What is a valence electron?
A valence electron is an electron in the outermost shell of an atom: the electrons that actually take part in chemical bonding. They decide how an element reacts, what charge its ions carry, and how many bonds it forms. For the main-group elements (groups 1, 2, and 13–18), the number of valence electrons follows a clean, memorable rule. Pick any element in the tool above to see its count and its dot symbol.
They sit in the outermost occupied shell, furthest from the nucleus and least tightly held, which is why they and not the inner electrons decide how an element behaves.
The group rule (main-group elements only)
Reading the group number straight off the periodic table tells you the valence count:
- Group 1 (alkali metals, plus hydrogen) → 1 valence electron
- Group 2 (alkaline earth metals) → 2
- Groups 13–18 → group number − 10, so 13→3, 14→4, 15→5, 16→6, 17→7, and 18→8
So carbon (group 14) has 4, oxygen (group 16) has 6, and chlorine (group 17) has 7. This count equals the number of electrons in the atom’s outermost shell, the same outer ring you see in the Bohr model, and the part of the electron configuration that sits outside the noble-gas core.
Two period-1 elements are special cases worth flagging to students. Hydrogen sits in group 1 and has 1 valence electron, as the rule predicts. Helium, however, is a noble gas with a full 1s² shell: a stable duet of 2, not the 8 that “group 18” would suggest. The tool draws helium with its two electrons as a single pair.
Every group, including the awkward ones
| Group | Family | Valence electrons | Examples | Note |
|---|---|---|---|---|
| 1 | Alkali metals (and hydrogen) | 1 | H, Li, Na, K | Hydrogen sits here for its one electron but is not an alkali metal. |
| 2 | Alkaline earth metals | 2 | Be, Mg, Ca | Two electrons in the outer s subshell. |
| 3 to 11 | Transition metals | usually 2 in the outer shell, but see the note | Fe, Cu, Ni | The outermost shell holds 2, but the d electrons underneath are close enough in energy to bond as well, so the useful count depends on the compound. Iron is commonly 2 or 3. |
| 12 | Zinc group | 2 | Zn, Cd, Hg | The clean case among the middle groups: the d subshell is completely full and stays out of the bonding, so only the 2 s electrons count. |
| 13 | Boron group | 3 | B, Al, Ga | Group number minus 10 from here on. |
| 14 | Carbon group | 4 | C, Si, Ge | Four electrons, four bonds, which is most of organic chemistry. |
| 15 | Pnictogens (nitrogen group) | 5 | N, P, As | Five valence electrons, so typically three bonds and one lone pair. |
| 16 | Chalcogens (oxygen group) | 6 | O, S, Se | Six valence electrons, so typically two bonds and two lone pairs. |
| 17 | Halogens | 7 | F, Cl, Br, I | One short of a full shell, which is why they are so reactive. |
| 18 | Noble gases | 8 (helium has 2) | He, Ne, Ar | A full outer shell, which is why they hardly react. Helium's shell is full at 2, not 8. |
The family names matter as much as the numbers, because that is how the questions are usually worded. Halogens are group 17, so 7. Noble gases are group 18, so 8, except helium at 2. Alkali metals are group 1, so 1.
The transition metals: harder, but not unanswerable
The transition metals (the d-block, groups 3 to 12) do not follow the tidy rule, and the widget greys them out so the rule you learn here stays exact. That is not the same as the question having no answer.
For groups 3 to 11, the outermost shell usually holds 2 electrons, but the (n−1)d subshell just beneath it is close enough in energy that its electrons bond too. So the number that matters depends on the compound rather than on the column: iron commonly forms both Fe²⁺ and Fe³⁺, which is exactly the ambiguity the group rule cannot capture.
Group 12 (zinc, cadmium, mercury) is the clean exception, and it has a settled answer: 2. Its d subshell is completely full, so those ten electrons sit below the bonding entirely and only the two outer s electrons count. That same full d subshell is why IUPAC’s strict definition of a transition metal, an element with an incomplete d subshell, leaves group 12 out of the category altogether.
Browse any of them on the full elements index.
Lewis electron-dot symbols
A Lewis (electron-dot) symbol shows valence electrons as dots around an element’s symbol. The convention: imagine four sides (top, right, bottom, left) and place one dot on each side singly first, then start pairing once all four sides have one. So nitrogen (5 valence electrons) shows one lone pair and three single dots; neon (8) is fully paired: an octet. These lone and shared electrons are exactly what later drive bonding and the patterns in periodic trends such as how readily an atom gains or loses electrons. The count also links to atomic mass only loosely: valence is about electrons, not the nucleus.
Here is one element from each main group, with its dots:
-
group 1
1 valence -
group 2
2 valence -
group 13
3 valence -
group 14
4 valence -
group 15
5 valence -
group 16
6 valence -
group 17
7 valence -
group 18
8 valence
Every dot is one valence electron, and only valence electrons are drawn. That is why sodium, which has 11 electrons in total, is written with a single dot, and chlorine, which has 17, is written with seven.
Counting the valence electrons of a whole molecule
A Lewis structure for a molecule starts by counting all of its valence electrons, not just one atom’s. Add up each atom, then adjust for charge:
- Add each atom’s valence electrons. Carbon dioxide is 4 from the carbon plus 6 from each oxygen.
- Add one electron for each negative charge, since a negative ion has gained them, and subtract one for each positive charge.
- That total is what your Lewis structure has to account for, as bonds and lone pairs.
| Species | Formula | Add up each atom | Total |
|---|---|---|---|
| Water | H₂O | (2 × 1) + 6 | 8 |
| Carbon dioxide | CO₂ | 4 + (2 × 6) | 16 |
| Ammonia | NH₃ | 5 + (3 × 1) | 8 |
| Methane | CH₄ | 4 + (4 × 1) | 8 |
| Nitrogen gas | N₂ | (2 × 5) | 10 |
| Carbonate ion | CO₃2− | 4 + (3 × 6) + 2 | 24 |
| Ammonium ion | NH₄+ | 5 + (4 × 1) − 1 | 8 |
| Sulfate ion | SO₄2− | 6 + (4 × 6) + 2 | 32 |
Carbonate is the row worth reading twice. The atoms alone give 4 + (3 × 6) = 22, and the 2− charge adds two more, for 24. Getting that sign backwards is the most common way a Lewis structure comes out two electrons short.
Using this with a class
Project the tool and have students predict the dot symbol before selecting an element, then check themselves. Or turn on color by valence and challenge them to explain why every column shares a color. You can embed this interactive free on your own site or LMS using the snippet below.