C3: Chemical Bonding and Structure

A metal atom can lose electrons and a non-metal atom can gain them, commonly reaching a noble-gas-like electron arrangement. The resulting opposite ions attract in an ionic bond.

A dot-and-cross diagram must conserve outer-shell electrons and show bracketed ion charges, as in $\ce{NaCl}$ and $\ce{MgCl2}$. Use one mark for electrons from one atom and another mark for electrons from the other atom. An ionic lattice has strong attractions in many directions, giving high melting points. It conducts only when ions can move in a melt or aqueous solution.

A covalent bond forms when non-metal atoms share an electron pair. Use dot-and-cross diagrams to account for $\ce{H2}$, $\ce{O2}$, $\ce{H2O}$, $\ce{CH4}$, and $\ce{CO2}$ from their valence electrons. A complete diagram shows every bonding pair and every non-bonding outer-shell electron, not only the bonds.

Simple molecular substances have strong bonds within molecules but weaker attractions between molecules, so many melt or boil at relatively low temperatures and do not conduct electricity.

An element contains one kind of atom. A compound contains different elements chemically joined in fixed proportions, while a mixture retains substances that are not chemically joined.

Metals are generally solid, malleable, good conductors of heat and electricity, and have high melting and boiling points. An alloy is a mixture of a metal with one or more other elements, as in brass or stainless steel. In particle diagrams, a pure metal has one kind of atom in a regular arrangement, while an alloy contains more than one kind of atom.