Valence electrons
Atoms on their own are not stable under normal circumstances (apart from the noble gases) and react with other atoms forming molecules or ions, depending on the difference in electronegativity between the reacting atoms.
They do this by using their outer shell electrons in such a way as to produce molecular shells with stable configurations of electrons, usually in shells of eight (an octet). Hence, the reactivity of an atom is determined mainly by its outer shell, or valence, electrons.
The vertical groups in the periodic table have the same number of valence electrons. It follows then, that vertical groups have similar chemical properties and, where dependent on the outer shell electrons, similiar physical properties.
|
group I
|
valence
|
group 17
|
valence
|
|
lithium
|
1
|
fluorine
|
7
|
|
sodium
|
1
|
chlorine
|
7
|
|
potassium
|
1
|
bromine
|
7
|
|
rubidium
|
1
|
iodine
|
7
|
|
caesium
|
1
|
|
|
Group trends
The outer electrons behaviour is modified by the attraction from the nucleus. As the nucleus increases in charge descending a group, it follows that there will be a gradual change in chemical characteristics, and any physical characteristics dependent on outer electrons, within a group as the group is descended.
These group properties are particularly noticeable in groups 1, 2 and 17, and for this reason they are usually the groups chosen to exemplify the concept of periodicity.
Chemical properties
The definition of a chemical reaction is a process in which a new substance is formed. The term 'chemical property' refers to any behaviour or characteristic involving a change in chemical identity and formation of new substances.
Chemical properties:
- Interaction with other substances causing chemical change (new substances formed).
- Energy of reaction.
- Speed or ease of reaction.
- Acid/base characteristics.
- Oxidation/reduction behaviour.
- Ease of decomposition - behaviour towards heat.
Example: The chemical properties of sodium hydroxide
|
Sodium hydroxide, NaOH, is a caustic, reactive, solid, white ionic compound that behaves like a typical strong base. It forms salts with acids and acidic oxides, often with the liberation of large amounts of energy (approximately 57 kJ mol-1 with a stong acid). NaOH + H2SO4 → Na2SO4 + H2O It reacts on warming with many types of organic compound such as esters and amides, hydrolysing them in the process: NaOH + CH3COOCH3 → CH3COONa + CH3OH Although it reacts rapidly and directly with aluminium releasing hydrogen gas, exemplifying the amphoteric nature of the metal, sodium hydroxide is not classified as a reducing or oxidising agent: 2NaOH + 2Al + 2H2O → 2NaAlO2 + 3H2 Sodium hydroxide dissolves chlorine gas causing it to disproportionate: 2NaOH + Cl2 → NaOCl + NaCl + H2O Sodium hydroxide is stable to heat, it does not decompose under normal circumstances. |
