Electronic Configuration and Periodic Trends - SS2 Chemistry Lesson Note

The electronic configuration of an atom describes the arrangement of electrons in its energy levels or shells. Electrons fill the shells and subshells according to the Aufbau principle, Pauli exclusion principle, and Hund's rule. The Aufbau principle states that electrons occupy the lowest energy levels first before filling higher energy levels. The Pauli exclusion principle states that no two electrons in an atom can have the same set of quantum numbers, meaning that each electron has a unique combination of four quantum numbers: the principal quantum number (n), azimuthal quantum number (l), magnetic quantum number (m), and spin quantum number (s). Hund's rule states that electrons will fill orbitals of the same energy level (degenerate orbitals) with parallel spins before pairing up.

The electronic configuration is typically written using the noble gas shorthand notation. The symbol of the nearest noble gas is used in brackets to represent the filled inner shells, followed by the configuration of the valence electrons. For example, the electronic configuration of oxygen (O) can be represented as [He] 2s² 2p⁴.

Periodic Trends:

Atomic Radius: Atomic radius refers to the size of an atom. It is defined as half the distance between the nuclei of two bonded atoms of the same element.

●     Across a period (from left to right), atomic radius generally decreases due to increased effective nuclear charge (the attraction between the positively charged nucleus and the negatively charged electrons) and the poor shielding effect of inner electrons.

●     Down a group (from top to bottom), atomic radius generally increases due to the addition of new energy levels or shells.

Ionization Energy: Ionization energy is the energy required to remove an electron from an atom or ion in the gas phase.

●     Across a period, ionisation energy generally increases because of the increased effective nuclear charge and decreased atomic radius.

●     Down a group, ionisation energy generally decreases due to the increased distance between the valence electrons and the nucleus.

Electronegativity: Electronegativity is the ability of an atom to attract shared electrons in a covalent bond.

●     Across a period, electronegativity generally increases due to the increased effective nuclear charge and decreased atomic radius.

●     Down a group, electronegativity generally decreases.

Metallic Character: Metallic character refers to the tendency of an element to exhibit metallic properties such as metallic lustre, conductivity, and malleability.

●     Across a period, metallic character generally decreases as atoms have a stronger attraction for electrons and are less likely to lose them to form positive ions.

●     Down a group, metallic character generally increases due to the larger atomic size and greater ease of losing electrons.