A relationship within the periodic table by which certain elements in the second period have a close chemical similarity to their diagonal neighbours in the next group of the third period. This is particularly noticeable with the following pairs.

Lithium and magnesium:

1. (1) both form chlorides and bromides that hydrolyse slowly and are soluble in ethanol;

2. (2) both form colourless or slightly coloured crystalline nitrides by direct reaction with nitrogen at high temperatures;

3. (3) both burn in air to give the normal oxide only;

4. (4) both form carbonates that decompose on heating.

Beryllium and aluminium:

1. (1) both form highly refractory oxides with polymorphs;

2. (2) both form crystalline nitrides that are hydrolysed in water;

3. (3) addition of hydroxide ion to solutions of the salts gives an amphoteric hydroxide, which is soluble in excess hydroxide giving beryllate or aluminate ions [Be(OH)₄]²⁻ and [Al(OH)₄]⁻;

4. (4) both form covalent halides and covalent alkyl compounds that display bridging structures;

5. (5) both metals dissolve in alkalis.

Boron and silicon:

1. (1) both display semiconductor properties;

2. (2) both form hydrides that are unstable in air and chlorides that hydrolyse in moist air;

3. (3) both form acidic oxides with covalent crystal structures, which are readily incorporated along with other oxides into a wide range of glassy materials.

The reason for this relationship is a combination of the trends to increase size down a group and to decrease size along a period, and a similar, but reversed, effect in electronegativity, i.e. decrease down a group and increase along a period.