Similarities between diagonal neighbours such as Li and Mg or Be and Al due to similar charge density.
Practise Atomic Structure & Periodicity All Atomic Structure & Periodicity topics
The diagonal relationship describes the similarity between elements that lie diagonally adjacent in the periodic table, such as lithium and magnesium, beryllium and aluminium, or boron and silicon. These pairs have similar charge density, because the increase in size and the decrease in charge offset each other, and they have similar electronegativity. They form compounds with similar properties, such as nitrides and carbonates that decompose on heating. The effect is strongest between lithium and magnesium. It explains otherwise surprising chemical similarities.
Key ideas: Pairs diagonal to each other in the table have similar properties because charge density and electronegativity are similar: Li-Mg, Be-Al, B-Si. Examples: Li and Mg form nitrides directly and have insoluble carbonates and fluorides; Be and Al are amphoteric with covalent chlorides; B and Si are acidic oxides and form network structures.
Common mistakes and connections: Students think the similarity applies beyond period 2 and 3 or that it is a perfect match; it is approximate. The cause is ionic potential (charge / radius) staying about equal moving across and down. It extends the periodic law and explains anomalous behavior of the first member of a group.
Diagonal neighbours have similar ionic radii and charge density, such as Li+ at 76 pm and Mg2+ at 72 pm, explaining their similar chemistry. The interactive lesson shows this as a live animation or chart.
Beryllium hydroxide and aluminium hydroxide are both amphoteric, dissolving in both acids and strong bases.
Similarities between diagonal neighbours such as Li and Mg or Be and Al due to similar charge density.
Diagonal Relationship is a intermediate topic in Atomic Structure & Periodicity. It builds on the vocabulary of beginner topics in this area.
Study these earlier topics in Atomic Structure & Periodicity first: Pauling Scale, Mulliken Electronegativity, Metallic Character.
The next topics in Atomic Structure & Periodicity are Inert Pair Effect and Lanthanide Contraction.
Yes. The interactive lesson includes a self-check question on Diagonal Relationship, and the Atomic Structure & Periodicity quiz draws on every topic in this area.
Diagonal Relationship is topic 94 of 115 in Atomic Structure & Periodicity.