Periodic Table and Chemical Bonding
Core ideas
The Periodic Table arranges elements by increasing proton number. Groups are vertical columns with the same number of outer electrons and similar properties; periods are horizontal rows. Group 1 (alkali metals) are reactive metals with one outer electron; Group 7 (halogens) are reactive non-metals with seven outer electrons; Group 0 (noble gases) have full outer shells and are unreactive. Metals lose electrons to form positive ions; non-metals gain electrons to form negative ions.
Ionic bonding occurs between metals and non-metals: electrons are transferred, forming oppositely charged ions held by strong electrostatic attraction. Ionic compounds have high melting points and conduct when molten or dissolved. Covalent bonding occurs between non-metals: electrons are shared. Simple molecular substances have low melting points; giant covalent structures (diamond, graphite) have high melting points. Metallic bonding involves positive ions in a sea of delocalised electrons, explaining conduction and malleability.
Traps to watch
- Ionic compounds do not conduct as solids because ions are fixed in the lattice.
- Graphite conducts because of delocalised electrons between layers, but diamond does not.
- Group number equals outer electrons, not total electrons.
- Reactivity in Group 1 increases down the group; in Group 7 it decreases down the group.
Rule of thumb
Metal + non-metal = ionic; non-metal + non-metal = covalent. For ionic formulas, balance charges to zero.
Worked example
Predict the formula of magnesium chloride. Mg forms Mg²⁺; Cl forms Cl⁻. Two chloride ions balance one magnesium ion, giving MgCl₂.