Key Concepts: Electronic Configurations

The electron configuration of an atom in its ground state describes the distribution of electrons into energy levels or “shells”. This pattern is crucial to understand how elements react chemically.
The first shell, closest to the nucleus, can hold up to 2 electrons. Further shells can hold up to 8 electrons.
Electrons tend to occupy the lowest possible energy levels available and are distributed from the innermost shell outward.
Each electron in an atom has its specific place in a shell, described by quantum numbers. This is based on the principle of quantum mechanics.
The order in which the shells fill is not always sequential. After the first 20 elements, electrons start filling higher energy levels before the lower ones are full.
The period number in the periodic table indicates the number of shells of electrons an element has.
Group number in the periodic table (for groups 1 to 8) often indicates the number of electrons in the outer shell.
Elements in the same group have the same number of electrons in their outer shell, hence exhibit similar chemical properties.
The noble gases, found in group 8 (or 0), have full outer electron shells (either two or eight electrons). They are generally very stable and do not readily react to form compounds.
Valence electrons, electrons in the outermost shell, are most involved in chemical reactions.
Elements strive to have their outer shell full, to reach a more stable configuration. This drive determines the chemical reactivity and bonding characteristics of an element.
Ionic bonding involves the transfer of electrons from one atom to another, resulting in ions with full outer shells.
Covalent bonding, on the other hand, involves sharing of electrons between atoms to achieve a fully-filled outer shell.
Electron configuration plays a critical role in predicting the physical properties of metals and non-metals. For example, metals, which tend to lose electrons, are good conductors of electricity while non-metals, which tend to gain or share electrons, act as insulators.
Knowledge of electron configuration can also assist in understanding the trends seen across the periodic table in terms of reactivity, atomic radius, ionisation energy, and electron affinity.