Chemistry happens on the outside
A sodium atom has 11 electrons, but when it reacts with chlorine, only the outermost one takes part; the 10 inner electrons sit undisturbed. An element’s chemistry is decided mostly by its outermost electrons.
Valence electrons
The electrons that take part in reactions and decide an element’s chemistry are its valence electrons价电子. For main-group elements, they’re the s and p electrons in the highest-numbered energy level:
| Element | Configuration | Valence electrons | Number |
|---|---|---|---|
| Na | [Ne]3s¹ | 3s¹ | 1 |
| Al | [Ne]3s²3p¹ | 3s²3p¹ | 3 |
| Cl | [Ne]3s²3p⁵ | 3s²3p⁵ | 7 |
| Br | [Ar]4s²3d¹⁰4p⁵ | 4s²4p⁵ | 7 |
Bromine’s 3d¹⁰ electrons belong to level 3, which is full, so they aren’t valence electrons. For main groups, the number of valence electrons matches the group (lesson 1.4.5: groups 1 and 2, then the group number minus 10 for groups 13–18). For transition metals, valence electrons also include the d electrons, such as iron’s 4s²3d⁶.
Lewis dot symbols
A Lewis dot symbol电子式 shows an atom’s valence electrons as dots around its element symbol. By convention, put one dot on each of the four sides (top, right, bottom, left) before pairing any. Here are the atoms of period 2:
Elements in the same group have the same number of dots, which is exactly why they behave alike.
The octet rule and ions
Noble gases have 8 electrons in their outer level (helium has 2) and are very stable. Atoms of other main-group elements tend to lose, gain, or share electrons until their outer level also holds 8. This is the octet rule八隅规则.
- A metal atom loses all its valence electrons to form a cation, uncovering a full level of 8 underneath. A cation’s Lewis symbol is just the ion symbol, with no dots, such as Na⁺ or Mg²⁺.
- A nonmetal atom gains electrons until it has 8, forming an anion. An anion’s Lewis symbol shows all 8 dots in square brackets, with the charge at the upper right:
These ions end up with the same electron arrangement as a noble gas. Na⁺, Mg²⁺, and O²⁻ all have neon’s 1s²2s²2p⁶, and Cl⁻ matches argon. Species with the same number of electrons are isoelectronic电子排布相同.
Pick an element and click around its symbol to place its valence electrons. Once it’s right, see the ion it forms:
Click the top, right, bottom, or left of the symbol. Each side holds at most two dots.
Example 1: Nitrogen and magnesium
Draw the Lewis symbols for nitrogen and magnesium atoms and for the ions they form.
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Find the valence electrons first. A Lewis symbol shows only the outer electrons. Nitrogen, [He]2s²2p³, has 5; magnesium, [Ne]3s², has 2.
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Nitrogen atom. 5 dots: one on each of the four sides, then the fifth pairs with one of them.
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Magnesium atom. 2 dots, one on each of two sides.
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The ions they form. Nitrogen gains 3 electrons to complete an octet, forming N³⁻, drawn with 8 dots in brackets and a 3− charge. Magnesium loses its 2 outer electrons to form Mg²⁺, which has no outer electrons left and is written simply as Mg²⁺.
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Think ahead. When the ions combine, the charges must balance:
3 Mg²⁺ for every 2 N³⁻ gives magnesium nitride, Mg₃N₂.
Example 2: Isoelectronic with neon
Which of O²⁻, F⁻, Na⁺, Mg²⁺, Al³⁺, K⁺, and Cl⁻ are isoelectronic with a neon atom?
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The method. Electrons = atomic number − ionic charge (a negative charge adds electrons). Neon has 10.
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Count each one.
Particle Electrons O²⁻ 8 + 2 = 10 F⁻ 9 + 1 = 10 Na⁺ 11 − 1 = 10 Mg²⁺ 12 − 2 = 10 Al³⁺ 13 − 3 = 10 K⁺ 19 − 1 = 18 Cl⁻ 17 + 1 = 18 -
Conclusion. O²⁻, F⁻, Na⁺, Mg²⁺, and Al³⁺ each have 10 electrons, arranged 1s²2s²2p⁶ like neon; K⁺ and Cl⁻ have 18 and are isoelectronic with argon.
- Na
[Ne]3s¹: 1 valence electron, group 1.
- Mg
[Ne]3s²: 2 valence electrons, group 2.
- Al
[Ne]3s²3p¹: 3 valence electrons, group 13.
- C
[He]2s²2p²: 4 valence electrons, group 14.
- N
[He]2s²2p³: 5 valence electrons, group 15.
- O
[He]2s²2p⁴: 6 valence electrons, group 16.
- Br
[Ar]4s²3d¹⁰4p⁵: the valence electrons are 4s²4p⁵, 7 in all; the full 3d¹⁰ doesn't count.
- Ga
[Ar]4s²3d¹⁰4p¹: the valence electrons are 4s²4p¹, 3 in all, group 13.
- Ne
[He]2s²2p⁶: 8 valence electrons, already a stable octet.
Common mistakes
Mistake 1: “Bromine has 17 valence electrons (4s²3d¹⁰4p⁵).” For main-group elements, count only the s and p electrons in the highest level. The 3d electrons belong to level 3, which is full, so bromine has 7 valence electrons.
Mistake 2: “An anion’s Lewis symbol is just the atom’s with a few more dots.” An anion’s Lewis symbol shows a full 8 dots, enclosed in square brackets, with the charge at the upper right.
Mistake 3: “Every atom ends up with 8 electrons.” Hydrogen, and the ions Li⁺ and Be²⁺, need only 2, like helium. The octet rule has other exceptions too, which chapter 2.4 covers.
Quick check
Which of these is isoelectronic with a neon atom?
Mg²⁺ has 12 − 2 = 10 electrons, like neon. K⁺, Cl⁻, and S²⁻ each have 18, like argon.
How many dots surround the symbol in sulfur's Lewis dot symbol?
Sulfur is [Ne]3s²3p⁴, with 6 valence electrons, so its Lewis symbol has 6 dots.
Summary
- A main-group element’s valence electrons are the s and p electrons in its highest level, and their number matches its group; full inner d sublevels don’t count.
- A Lewis dot symbol shows valence electrons as dots, one per side before pairing. A cation is written as its ion symbol; an anion shows 8 dots in brackets, with its charge.
- The octet rule: main-group atoms lose, gain, or share electrons to reach 8 in their outer level (2 for hydrogen and helium), so their ions are isoelectronic with noble gases.
On to AP (for later)
AP topic 1.8 uses valence electrons to predict ion charges: group 1 forms +1, group 2 forms +2, group 17 forms −1, and group 16 forms −2. AP Lewis dot symbols are drawn exactly as here, and topic 2.5 builds them into Lewis structures for molecules, the subject of chapter 2.4.