Appearance

All the Group 1A elements are silvery-colored metals. They are soft, and can be easily cut with a knife to expose a shiny surface, which dulls on oxidation.

General Reactivity

These elements are highly reactive metals. The reactivity increases on descending the Group from Lithium to Cesium. There is a closer similarity between the elements of this sub-Group than in any other Group of the Periodic Table.

Occurrence and Extraction

These elements are too reactive to be found free in nature. Sodium occurs mainly as NaCl (salt) in seawater and dried-up seabeds. Potassium is more widely distributed in minerals such as sylvite, KCl, but is also extracted from seawater. The alkali metals are so reactive that they cannot be displaced by another element, so are isolated by electrolysis of their molten salts.

Physical Properties

The alkali metals differ from other metals in several ways. They are soft, with low melting and boiling temperatures. They have low densities - Li, Na and K are less dense than water. They show relatively weak metallic bonding, as only one electron is available from each atom.

Alkali metals color flames. When the element is placed in a flame the heat provides sufficient energy to promote the outermost electron to a higher energy level. On returning to ground level, energy is emitted and this energy has a wavelength in the visible region: Li - red; Na - yellow; K - lilac; Rb - red; Cs - blue.

The ionic radii of the alkali metals are all much smaller than the corresponding atomic radii. This is because the atom contains one electron in an s-level relatively far from the nucleus in a new quantum shell, and when it is removed to form the ion the remaining electrons are in levels closer to the nucleus. In addition, the increased effective nuclear charge attracts the electrons towards the nucleus and decreases the size of the ion.

Date: 2015-01-12; view: 679