Make the following into good but brief study notes: Atoms are the building blocks of the chemical elements. They are, therefore, also the building blocks of compounds and mixtures.... Make the following into good but brief study notes: Atoms are the building blocks of the chemical elements. They are, therefore, also the building blocks of compounds and mixtures. For thousands of years, alchemists and scientists have searched for patterns in the substances that make up the universe. Many of them succeeded to some extent. But the discovery by Lord Rutherford in 1911 that most of the atom was empty space, and subsequent discoveries about the particles inside that atom by Niels Bohr and other scientists, provided the missing links in the patterns. The periodic table is organised on the basis of atomic numbers. The atomic number of an element is the number of protons present in each atom. Atoms with the same atomic number have identical chemical properties. Because atoms are electrically neutral, the number of protons in an atom is the same as the number of electrons. The mass number of an atom is the sum of the number of protons and neutrons in the atom. The number of neutrons in an atom can therefore be calculated by subtracting the atomic number from the mass number. This information is usually shown in the following way: For example, the element iron has a mass number of 56 and an atomic number of 26. It can be represented as follows: 56 26Fe. Once you know the atomic number and mass number of an element, you can work out how many electrons and neutrons it has. The atomic number of iron is 26 because all iron atoms have 26 protons. Iron’s mass number of 56 indicates that most iron atoms have a total of 56 protons and neutrons. To calculate the number of neutrons, the atomic number is subtracted from the mass number to give 30 neutrons. Since atoms are electrically neutral and protons have a positive charge, each iron atom has 26 electrons. Measuring and comparing the masses of atoms is difficult because of their extremely small size. Chemists solve this problem by comparing equal numbers of atoms, rather than trying to measure the mass of a single atom. A further problem arises because not all atoms of an element are identical. Although all atoms of a particular element have the same atomic number, they can have different numbers of neutrons. Hence, some elements contain atoms with slightly different masses. These different masses are used to calculate an average or weighted mean, which is based on the relative amounts of each type of atom. This number is referred to as the relative atomic mass and is usually not a whole number. The mass number (A) of an element can usually be found by rounding the relative atomic mass. The periodic table contains eight groups (or families) of elements, some of which have been given special names. (Remember that these groups form columns in the periodic table.) Group 1 elements are known as alkali metals. The alkali metals all react strongly with water to form basic solutions. Group 2 elements are referred to as alkaline earth metals. Group 17 elements are known as halogens. The halogens are brightly coloured elements. Chlorine is green, bromine is red-brown and iodine is silvery-purple. Group 18 elements are known as noble gases. The noble gases are inert and do not readily react with other substances. The block of elements in the middle of the table is known as the transition metal block. The line that zigzags through the periodic table separates the metals from the non-metals. About three-quarters of all elements are classified as metals, which are found on the left-hand side of the table. The non-metals are found on the upper right-hand side of the table. Eight elements that fall along the line between metals and non-metals have properties belonging to both. They are called metalloids. Metals have several features in common. They are solid at room temperature, except for mercury which is a liquid. They can be polished to produce a high shine or lustre. They are good conductors of electricity and heat. They can all be beaten or bent into a variety of shapes. We say they are malleable. They can be made into a wire. We say they are ductile. They usually melt at high temperatures. Mercury, which melts at −40° C, is one exception. Only 22 of the elements are non-metals. At room temperature, eleven of them are gases, ten are solid and one is liquid. The solid non-metals have most of the following features in common. They cannot be polished to give a shine like metals; they are usually dull or glassy. They are brittle, which means they shatter when they are hit. They cannot be bent into shape. They are usually poor conductors of electricity and heat. They usually melt at low temperatures. Many of the non-metals are gases at room temperature. Some of the elements in the non-metal group look like metals. One example is silicon. While it can be polished like a metal, silicon is a poor conductor of heat and electricity and cannot be bent or made into wire. Elements that have features of both metals and non-metals are called metalloids. There are eight metalloids altogether: boron, silicon, arsenic, germanium, antimony, polonium, astatine and tellurium. There are a number of repeating patterns in the periodic table. The most obvious is the change from metals on the left of each period to non-metals on the right. Other patterns exist in the physical and chemical properties of elements in the same group or period. When atoms come into contact with one another, they often join together to form molecules. Other atoms join together to form giant crystals that contain billions of atoms. The electrons in each atom account for the chemical behaviour of all matter because they form the outermost part of the atom. Drawing an accurate picture of an atom using a diagram is difficult because electrons cannot be observed like most particles. Their exact location within the atom is never known — they tend to behave like a ‘cloud’ of negative charge. Furthermore, an atom is many times larger than its nucleus so it is not practical to draw a diagram to scale. Nonetheless, diagrams are useful because they help us to understand how atoms combine. An electron shell diagram is a simplified model of an atom. In these diagrams, the nucleus of the atom, containing protons and neutrons, is drawn in the middle. Electrons are arranged in a series of energy levels around the nucleus. These energy levels are called shells and are drawn as concentric rings around the nucleus. The electrons in the inner shells are more strongly attracted to the nucleus than those in the outer shells. Each shell contains a limited number of electrons. The first (or K) shell holds a maximum of two electrons. The second (or L) shell holds up to eight electrons. The third (M) shell holds up to 18 electrons. The fourth (N) shell holds up to 32 electrons. The maximum number of electrons in each shell can be calculated using the rule: The nth shell holds a maximum of 2n2 electrons. For example, the fourth shell holds a maximum of 2 × 42 electrons, which is 32 electrons. Electron configuration The electron configuration of an element is an ordered list of the number of electrons in each shell. The electron configuration is determined from the atomic number of the element, which is the same as the number of protons in the nucleus of each atom. In a neutral atom, the total number of electrons is the same as the number of protons. To work out the electron configuration of a particular atom, you need to remember that electrons occupy the innermost shells first. Once the first two shells are filled, the remaining electrons begin to fill the third shell. For example, the element sodium has an atomic number of 11. Each atom has 11 protons and 11 electrons. The electrons will fill the two innermost shells first — two in the first shell and eight in the second shell. That accounts for ten. The remaining electron must be in the third shell because the first two have already been filled. The electron configuration of an atom is written by showing the number of electrons in each shell separated by commas. For example: sodium 2, 8. The periodic table explained When Mendeleev and Meyer grouped elements on the basis of their similar chemical properties, they were not aware of the existence of electrons. We can now explain many of their observations using our understanding of electron shells. Atoms in the same group of the periodic table have similar properties because they have the same number of electrons in their outer shells. (The outer shell is the last shell to be filled by electrons.) The number of electrons in the outer shell relates to the group number in the periodic table. Hence, all elements in group 1 have one electron in their outer shell and all elements in group 18 (with the exception of helium) have eight electrons in their outer shell. Filling up in turn. The largest atoms contain up to seven shells of electrons. Thus, there are seven periods (rows) in the periodic table. (Look at the periodic table in subtopic 4 .2 to confirm this.) The period number tells you the number of shells containing electrons. The first shell can hold up to two electrons, so there are two elements in the first period, with hydrogen containing one electron and helium containing two. The second shell holds up to eight electrons, so there are eight elements in the second period. Even though the third shell can hold up to 18 electrons, there are only eight elements in the third period. This is because the outer shell of an atom can never hold more than eight electrons as the atom would then become unstable. Therefore, while the third shell is yet to be filled completely, electrons begin to fill the fourth shell in both potassium and calcium atoms. This stabilises the atoms because the third shell is no longer the outer shell. The filling of the third shell resumes in the block of elements from scandium to zinc (the transition metals). Once the third shell is full, the fourth shell continues to fill from gallium to xenon. Note that the fourth shell of the potassium atom begins filling before the third shell is full. It’s the shell structure that counts Knowledge of the electron shell structures of atoms helps us to understand how compounds such as sodium chloride (table salt) form. When atoms react with each other to form compounds, it is the electrons in the outer shell that are important in determining the type of reaction that occurs. It’s great to be noble. In 1919, Irving Langmuir suggested that the noble gases do not react to form compounds because they have a stable electron configuration of eight electrons in their outer shell. Most other atoms react because their electron arrangements are less stable than those of the noble gases. Atoms become more stable when they attain an electron arrangement that is the same as that of the noble gases. Chemical reactions can allow atoms to obtain this arrangement. The atoms of other elements must gain or lose electrons to attain full outer shells. In this way they become more stable, ending up with the electron arrangement of the nearest noble gas in the periodic table. Atoms that have lost or gained electrons and therefore carry an electric charge are called ions. Metal atoms, such as sodium, magnesium and potassium, have a small number of outer shell electrons. They form ions by losing the few electrons in their outer shell. This means that metal ions have more protons than electrons and so are positively charged. For example, the magnesium atom loses its two outer shell electrons to become a positively charged magnesium ion. The symbol for the magnesium ion is Mg2+. The ‘2+’ means that two electrons have been lost to form the ion. Positively charged ions are called cations. Non-metal atoms form ions by gaining electrons to fill their outer shell. These ions contain more electrons than protons, so they are negatively charged. For example, the chlorine atom gains one electron to fill its outer shell, becoming a negatively charged chloride ion. Its symbol is Cl−. The ‘−’ means that one electron has been gained to form the ion. Negatively charged ions are called anions. The diagram below shows how sodium and chlorine atoms lose and gain electrons respectively to form ions. Note that the sodium atom becomes a sodium ion and that the chlorine atom becomes a chloride ion. (When non-metals form ions, the suffix ‘-ide’ is used.) Compounds such as sodium chloride, copper sulphate, calcium carbonate and sodium hydrogen carbonate all form when atoms come in contact with each other and lose or gain electrons. Compounds formed in this way are called ionic compounds. Ionic compounds form when metal and non-metal atoms combine. A sodium atom loses an electron to form an ion and a chlorine atom gains an electron to form an ion. The electrons are transferred from one atom to the other, and the oppositely charged ions produced attract each other and form a compound. This electrical force of attraction between the ions is called an ionic bond. The diagram on the following page shows some examples of the transfer of electrons that occurs when ionic compounds are formed. Note that more than two atoms may be involved to ensure that all the elements achieve eight electrons in their outer shell. For example, when magnesium reacts with chlorine to form magnesium chloride, each magnesium atom loses two electrons. Since each chlorine atom needs to gain only one electron, a magnesium atom gives one electron to each of two chlorine atoms. The resulting Mg2+ and Cl− ions are attracted to each other to form the compound MgCl2. Ionic compounds have the following properties. They are made up of positive and negative ions. They are usually solids at room temperature. They normally have very high melting points because the electrostatic force of attraction between the ions is very strong. They usually dissolve in water to form aqueous solutions. Their aqueous solutions normally conduct electricity. Ionic compounds form when atoms lose or gain electrons. Atoms can also achieve stable electron configurations by sharing electrons with other atoms to gain a full outer shell. When two or more atoms share electrons, a molecule is formed. A chemical bond formed by sharing electrons is called a covalent bond. The compounds formed are called covalent or molecular compounds. Non-metal atoms share electrons to form covalent bonds. Molecules can be made of more than one type of atom or made of atoms of the same element. For example, oxygen gas consists of molecules formed when two oxygen atoms share electrons. Individual atoms of oxygen are not stable and become more stable by sharing electrons with each other. It is possible to draw diagrams to show how elements share electrons to form covalent compounds. These diagrams are called electron dot diagrams. They show the symbol for the atom and dots for the electrons in the outer shell of atoms. The table at right shows electron dot diagrams for some elements. Note that the electrons in the diagrams are arranged in four regions around the atom. Wherever possible, they are grouped in pairs. When elements combine to form covalent compounds, they share electrons to achieve a full outer shell with eight electrons. Hydrogen has a full outer shell when it has two electrons, but all the other elements in the table need eight electrons to fill the outer shell. Most covalent compounds have the following properties. They exist as gases, liquids or solids with low melting points because the forces of attraction between the molecules are weak. They generally do not conduct electricity because they are not made up of ions. They are usually insoluble in water. Have you ever wondered why gold can be found lying near the surface of the Earth and yet we need to mine iron ore and smelt it in large furnaces before we can obtain iron? The answer lies in the reactivity of the metals. Gold is a very unreactive element. It does not combine readily with other elements to form compounds. Most metals are much more reactive than gold. When the Earth formed, the more reactive metals — including aluminium, copper, zinc and iron — reacted with other elements to form ionic compounds. These compounds are the mineral ores from which the metal elements are obtained. Iron ores include haematite (Fe 2O 3 ), magnetite (Fe 3O 4 ), siderite (FeCO 3 ), pyrite (FeS 2 ) and chalcopyrite (CuFeS 2). The reactivity of metals is dependent on how easily they are able to give up their outer shell electrons. For example, it is easier for an atom to give up a single electron from an outer shell than to give up two electrons. The reactivity of metals can be investigated by observing their reactions with acids. A metal reacts with hydrochloric acid according to the equation: metal + hydrochloric acid ⟶ salt + hydrogen gas. In these reactions, electrons are transferred away from the metal atoms to the hydrogen in the acid, forming positive metal ions and hydrogen gas. The metal is said to have displaced the hydrogen from the acid. For this reason, these reactions are also displacement reactions. The chemicals used in your school science laboratory are usually identified by both a name and a formula. Most people are able to recognise the formula of common compounds such as water (H2O) and carbon dioxide (CO2). A chemical formula (plural formulae) is a shorthand way to write the name of an element or compound. It tells us the number and type of atoms that make up an element or compound. Writing the correct formula is of paramount importance in chemistry. Most chemical problems cannot be solved without the knowledge of chemical formulae. Often, the formula of a substance is simply the symbol for the element. Metals such as iron and copper, which contain only one type of atom, are identified simply by the symbol for that element (for example, Fe and Cu). Noble gases such as neon (Ne) have a similar formula. Some non-metal elements such as hydrogen, oxygen and nitrogen exist as simple molecules. These molecules form when atoms of the same nonmetal are joined together by covalent bonds. For example, the formula for the element hydrogen is H2, indicating that two hydrogen atoms are joined together to make each molecule of hydrogen. A molecular formula is a way to describe the number and type of atoms that join to form a molecule. The formula of a compound shows the symbols of the elements that have combined to make the compound and the ratio in which the atoms have joined together. For example, the chemical formula for the covalent compound methane, a constituent of natural gas, is CH4 — one carbon atom for every four hydrogen atoms. The formula for the ionic compound calcium chloride, which is used as a drying agent, is CaCl2 — two chlorine ions for every calcium ion. Knowledge of the valency of an element is essential if we wish to write formulae correctly. The valency of an element is equal to the number of electrons that each atom needs to gain, lose or share to fill its outer shell. For example, the chlorine atom has only seven electrons in its outer shell, which can hold eight electrons. By gaining one electron, its outer shell becomes full. Chlorine therefore has a valency of one. The magnesium atom has two electrons in its outer shell. By losing two electrons, it is left with a full outer shell. Magnesium therefore has a valency of two. A simple guide to remembering the valency of many elements is to remember which group in the periodic table they belong to. The number of outer shell electrons allows you to work out how many electrons are required to fill the outer shell. The table at right provides a simple guide to the valency of many elements. To write the formula of a non-metal compound made up of only two elements, use the valency of each element. The formulae for ionic compounds can be written from knowledge of the ions involved in making up the compound. In ionic compounds, metal ions combine with non-metal ions. The tables below list common positive and negative ions and their names. Metal atoms usually form positive ions. The number of positive charges on the ion is called its electrovalency. For example, a sodium ion has one positive charge (Na+), the calcium ion has two positive charges (Ca2+) and the aluminium ion has three (Al3+). Note that some of the transition metals (e.g. iron) have more than one valency, as shown in the table above right. The Roman numerals in brackets after iron and copper identify the valency. Non-metals usually form negative ions. The number of negative charges is the electrovalency of the ion. For example, chloride has one negative charge (Cl–), oxide has two negative charges (O2–) and phosphide has three (P3–). There are also some more complex negative ions called molecular ions, such as hydroxide ions (OH–) and sulphate ions (SO42–). These groups of atoms have an overall negative charge and are treated as a single entity. Note that the hydrogen ion, although a non-metal ion, exists as a positive ion. In order to communicate with each other easily about chemical reactions, scientists all over the world need to use the same language. That language involves chemical symbols, formulae and equations. Word equations provide a simple way to describe chemical reactions by stating the reactants and products. Chemical equations that use formulae provide more information. They show how the atoms in the reactants combine to form the products. Writing chemical equations involves some simple mathematics and a knowledge of chemical formulae. Chemical equations are set out in the same way as word equations, with the reactants to the left of the arrow and products to the right. However, they are different from word equations in three ways: • Formulae are used to represent the chemicals involved. • The physical states of the chemicals are often included. • Numbers are written in front of the formulae in order to balance the numbers of atoms on each side of the equation. The rules of a ‘game’ of balancing equations are described below. Read through the rules very carefully before you play the game. When table salt (sodium chloride) is dissolved in water to form an aqueous solution, it seems to disappear. The ions in the salt no longer bond together as a large array of positive and negative ions like they do as a solid. The sodium ions and the chloride ions separate when they dissolve. Sodium chloride dissolving in water can be represented by the equation: NaCl(s) ⟶ (H2O) Na+(aq)+Cl−(aq) Ions in aqueous solutions are therefore separate entities and are able to react independently. Ionic compounds dissolve in water to varying degrees. Some are soluble, others slightly soluble and others insoluble. The box on the next page outlines some handy rules for predicting whether or not a compound is soluble. When two solutions containing dissolved ions are mixed together, the ions are able to come into contact with each other. Oppositely charged ions attract. In some cases, the attraction is strong enough to form ionic bonds and hence a new ionic compound. Some of these compounds are insoluble (unable to dissolve in water) and so a solid called a precipitate forms. Chemical reactions in which precipitates form are called precipitation reactions. When colourless lead nitrate solution and colourless potassium iodide solution are added together, a brilliant yellow precipitate is formed. Another example of a precipitation reaction occurs between silver nitrate solution and sodium chloride solution. When these two colourless solutions are added together in a test tube the contents become cloudy, indicating that a precipitate has formed. If the tube is allowed to stand for a while, the solid settles to the bottom and we can see that a clear solution is also present. The products of the reaction are insoluble solid silver chloride (the precipitate) and sodium nitrate (not visible because it is soluble in water). This reaction can be represented by the equation: silver nitrate + sodium chloride ⟶ silver chloride + sodium nitrate AgNO3(aq) + NaCl(aq) ⟶ AgCl(s) + NaNO3(aq) Silver nitrate, sodium chloride and sodium nitrate all dissolve in water. Therefore, they have the symbol (aq). Silver chloride does not dissolve in water, so it has the symbol (s) to indicate that it is solid. The equation shows that the ions in the reactants have changed partners. The silver ion is paired with the chloride ion on the product side of the reaction and the sodium ion is paired with the nitrate ion. The opposite is the case on the reactant side. A positive ion can pair up only with a negative ion because oppositely charged ions are attracted to each other. When writing the formula of any new compound, the positive ion is always written first. In a world where countless chemical reactions take place, it is helpful to classify the reactions. They can be classified according to whether they release or absorb energy. They can also be grouped according to the nature of the reactants, the nature of the products, the way in which charged particles in atoms rearrange themselves, or even the number of reactants. Any one reaction can fall into several different groups. Corrosion is a chemical reaction in which a metal is ‘eaten away’ by substances in the air or water. The tarnishing of silver jewellery and cutlery, rust, and the green coating that appears on copper are all examples of corrosion. In the classroom laboratory, waste solutions containing silver ions are never poured down the sink. They are collected and sent to commercial laboratories where the valuable silver is recovered from the solutions. Silver metal can be recovered from silver nitrate solution simply by adding a piece of copper wire. This happens according to the equation: Cu(s) + 2AgNO3(aq) ⟶ 2Ag(s) + Cu(NO3)2(aq) atoms ions atoms ions Reactions of this type, where an element displaces another element from a compound, are called displacement reactions. In this example, copper has displaced the silver from the silver nitrate solution. The reactions of metals with acids are examples of displacement reactions. Combustion reactions are those in which a substance reacts with oxygen, and heat is released. Examples of combustion reactions include the burning of petrol in a motorcycle engine, wax vapour in a candle flame and natural gas in a kitchen stove. In each of these cases hydrocarbons (compounds containing only the elements carbon and hydrogen) combine with oxygen in the air to form carbon dioxide gas and water vapour. This is shown in the following equation for the burning of methane (natural gas) in a gas jet. CH4(g) + 2O2(g) ⟶ CO2(g) + 2H2O(g) methane molecule In decomposition reactions one single compound breaks down into two or more simpler chemicals. An example of this is the decomposition of zinc carbonate. This is represented by the equation: ZnCO3(s) ⟶ Zn(s) + CO2(g) Often two elements combine in chemical reactions to form a compound. Such reactions are called combination reactions. The reaction of magnesium with oxygen is a spectacular example. Magnesium burns in air, producing a brilliant flash of white light. The equation for this combination reaction is: 2Mg(s) + O2(g) ⟶ 2MgO(s) atoms molecules molecules Notice that this combination reaction is also a combustion reaction. It is also an exothermic reaction because it transfers energy to the surroundings. (Endothermic reactions are chemical reactions that absorb energy from the surroundings.) In many chemical reactions, electrons are either completely or partially moved from one atom, ion or molecule to another. This process is known as electron transfer. Chemical reactions that involve electron transfer are called redox reactions. Redox reactions are extremely important in industry and in our everyday lives. A redox reaction is really two reactions occurring simultaneously. In the electron transfer process, one reactant loses electrons and another gains electrons. Loss of electrons is known as oxidation. Gain of electrons is called reduction. Oxidation and reduction always occur together, thus the two words are combined to form the word redox, which is used to describe reactions where electrons are transferred. The mnemonic OIL RIG may help you to remember these processes: oxidation is loss, reduction is gain. Each of the corrosion, displacement, combustion and combination reactions described earlier are examples of redox reactions. Oxidation and reduction can be clearly seen in the reaction when zinc corrodes. Neutralisation is the name given to the chemical reaction in which an acid and a base react with each other to produce water. The other substance produced in a neutralisation reaction is called a salt. Many neutralisation reactions occur in water. These reactions are said to occur 'in solution’. Your stomach contains hydrochloric acid, which helps to break up food for digestion. Too much acid, however, can be a problem. If your stomach produces too much acid, you may need to take an antacid such as milk of magnesia. This medicine has the solid base magnesium oxide (MgO) suspended in it. This base reacts with the hydrochloric acid in your stomach according to the equation: MgO(s) + 2HCl(aq)⟶MgCl2(aq)+H2O(I) The products are the salt magnesium chloride and water. The salt contains the positive metal ion from the base and the negative non-metal ion from the acid. The base sodium hydrogen carbonate, commonly known as bicarb, is a component of baking powder. It has the formula NaHCO3 and contains the hydrogen carbonate ion HCO3–. When bases containing this ion react with acids, carbon dioxide gas is produced as well as salt and water. When hydrochloric acid and bicarb are mixed together, the following reaction takes place: NaHCO2(s) + HCl(aq)⟶NaCl(aq)+CO2(g) + H2O(I) In both of the reactions mentioned, the salts formed were metal chlorides because they contained the chloride ion (Cl−) from the hydrochloric acid. Neutralisation reactions between many different acids and bases are possible; therefore, it is possible to produce many different salts. Some of these reactions are summarised in the table below. Another way to increase the rate of a reaction is to use a catalyst. Catalysts are not changed by the reaction. There is always as much catalyst present at the end of a reaction as there was at the start. Catalysts work by helping bonds to break more easily; therefore, the reactants need less energy to react and the reaction is faster. A catalyst can be recovered and used again and again. We all make use of catalysts every day. Cars have catalytic converters; contact lenses are cleaned using a catalysed chemical reaction; and there are catalysts in the food you eat every day. There are also thousands of catalysts in your body without which you could not live. These biological catalysts are called enzymes. Industry makes use of many catalysts. For example: Compound New compound New products • Iron and iron oxide are used to catalyse the production of ammonia gas. Ammonia is used to make fertilisers and explosives. • vanadium oxide (V2O5) is used in the production of sulfuric acid. One important reaction in this process, between sulphur dioxide gas and oxygen, has a very slow rate at room temperature. However, it proceeds rapidly at 450 °C in the presence of a vanadium oxide catalyst according to the equation: V2O5 450°C 2SO2(g) + O2(g) ⟶ 2SO3(g). Read more