Periodicity 2

Trends Across Period 3 Atomic radii decreases from left to right across period 3 due to the increasing attraction experienced by the outer-shell electrons. The first ionization energy and electronegativity increases from left to right across period 3.

Chemical Properties of Elements and Their Oxides Just as the physical properties exhibit a periodicity within groups and periods, so do the chemical properties of the elements and some of their compounds.

Trends Within a Group The alkali metals are known for their reactive nature. ▫Their tendency to react, sometimes violently, with water means that they must be stored under oil. ▫The reactivity of the alkali metals with water increases down group 1. 2Li (s) + H 2 O ( l ) Li 2 O (aq) + H 2(g)

Trends Within a Group Lithium, sodium and potassium all produce alkaline solutions when they react with water. Li 2 O (s) + H 2 O ( l ) 2LiOH (aq) The increase in the reactivity can be explained by the decrease in the electrostatic attraction between the outer-shell electron and the positive nucleus of the alkali metal.

Trends Within a Group The increasing reactivity of the alkali metals can also be seen in their reaction with halogens. As Li, Na and K all lose electrons easily (are good reducing agents) and the halogens gain electrons easily (are good oxidizing agents), these reactions are predictably violent. 2Na (s) + Cl 2(g) 2NaCl (s)

Trends Within a Group The smaller the halogen atom, the greater is its ability to gain electrons. This can be explained by the closeness of the outer shell to its nucleus.

The Oxides of Period 3 Elements Each of the period 3 elements reacts with oxygen. The large difference in electronegativity between the metals and oxygen produces an ionic compound, whereas the non-metals form a covalent compound with oxygen.

The Oxides of Period 3 Elements Na burns in air to produce Na 2 O, which reacts easily with water to produce a strongly alkaline solution of NaOH. To the right of Na is Mg, which reacts the same. The decrease in metallic nature from left to right across period 3 can be seen from the very different behaviour of Al. ▫Al 2 O 3 can behave as an acid or a base. ▫Al 2 O 3 does not dissolve in water, but will react with acids or bases.

The Oxides of Period 3 Elements Further to the right in period 3, P burns violently in air to make P 4 O 10, which then reacts violently with water to make a weakly acidic solution. We generalize this trend: ▫Non-metals form acidic oxides ▫Metals form basic oxides ▫Al forms an amphoteric oxide