductivity Molecular compounds are poor conductors of electricity as there are no free ions or electrons to carry the charge. Most covalent compounds do not conduct at all in the solid state and are thus insulators Common insulators include the plastic coating around household electrical wiring, rubber and wood Page 31 of 40 © 2015-2021 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to savemyexams.co.uk for more awesome resources YOUR NOTES  The plastic coating around electrical wires is made from covalent molecules that do not allow a flow of charge Page 32 of 40 © 2015-2021 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to savemyexams.co.uk for more awesome resources 2.4 Giant Structures YOUR NOTES  2.4.1 Diamond & Graphite Structure of Graphite & Diamond Diamond and graphite are allotropes of carbon which have giant covalent structures Both substances contain only carbon atoms but due to the differences in bonding arrangements they are physically completely different Giant covalent structures contain billions of non-metal atoms, each joined to adjacent atoms by covalent bonds forming a giant lattice structure Diamond In diamond, each carbon atom bonds with four other carbons, forming a tetrahedron All the covalent bonds are identical, very strong and there are no intermolecular forces Diagram showing the structure and bonding arrangement in diamond Graphite Each carbon atom in graphite is bonded to three others forming layers of hexagons, leaving one free electron per carbon atom which Page 33 of 40 © 2015-2021 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to savemyexams.co.uk for more awesome resources becomes delocalised YOUR NOTES The covalent bonds within the layers are very strong, but the layers are attracted to  each other by weak intermolecular forces The structure and bonding in graphite Page 34 of 40 © 2015-2021 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to savemyexams.co.uk for more awesome resources Uses of Graphite & Diamond YOUR NOTES Properties of Diamond  Diamond has the following physical properties: It does not conduct electricity It has a very high melting point It is extremely hard and dense All the outer shell electrons in carbon are held in the four covalent bonds around each carbon atom, so there are no freely moving charged particles to carry the current thus it cannot conduct electricity The four covalent bonds are very strong and extend in a giant lattice, so a very large amount of heat energy is needed to break the lattice thus it has a very high melting point Diamond ́s hardness makes it very useful for purposes where extremely tough material is required Diamond is used in jewellery due to its sparkly appearance and as cutting tools as it is such a hard material The cutting edges of discs used to cut bricks and concrete are tipped with diamonds Heavy-duty drill bits and tooling equipment are also diamond-tipped  Exam Tip Diamond is the hardest naturally occurring mineral, but it is by no means the strongest. Students often confuse hard with strong, thinking it is the opposites of weak. Diamonds are hard, but brittle – that is, they can be smashed fairly easily with a hammer. The opposite of saying a material is hard is to describe it as soft. Properties of Graphite Each carbon atom is bonded to three others forming layers of hexagonal-shaped forms, leaving one free electron per carbon atom These free (delocalised) electrons exist in between the layers and are free to move through the structure and carry charge, hence graphite can conduct electricity The covalent bonds within the layers are very strong but the layers are connected to each other by weak forces only, hence the layers can slide over each other making graphite slippery and smooth Graphite thus: Conducts electricity Has a very high melting point Is soft and slippery, less dense than diamond Graphite is used in pencils and as an industrial lubricant, in engines and in locks It is also used to make non-reactive electrodes for electrolysis Page 35 of 40 © 2015-2021 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to savemyexams.co.uk for more awesome resources YOUR NOTES  Exam Tip Don’t confuse pencil lead with the metal lead – they have nothing in  common. Pencil lead is actually graphite, and historical research suggests that in the past, lead miners sometimes confused the mineral galena (lead sulfide) with graphite; since the two looked similar they termed both minerals ‘lead’. The word graphite derives from the Latin word ‘grapho’ meaning ‘I write’, so it is a well named mineral! Page 36 of 40 © 2015-2021 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to savemyexams.co.uk for more awesome resources 2.4.2 Silicon(IV) Oxide YOUR NOTES  Structure of Silicon(IV) Oxide EXTENDED Silicon(IV) oxide (also known as silicon dioxide or silica), SiO2, is a macromolecular compound which occurs naturally as sand and quartz Each oxygen atom forms covalent bonds with 2 silicon atoms and each silicon atom in turn forms covalent bonds with 4 oxygen atoms A tetrahedron is formed with one silicon atom and four oxygen atoms, similar to diamond Diagram showing the structure of SiO2 with the silicon atoms in blue and the oxygen atoms in red Page 37 of 40 © 2015-2021 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to savemyexams.co.uk for more awesome resources Comparing Diamond & Silicon(IV) Oxide YOUR NOTES EXTENDED  SiO2 has lots of very strong covalent bonds and no intermolecular forces so it has similar properties to diamond It is very hard, has a very high boiling point, is insoluble in water and does not conduct electricity SiO2 is cheap since it is available naturally and is used to make sandpaper and to line the inside of furnaces Page 38 of 40 © 2015-2021 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to savemyexams.co.uk for more awesome resources 2.4.3 Metallic Bonding YOUR NOTES  Metallic Bonding EXTENDED Metal atoms are held together strongly by metallic bonding in a giant metallic lattice Within the metallic lattice, the atoms lose the electrons from their outer shell and become positively charged ions The outer electrons no longer belong to a particular metal atom and are said to be delocalised They move freely between the positive metal ions like a 'sea of electrons' Metallic bonds are strong and are a result of the attraction between the positive metal ions and the negatively charged delocalised electrons Diagram showing metallic lattice structure with delocalised electrons Page 39 of 40 © 2015-2021 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to savemyexams.co.uk for more awesome resources Properties of Metals YOUR NOTES EXTENDED  Metals have high melting and boiling points There are many strong metallic bonds in giant metallic structures between the positive metal ion and delocalised electrons A lot of heat energy is needed to break these bonds Metals conduct electricity There are free electrons available to move through the structure and carry charge Electrons entering one end of the metal cause a delocalised electron to displace itself from the other end Hence electrons can flow so electricity is conducted Metals are malleable and ductile Layers of positive ions can slide over one another and take up different positions Metallic bonding is not disrupted as the outer electrons do not belong to any particular metal atom so the delocalised electrons will move with them Metallic bonds are thus not broken and as a result metals are strong but flexible They can be hammered and bent into different shapes or drawn into wires without breaking  Exam Tip When explaining why metals can conduct electricity, be careful of the terminology you use. Don't get confused with ionic compounds. Metals can conduct electricity as they have free electrons that can carry charge whereas molten or aqueous ionic compounds can conduct electricity because they have free ions that can carry charge. Page 40 of 40 © 2015-2021 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers