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# Group 1: Alkali metals

## Physical properties of alkali metals

Lithium (left), sodium (centre) and potassium (right) are the first three members of group 1.

Alkali metals are the elements in group 1. They are soft metals with relatively low melting points and low densities compared to other metals.

Alkali metals are soft enough to be cut using a knife.

Alkali metals have low melting points because of weak metallic bonding compared to most metals.

Metallic bond strength increases with increasing number of delocalised electrons in the electron sea. This is due to the greater electrostatic attraction between the delocalised sea of electrons and metal cations.

Each alkali metal atom donates only one electron. The electron sea in alkali metals is less dense. Therefore, the metallic bonding in alkali metals is weaker.

The low density of alkali metals is due to their greater atomic radius (and hence larger volume) and lower mass compared to other atoms in their respective periods.

Lithium, sodium and potassium are the only metals that can float in water.

## Trends in physical properties of alkali metals

The melting (and boiling) point decreases down the group.

As we move from lithium to potassium, the atomic radius decreases. The nuclei of heavier alkali metals are further away from the delocalised sea of electrons in metallic bonding.

The metallic bond strength is weaker for heavier alkali metals than for lighter ones.

Densities increase down the group, with the exception of potassium.

Atomic mass increases as we move from lithium to potassium. Atoms of heavier alkali metals have more volume than atoms of lighter alkali metals.

The following table lists the melting point temperatures (expressed in Kelvins, or $K$) and the densities of alkali metals.

Alkali metal Melting point ($^{\circ}\text{C}$) Density ($\text{g}/\ccm$)
Lithium ($\ce{Li}$) 180.54 0.534
Sodium ($\ce{Na}$) 97.72 0.968
Potassium ($\ce{K}$) 63.38 0.89
Rubidium ($\ce{Rb}$) 39.32 1.532
Caesium ($\ce{Cs}$) 28.44 1.93

## Trends in chemical properties of alkali metals

An alkali metal atom easily loses its valence electron and is thus highly reactive.

All alkali metals react with water to form bases and hydrogen gas ($\ce{H2}$). The following shows the general chemical equation of the reaction between alkali metal (represented by "M") and water.

$$\ce{2M {(s)} + 2H2O {(l)} -> 2MOH {(aq)} + H2 {(g)}}$$
Potassium reacts explosively with cold water.

Heavier alkali metals are more reactive than lighter ones as heavier metals lose their valence electrons more easily.

Inner electron shells "shield" valence electrons from the positive nucleus, decreasing the attractive force acting on the valence electrons in the outer shell.

Heavier metals have more electron shells and hence, experience more shielding. Heavier alkali metals are more reactive with water than lighter ones.