Atomic structure and the Periodic Table

• An atom is made up of 3 sub-atomic particles: Protons, neutrons (nucleus) and electrons (shells)
• Proton: Its charge is +1 and its relative mass is 1. This gives an atom its identity
• Neutron: Its charge is 0 and its relative mass is 1.
• Electron: Its charge is -1 and its relative mass is 1/1840 (negligible)
• Proton (atomic) number (Z) = No of protons
• Nucleon (mass) number (A) = No of protons + no of neutrons
• No of electrons = proton number
• No of neutrons = A - Z
• Electrons orbit around the nucleus in shells with different energies (the farther, the more). The first shell is first filled. 1st shell only can have 2 electrons; others can have up to 8.
• Noble gases are stable arrangements of electrons with full shells
• Isotopes: atoms of the same element which have the same Z number but different A number. Therefore isotopes have the same chemical properties because they have the same electron structure. Radioisotopes are isotopes with unstable nuclei

• An ion is an electrically charged atom that has lost or gained electrons. This happens so to gain a full outer shell of electrons, thereby having a more stable electron arrangement.
• Metals lose electrons to become cations while non-metals gain electrons to become anions.

Formation of ionic bonds between Group I and Group VII

NaCl: Sodium is a group 1 metal so will lose one electron to gain a full outer shell, forming Na+ ion. Chlorine is a group 7 non-metal so will need to gain an electron to have a full outer shell. One electron is transferred from outer shell of Na to outer shell of Cl, so a Cl atom will gain an electron to form Cl-

Ionic bonds between metallic & non-metallic elements

Mg is a group 2 metal so will lose 2 electrons to another atom (in this case, to O₂) to have a full outer shell, and a cation with charge +2 is formed. O₂ is group 6 non-metal so will need to gain 2 electrons for a full outer shell. It then forms an anion with charge -2

The Lattice Structure of Ionic Compounds: They are a regular arrangement of alternating positive and negative ions held together by strong electrostatic forces of attraction, for example NaCl

• Non-metals form simple molecules with covalent bonds between atoms. The bond is formed by the sharing of a pair of electrons between 2 atoms (each of them contributing one electron to each bond), forming a molecule. This makes each atom gain a full outer shell.

Differences between ionic and covalent compounds

Ionic compounds have high m.p. and b.p. because the ions are attracted to each other by strong electrostatic forces, thus large amounts of energy are needed to separate them. They are not volatile, they are usually soluble in water and they conduct electricity when molten or in solution.

Covalent compounds have low m.p. and b.p. because, as the intermolecular forces between molecules are weak, less energy is needed to break them apart; though the covalent bonds between the atoms are strong. Thus, they are usually liquids or gases at room temp. They are volatile, are soluble in organic solvents, but they don't conduct electricity

• There are several different forms of carbon, including diamond and graphite
• Diamond: Each atom is attached to four others (they are arranged tetrahedrally).They have strong covalent bonds without weak intermolecular forces. It has very high m.p., does not conduct electricity (there are no free electrons), and dense (3.51 g/cm³)
• It is very hard because the bonding extends throughout the whole structure, so it is used in cutting tools. It is colourless and its crystals sparkle in light, so it is used in jewellery and ornamental objects
• Graphite: The carbon atoms are arranged in flat layers of linked hexagons, each layer is a 2D giant molecule, in which each atom is attached to 3 others by strong covalent bonds
• There are weak intermolecular forces, so the layers can easily slide over each other, making it feel slippery. It can be used as a lubricant
• It conducts electricity because it has free electrons (not those used for covalent bonding) that can move between layers, carrying charge. It is used as electrodes and for the brushes in electric motors
• Silicon(IV) oxide (silicon dioxide, SiO₂): It is similar to diamond: tetrahedron, very hard, high m.p. Each O₂ atom is attached to 2 Si atoms, and each Si atom is attached to 4 O₂; held together by strong covalent bonds. Sand and quartz are examples