Which elements of a group show anomalous behaviour? Explain with examples.

(N/A) The first element of each group in the $s$-block and $p$-block shows anomalous behaviour compared to the other members of the same group.
Examples:
$1$. In group-$1$,$Li$ shows anomalous behaviour.
$2$. In group-$2$,$Be$ shows anomalous behaviour.
$3$. Elements from $B$ to $F$ (group-$13$ to $17$) show anomalous behaviour.
Reasons for anomalous behaviour:
- Small size and high electronegativity.
- High ionization enthalpy.
- Absence of $d$-orbitals in the valence shell.
Diagonal Relationship:
Lithium and beryllium exhibit properties similar to the second element of the next group (magnesium and aluminium,respectively). This similarity is known as the diagonal relationship.