Lewis Dot Symbols for Atoms and Ions

Before you can draw the structure of a molecule, you need a quick way to show what each atom brings to the table. That’s what a Lewis dot symbol does. Invented by the American chemist Gilbert N. Lewis in 1916, it is simply the element’s symbol surrounded by dots, one dot for each valence electron. It’s the simplest diagram in chemistry, and it’s the foundation for Lewis dot structures of whole molecules.

What a Lewis symbol shows, and what it leaves out

A Lewis symbol shows:

• the element (its chemical symbol stands for the nucleus plus all the inner, core electrons);
• the valence electrons, drawn as dots around the symbol.

It deliberately leaves out the core electrons, orbital shapes and energy levels. That’s the point: chemistry happens with the outer electrons, so the symbol shows only those.

Step 1: find the number of valence electrons

For main-group elements, use the group number:

• Groups 1 and 2: valence electrons = group number (1 or 2).
• Groups 13 to 18: valence electrons = group number minus 10 (3 to 8).
• Helium is the exception in group 18: it has just 2.

Element	Group	Valence electrons
H	1	1
Mg	2	2
B	13	3
C	14	4
N	15	5
O	16	6
Cl	17	7
Ar	18	8

If you’d like the reasoning behind this, see how to find valence electrons.

Step 2: imagine four sides around the symbol

Picture the element symbol sitting in a small square with four sides: top, right, bottom and left. Each side can hold up to two dots, so the maximum is eight, matching the octet rule.

Step 3: place the dots one at a time, then pair them

Put one dot on each side before you start doubling up. This mirrors Hund’s rule: electrons spread out before they pair. Then add second dots to make pairs.

Here is how the dots build up across period 2, written in text form (the positions of dots are shown with words, since the exact side you start on doesn’t matter):

• Li: 1 dot (one side).
• Be: 2 dots, on two different sides.
• B: 3 dots, on three sides.
• C: 4 dots, one on each side. This shows why carbon usually forms four bonds.
• N: 5 dots: one pair plus three single dots. The three unpaired dots show that nitrogen usually forms three bonds.
• O: 6 dots: two pairs and two singles, so oxygen usually forms two bonds.
• F: 7 dots: three pairs and one single, so fluorine forms one bond.
• Ne: 8 dots: four pairs, a full octet, and no bonds.

The unpaired dots tell you how many bonds an atom tends to form in simple compounds, a very handy shortcut.

A note on beryllium and boron: strictly, the ground-state atoms have their s electrons paired (Be is 2s², B is 2s² 2p¹). The usual Lewis convention still spreads the dots out, because that matches how these atoms bond (Be forms two bonds, B forms three). Lewis symbols are a bonding tool, not an exact picture of orbitals.

Step 4: draw ions by adding or removing dots and showing the charge

Ions form when atoms lose or gain electrons (see ions: how atoms gain and lose electrons).

For a positive ion (cation): remove the dots for the electrons lost. Usually that removes all of them. Write the symbol in square brackets and put the charge outside at the top right.

• Sodium atom: Na with 1 dot → sodium ion: [Na]⁺ with no dots.
• Magnesium atom: Mg with 2 dots → [Mg]²⁺ with no dots.

The ion’s remaining electrons form a full inner shell, which the symbol doesn’t show.

For a negative ion (anion): add dots for the electrons gained until there are eight, then add brackets and the charge.

• Chlorine atom: Cl with 7 dots → chloride ion: [Cl]⁻ with 8 dots (four pairs).
• Oxygen atom: O with 6 dots → oxide ion: [O]²⁻ with 8 dots.
• Nitrogen atom: N with 5 dots → nitride ion: [N]³⁻ with 8 dots.

The square brackets make it clear that the charge belongs to the whole ion, electrons included.

Step 5: use the symbols to predict ionic formulas

Lewis symbols make electron transfer easy to picture. Take sodium and chlorine:

1. Na has 1 valence electron; Cl has 7.
2. Sodium gives its electron to chlorine.
3. Result: [Na]⁺ and [Cl]⁻ with eight dots. Formula: NaCl.

Now magnesium and chlorine:

1. Mg has 2 valence electrons to give; each Cl can accept only 1.
2. So two chlorine atoms are needed.
3. Result: one [Mg]²⁺ and two [Cl]⁻ ions. Formula: MgCl₂.

And aluminium with oxygen:

1. Al gives 3 electrons; O takes 2.
2. The lowest common multiple is 6: two Al atoms give 6 electrons, and three O atoms take them.
3. Formula: Al₂O₃.

This is exactly what you’d get with the charge-balancing method in how to name ionic compounds; the dots just make the bookkeeping visible.

Step 6: use the symbols to predict simple covalent molecules

Non-metals share electrons instead of transferring them. Pair up the unpaired dots between atoms: each shared pair is a covalent bond.

• H₂: each H has one unpaired dot; they pair to form one bond (H–H).
• Cl₂: each Cl has one unpaired dot; they share it to form Cl–Cl, and each Cl ends up with eight electrons around it.
• H₂O: O has two unpaired dots, so it bonds to two H atoms. Two lone pairs are left on oxygen.
• NH₃: N has three unpaired dots, so three H atoms, plus one lone pair on N.
• CH₄: C has four unpaired dots, so four H atoms and no lone pairs.

These lone pairs become important later: they shape molecules (VSEPR) and make ammonia a base.

Common pitfalls

• Drawing core electrons. Only valence electrons get dots. Sodium has 11 electrons but gets just 1 dot.
• Pairing too early. For carbon, four single dots, not two pairs. Pairing early hides carbon’s four bonds.
• Forgetting brackets and charges on ions. “Cl with 8 dots” without brackets and a minus sign looks like an impossible atom.
• Adding dots to cations. Metal cations usually lose all their valence dots; they don’t gain any.
• Using group numbers for transition metals. Lewis symbols are mainly for main-group elements; transition metals don’t follow the simple valence-electron rule.
• Treating the symbol as a real picture. Electrons don’t sit at four corners. The dots are bookkeeping, not positions (see electron probability clouds).

Quick practice

Draw the Lewis symbol for each, then check below.

1. Silicon
2. Phosphorus
3. Sulfide ion
4. Calcium ion
5. Predict the formula of the compound formed by potassium and sulfur.

Answers: (1) Si with 4 single dots. (2) P with 5 dots: one pair and three singles. (3) [S]²⁻ with 8 dots. (4) [Ca]²⁺ with no dots. (5) Each K gives one electron and S needs two, so the formula is K₂S.

Key takeaways

• A Lewis symbol is an element symbol with one dot per valence electron.
• Place dots one per side first, then pair them; unpaired dots show how many bonds the atom usually forms.
• Cations lose their dots and gain a positive charge; anions gain dots up to eight and a negative charge, and both get square brackets.
• Transfer dots to predict ionic formulas, and share unpaired dots to predict simple covalent molecules.