Horizontal trends

The chief horizontal trends towards the right in any block are a general increase of ionization energy (that is reflected in an increase in electronegativity), a contraction in size, and an increment in the number of electrons in the valence shell. The influence of changing electronegativity is obvious in determining the metal-nonmetal borderline, in main groups. The number of valence electrons has a clear affect on the stoichiometry of compounds formed (NaF, MgF₂, AlF₃, etc.). Main group elements generally form ions with closed shell configurations: therefore cations (Na⁺, Mg²⁺, Al³⁺) where all electrons have been lost from the valence shell and anions (F^-, O^2-) where the valence shell has been filled.

This observation proposes some 'special stability' of filled shells, but like in atomic structure, such type of an interpretation is misleading. The stoichiometry of the stable ionic compounds are depends on the balance among the energy needed to form ions and the lattice energy, that provides the bonding. Such type of an approach provides a better understanding not only of why closed-shell ions are frequently found but also of cases where they are not, as occurs frequently in the d block.

In covalent compounds some regularities in stoichiometry can also be understood from the increasing number of valence electrons. So the simple hydrides of the groups 14, 15, 16 and 17 elements have the formulae EH₄, EH₃, EH₂ and EH, correspondingly, reflecting the octet rule. Filling the valence shell forms progressively more nonbonding electrons and limits the ability for bonding. Such type of nonbonding electrons also affect the geometrical structures of the molecules.

The general increase of electronegativity or decline in electropositive character and contraction in size is apparent also in d-block chemistry. The development of closed-shell ions (Sc³⁺, Ti⁴⁺, etc.) is a characteristic of only the early groups. As ionization energies increment more electrons are prohibited from involvement in bonding. Non-bonding d electrons also affect the structures and stabilities of compounds, but due to the different directional properties of d orbitals compared with p, these influences are best understood by a distinct approach that of ligand field theory.