Chemical Bonding
The paper tests why atoms combine, how ionic and covalent bonds form, how to draw electron-dot structures, and how bonding explains melting points and electrical conductivity.
Why atoms combine
Atoms react to reach a stable noble-gas configuration, usually eight electrons in the outermost shell. They do this by losing, gaining or sharing electrons.
Ionic (electrovalent) bonding
A metal transfers electrons to a non-metal, and the resulting ions are held by strong electrostatic attraction. For example, Na (2, 8, 1) becomes Na+ (2, 8) and Cl (2, 8, 7) becomes Cl− (2, 8, 8), giving NaCl. Magnesium (2, 8, 2) forms Mg2+ and needs two chloride ions, so the formula is MgCl2. Ionic compounds have high melting points, are usually soluble in water, and conduct electricity when molten or in solution but not when solid, because the ions are locked in the lattice.
Covalent bonding
Non-metals share electron pairs. One shared pair is a single bond, two a double bond, three a triple bond. H2, Cl2, O2, N2, H2O, CH4 and NH3 are covalent. Covalent compounds have low melting points and generally do not conduct electricity because there are no free ions or electrons. In a dot structure the shared pair is drawn between the symbols and each atom's octet is completed with dots.
Worked example
For water: oxygen has 6 valence electrons and each hydrogen 1, giving 6 + 2 = 8 valence electrons. Oxygen shares one electron with each hydrogen, forming two single bonds and leaving two lone pairs on oxygen, so every atom has a full outer shell.
Traps
Ionic solids do not conduct when solid, a very common wrong answer; use the word molecule for covalent substances and formula unit for ionic ones; MgCl2, not MgCl, because of the 2+ and 1− charges.