Electronic Configuration

Summary: Electronic configuration describes how electrons are arranged in shells around the nucleus, following the 2.8.8 rule for the first 20 elements, and determines an element’s chemical properties including its group and period. Tags: igcse chemistry Created: 2026-07-18

Electronic configuration (also called electron arrangement or electronic structure) is the way electrons are distributed among the electron shells (energy levels) surrounding the nucleus of an atom. For the first 20 elements (proton numbers 1 to 20), electrons fill shells according to a simple rule: the first shell holds a maximum of 2 electrons, the second shell holds a maximum of 8 electrons, and the third shell holds a maximum of 8 electrons (for these elements). The electronic configuration is written as a series of numbers separated by dots or commas — for example, sodium (Z = 11) is 2.8.1, meaning 2 electrons in shell 1, 8 in shell 2, and 1 in shell 3. The configuration directly determines an element’s group number (from the number of valence electrons in the outer shell) and its period number (from the number of occupied shells). When atoms form ions, the electronic configuration changes — metals lose outer-shell electrons to achieve a full outer shell, while non-metals gain electrons to do the same, both aiming for the stable configuration of the nearest noble gas.

Shell Filling Rules (Elements 1–20)

For the first 20 elements, electrons fill shells in order, starting from the shell closest to the nucleus:

The order of filling is always: fill shell 1 first, then shell 2, then shell 3. Each shell must be full before electrons enter the next shell.

Electronic Configurations of Elements 1–20

Electronic Configuration of Ions

When atoms form ions, they gain or lose electrons to achieve a full outer shell — the stable electronic configuration of the nearest noble gas. This is called the octet rule.

Metals lose electrons to form positive ions (cations):

• Na (2.8.1) loses 1 electron to become Na^+ (2.8) — same configuration as neon
• Mg (2.8.2) loses 2 electrons to become Mg^2+ (2.8) — same configuration as neon
• Al (2.8.3) loses 3 electrons to become Al^3+ (2.8) — same configuration as neon
• K (2.8.8.1) loses 1 electron to become K^+ (2.8.8) — same configuration as argon
• Ca (2.8.8.2) loses 2 electrons to become Ca^2+ (2.8.8) — same configuration as argon

Non-metals gain electrons to form negative ions (anions):

• F (2.7) gains 1 electron to become F^- (2.8) — same configuration as neon
• O (2.6) gains 2 electrons to become O^2- (2.8) — same configuration as neon
• Cl (2.8.7) gains 1 electron to become Cl^- (2.8.8) — same configuration as argon
• S (2.8.6) gains 2 electrons to become S^2- (2.8.8) — same configuration as argon

The atomic number (Z) does not change during ion formation — only the electron count changes.

Sources

• Cambridge IGCSE Chemistry 0620 Syllabus (2023-2025) — Section 2.2 Core 5, Cambridge Assessment International Education

Common Misconceptions