Determine the electronic configuration of elements and their ions with proton number 1 to 20, e.g. 2,8,3

Atoms, Elements, and Compounds – Atomic Structure & Periodic Table

What is an Atom?

Think of an atom as a tiny solar system. The nucleus is the sun, packed with protons (positive) and neutrons (neutral). Electrons orbit the nucleus like planets, but they’re much smaller and move very fast.

🔬 Analogy: Imagine a mini city: the nucleus is the city center, and the electrons are the cars driving around the streets.

Protons, Neutrons, Electrons

ParticleChargeMass (approx.)
Proton+11 u
Neutron01 u
Electron–10.0005 u

Electronic Configuration Basics

The electronic configuration tells us how electrons are arranged in shells (energy levels) and subshells (orbitals). The Aufbau principle says electrons fill the lowest energy orbitals first.

🔢 Rule of thumb: Fill in the order 1s, 2s, 2p, 3s, 3p, 4s, 3d, 4p, …

Each orbital can hold 2 electrons. Use the notation \$n\ell^{\,x}\$ where \$n\$ is the shell number, \$\ell\$ is the orbital type (s, p, d, f), and \$x\$ is the number of electrons.

Determining Configurations for Elements 1–20

Below is a quick reference table. Use the proton number (atomic number) to find the configuration.

ZElementSymbolElectronic Configuration
1HydrogenH\$1s^1\$
2HeliumHe\$1s^2\$
3LithiumLi\$1s^2\,2s^1\$
4BerylliumBe\$1s^2\,2s^2\$
5BoronB\$1s^2\,2s^2\,2p^1\$
6CarbonC\$1s^2\,2s^2\,2p^2\$
7NitrogenN\$1s^2\,2s^2\,2p^3\$
8OxygenO\$1s^2\,2s^2\,2p^4\$
9FluorineF\$1s^2\,2s^2\,2p^5\$
10NeonNe\$1s^2\,2s^2\,2p^6\$
11SodiumNa\$1s^2\,2s^2\,2p^6\,3s^1\$
12MagnesiumMg\$1s^2\,2s^2\,2p^6\,3s^2\$
13AluminiumAl\$1s^2\,2s^2\,2p^6\,3s^2\,3p^1\$
14SiliconSi\$1s^2\,2s^2\,2p^6\,3s^2\,3p^2\$
15PhosphorusP\$1s^2\,2s^2\,2p^6\,3s^2\,3p^3\$
16SulfurS\$1s^2\,2s^2\,2p^6\,3s^2\,3p^4\$
17ChlorineCl\$1s^2\,2s^2\,2p^6\,3s^2\,3p^5\$
18ArgonAr\$1s^2\,2s^2\,2p^6\,3s^2\,3p^6\$
19PotassiumK\$1s^2\,2s^2\,2p^6\,3s^2\,3p^6\,4s^1\$
20CalciumCa\$1s^2\,2s^2\,2p^6\,3s^2\,3p^6\,4s^2\$

Ions and Their Configurations

When an atom gains or loses electrons, it becomes an ion. The electronic configuration changes by removing or adding electrons from the outermost shell.

  • Na⁺ (sodium ion) – loses one electron: \$1s^2\,2s^2\,2p^6\,3s^1 \;\rightarrow\; 1s^2\,2s^2\,2p^6\$ (same as Ne).

  • Cl⁻ (chloride ion) – gains one electron: \$1s^2\,2s^2\,2p^6\,3s^2\,3p^5 \;\rightarrow\; 1s^2\,2s^2\,2p^6\,3s^2\,3p^6\$ (same as Ar).

  • Ca²⁺ (calcium ion) – loses two electrons: \$1s^2\,2s^2\,2p^6\,3s^2\,3p^6\,4s^2 \;\rightarrow\; 1s^2\,2s^2\,2p^6\,3s^2\,3p^6\$ (same as Ar).

⚡ Tip: Remember that electrons are always removed from the highest energy level first.

Exam Tips & Quick Reference

Use the table above as a cheat sheet for the first 20 elements. When you see an ion, think “remove from the outermost shell” or “add to fill the p‑orbital.”

  1. Write the full configuration first before simplifying.

  2. Check that the total number of electrons equals the atomic number minus the charge.

  3. Remember the noble gas shorthand – e.g., \$[Ne]\,3s^1\$ for Na.

📝 Practice Question: What is the electronic configuration of Mg²⁺?
Answer: \$1s^2\,2s^2\,2p^6\$ (same as Ne).