Electron Configuration

Electron configuration shows how an atom’s electrons are arranged in shells around the nucleus. For the first 20 elements, we use simple shell numbers like $2,8,1$. This helps predict reactivity and position in the Periodic Table.

Key rules (elements 1–20)

• Fill inner shells first (closest to the nucleus).
• 1st shell holds up to 2 electrons.
• 2nd shell holds up to 8 electrons.
• 3rd shell holds up to 8 electrons (for $Z=1$ to $20$).
• In a neutral atom, number of electrons = proton (atomic) number.

How to write it

1. Find the proton number (from the Periodic Table).
2. Place electrons into shells using the 2, 8, 8 rule.
3. Write as numbers separated by commas, e.g. $2,8,1$.

Links to the Periodic Table

• Period number = number of occupied shells.
• For Groups I–VII, group number = number of outer (valence) electrons.
• Group VIII (also called Group 18) noble gases have a full outer shell and are unreactive.

Ions and stability

Atoms often gain or lose electrons to get a full outer shell. Metals usually lose electrons to form positive ions; non-metals usually gain electrons to form negative ions.

• $\text{Na}$ ($2,8,1$) → $\text{Na}^+$: loses 1 electron → $2,8$
• $\text{Cl}$ ($2,8,7$) → $\text{Cl}^-$: gains 1 electron → $2,8,8$
• $\text{Ca}$ ($2,8,8,2$) → $\text{Ca}^{2+}$: loses 2 electrons → $2,8,8$

Common misconceptions

• Electrons are not in the nucleus; they occupy shells around it.
• For elements 1–20, use $2,8,8$ (not $2,8,18$).
• Forming ions changes electrons, not protons (the proton number stays the same).