9.3 Chemical Structures and Properties PDF

9.3: Chemical Structures and Properties Key topics: Understanding the Periodic Table Properties of Group 1 elements (The Alkali Metals) Properties of Group 7 elements (The Halogens) Noble Gases Giant structures The Periodic Table: Development The...

9.3: Chemical Structures and Properties Key topics: Understanding the Periodic Table Properties of Group 1 elements (The Alkali Metals) Properties of Group 7 elements (The Halogens) Noble Gases Giant structures The Periodic Table: Development The periods The groups Density Development: Before the discovery of protons, neutrons, and electrons, the Periodic Table was arranged mainly based on the atomic masses of elements. This led to issues where elements with similar properties were not always grouped. After the discovery of isotopes (the same elements with different masses) in 1913, this issue was fixed. Dimitri Mendeleev was a Russian chemist who organized the Periodic Table based on their atomic masses and in similar categories, and also cleverly left gaps in his table to predict future elements to be discovered. He created a systematic way to sort the elements while still allowing space for future elements. The periods & the groups: Also known as the rows and columns They represent how elements are organized based on their atomic structure and properties The Groups: Elements in the same group have the same number of valence electrons (electrons in their outermost shell) Elements in a group have similar chemical properties because they interact with other elements in the same way during chemical reactions. The Periods: Elements in the same period have the same number of electron shells. As you move across a period from left to right: The number of protons and electrons increases.  Elements become less metallic and more non-metallic in character. Density: Density → The amount of matter per unit volume 1. Volume a. increased volume - more space, less density 2. Shape a. only matters if the shape changes volume 3. Density a. density increases - the material sinks Properties of Group 1 elements: Trends in reactivity & reasons Trends in density & MP and BP Chemical reactions in water Overview: The group 1 elements are known as the Alkali Metals All the elements in this group have one electron on their outer shell They are very reactive, but not very dense, and can conduct electricity & heat well.  The elements are; Lithium (Li), Sodium (Na), Potassium (K), Rubidium (Rb), Casesium (Cs), and Francium (Fr). Trends in reactivity: As you go down the group 1 elements, the reactivity of each element increases. This happens because the atoms increase in size, weakening the connection between the electron on the valence shell (outermost shell) and the nucleus. And since group 1 elements want to lose an electron to have an empty valence shell, having bigger atoms (and a weaker force holding them together) makes it easier for the elements to react (bond) with other elements. In short; the further down the group an element is, the more reactive. This happens because the further from the nucleus the outer electron is, the more easily the electron is lost. Trends in density & MP and BP: Density increases as you go down group 1 This is because of the atoms getting bigger, and a bigger atom equals a bigger mass, so it is more dense The melting point (MP) and the boiling point (BP) decreases as you go down the group This happens because the atoms get bigger, weakening the metallic bond, and making it easier to break down the atom. Chemical reactions in water: All group 1 elements react with water to form an alkali and a hydrogen. They all form ionic compounds; white solids that dissolve in water to form colorless solutions (e.g. salt water) For example: Lithium (Li) + Water (H2O) = Lithium Hydroxide (LiOH) + Hydrogen Gas (H2) Sodium (Na) + Water (H2O) = Sodium Hydroxide (NaOH) + Hydrogen Gas (H2) Properties of Group 7 elements: Trends in reactivity & reasons Trends in density & MP and BP Overview: Group 7 elements are also known as the Halogens All the elements are molecules containing two atoms (diatomic) Each atom is 1 electron short of a noble gas electron (All group 7 elements have 7 electrons in their outer shell) They share atoms in a covalent bond to form full valence shells  The Group 7 elements are; Florine (F), Chlorine (Cl), Bromine (Br), Iodine (I), and Astatine(At) Sometimes Tennesine (Ts) is considered. Trends in reactivity: The elements are less reactive as you go down Group 7 This is because the atoms get larger, thus increasing the distance between the nucleus and the outermost electrons, where the incoming electron would be added. Since all the Halogens want to gain an electron to have a full valence shell, larger atoms reduce the attraction of the positively charged nucleus for an additional negatively charged electron. Shielding by the inner electron shells also reduces the attraction from the nucleus, and that's why it is harder to pull in an electron as there is an increase in force. Shielding → when inner electrons block the outer electrons from feeling the full pull of the nucleus. This happens because the inner electrons repel the outer ones, making the nucleus's attraction weaker for the outermost electrons Trends in density & MP and BP: Density increases as you go down Group 7 This happens because the atomic mass of halogens increases as you go down the group. Heavier atoms have more protons, neutrons, and electrons. Also, as a result, the overall density (mass per unit volume) becomes higher. The MP and BP increases as you go down the group This happens because as you go down the group, the atoms increase in size. Larger atoms lead to larger diatomic molecules (e.g., F₂, Cl₂, Br₂, I₂), which increases their molecular mass. And since there is a larger mass, there is a stronger force holding the atoms together- and are harder to break down. Noble gases: Key properties Uses Overview:  The elements in Group 8 / 18 are called the Noble Gases The noble gases are; Helium (He), Neon (Ne), Argon (Ar), Krypton (Kr), Xenon (Xe), and Radon (Rn) Organessen (Og) could be considered but it is a synthetic element. Key properties: They are / have: Colorless & Odorless Non- flammable Very unreactive (inert) gases Low BP and MP Non-metals Not very dense Full valence shells Other properties: They exist as single atoms (monatomic) They only have 1 atom (as they have full outer shells, making them very stable & inert) When exposed to electric current, they glow with distinct colors (conducts electricity) As you go down the group the elements get more dense The BP and MP increases as you go down the group Uses: Helium party balloons, airships Krypton used in lasers, lamps, and screens Argon used in lightbulbs as they last longer  Xenon used in lamps, and also as a natural anesthetic Radon used in radiotherapy as very radioactive and reactive Giant structures: Diamonds Graphite Silicon Dioxide Giant Ionic Structures Simple Covalent Structures RFM Diamonds: In a diamond, all the electrons in the outer shell of each carbon atom are included in forming covalent bonds This is why diamonds cannot conduct electricity They have 4 Carbon-Carbon bonds Diamonds have a high MP and BP and are brittle They are used in jewelry, industrial applications, and drill bits Graphite: In graphite, only 3 of the 4 electrons in the outer shell are involved in covalent bonds. The 4th electron becomes delocalized and can move between layers  This is why graphite can be moved so easily- and can conduct electricity (as there is a spare electron) There are 3 Carbon-Carbon bonds Graphite also has a high MP and BP Used in pencils, polishes, and coes of nuclear reactors Silicon Dioxide: In the diagram, the red dots represent oxygen atoms and the black dots represent silicon atoms Each silicon atom is bonded to 4 oxygen atoms & each oxygen atom is bonded to 2 silicon atoms There are 2 different arrangements on the different sides of the atoms, making them weaker Has high MP and BP Used in insulators, concrete, and glass Giant Ionic Structures: Metals and Non-metals bond together to form giant ionic structures made from many oppositely charged ions These structures usually have high MP and BP (meaning they’re mostly solid at room temperature), and they have strong bonds They are also mostly soluble in water They can conduct electricity when molten (dissolved in liquid) Ion → an atom with an electric charge Simple Covalent Structures: Non-metal elements are simple molecules (e.g oxygen) Covalent compounds form simple molecules (e.g carbon dioxide) Simple molecules have: Low MP and BP Poor electrical conductivity & are often gases or liquids at room temperature Relative Formula Mass (RFM): RFM → the combined mass of all the atoms added up E.g. Oxygen (O2) = 16 + 16 = 32