Where do the electrons go?
Lesson 1.3.6 already borrowed a few results about electron shells to explain ions: sodium’s 11 electrons are arranged 2, 8, 1, and a full outer shell of 8 is stable, which is why sodium tends to lose one electron. But why 2, 8, 1, and not 3, 8 or all 11 together? This lesson fills in the complete rules: it works through the arrangements of the first 20 elements and shows you how to draw them.
Electron shells
The electrons around a nucleus are arranged in layers. Low-energy electrons stay close to the nucleus; higher-energy electrons move farther out. These layers are called electron shells电子层, numbered 1, 2, 3, and so on from the inside out.
In the first 20 elements, electrons fill the shells by these rules:
- Electrons go into the inner, lowest-energy shell first, and move outward only when it’s full.
- The first shell holds at most 2 electrons; the second holds at most 8.
- The outer shell holds at most 8 electrons (or 2, if there’s only one shell).
The last rule explains potassium’s 2, 8, 8, 1. Once the third shell has 8 electrons, a ninth electron there would give the outer shell more than 8, so electron 19 starts the fourth shell.
The electrons in the outer shell are called valence electrons最外层电子（价电子）. Sodium (2, 8, 1) has 1 valence electron; chlorine (2, 8, 7) has 7.
Bohr diagrams
A Bohr diagram原子结构示意图（玻尔模型） shows how an atom’s electrons are arranged. Here are sodium, chlorine, and neon:
Take sodium:
- The center circle is the nucleus, and +11 is its nuclear charge (the number of protons).
- Each ring is a shell: first, second, and third from the inside out.
- The dots are electrons: 2 in the first shell, 8 in the second, 1 in the third. In writing, this arrangement is listed as 2, 8, 1 (sometimes 2-8-1).
Chinese textbooks draw the same information in a shorter form: a circle for the nucleus, an arc for each shell, and a number on each arc.
Pick an element and place its electrons one at a time. Break a rule on purpose and see what happens:
Valence electrons decide the chemistry
Valence electrons are the ones that take part in reactions, which explains the pattern from lesson 1.3.6:
| Atoms | Valence electrons | Tendency in reactions |
|---|---|---|
| Noble gases | 8 (helium: 2) | Stable; don’t readily gain or lose electrons |
| Metals | Usually fewer than 4 | Lose their valence electrons, forming positive ions |
| Nonmetals | Usually 4 or more | Gain electrons to reach 8, forming negative ions |
Ions get Bohr diagrams too. Sodium loses its 1 valence electron to become a sodium ion; chlorine gains 1 electron to become a chloride ion. The nucleus doesn’t change, only the electrons do:
To tell whether a particle is an atom or an ion, compare its protons (nuclear charge) with its electrons:
- Protons = electrons: a neutral atom.
- More protons than electrons: a positive ion (cation).
- Fewer protons than electrons: a negative ion (anion).
Example 1: Draw a Bohr diagram
Draw the Bohr diagram for element 17 and predict how it behaves in reactions.
- Nuclear charge. Element 17 is chlorine, so the nucleus is +17.
- Fill the shells. 17 electrons: 2 in the first shell, 8 in the second, and 17 − 2 − 8 = 7 in the third: 2, 8, 7.
- Look at the valence electrons. With 7, chlorine needs just 1 more to reach 8, so it tends to gain 1 electron and form (2, 8, 8).
Example 2: Atom or ion?
Which of these particles is an atom, which is a cation, and which is an anion?
- Count protons and electrons. (1) 12 protons, 2 + 8 = 10 electrons; (2) 10 protons, 10 electrons; (3) 8 protons, 10 electrons.
- (1) More protons than electrons. A charge of 2+, so it’s a cation: the magnesium ion, .
- (2) Protons = electrons. Neutral, so it’s an atom: neon.
- (3) Fewer protons than electrons. A charge of 2−, so it’s an anion: the oxide ion, .
- Watch out. All three have the same electron arrangement, 2, 8. You have to read the nuclear charge to tell them apart.
- Sodium (2, 8, 1)
1 valence electron, easily lost, forming .
- Chlorine (2, 8, 7)
7 valence electrons, so it gains 1, forming .
- Neon (2, 8)
8 valence electrons: a stable arrangement.
- Magnesium (2, 8, 2)
2 valence electrons, easily lost, forming .
- Oxygen (2, 6)
6 valence electrons, so it gains 2, forming .
- Argon (2, 8, 8)
8 valence electrons: a stable arrangement.
- Potassium (2, 8, 8, 1)
1 valence electron, easily lost, forming .
- Sulfur (2, 8, 6)
6 valence electrons, so it gains 2, forming .
- Helium (2)
Helium has only one shell, and 2 electrons fill it: also a stable arrangement.
Common mistakes
Mistake 1: “Potassium’s arrangement is 2, 8, 9.” The outer shell holds at most 8, so potassium is 2, 8, 8, 1 and calcium is 2, 8, 8, 2.
Mistake 2: “The number in the center is the number of electrons.” It’s the nuclear charge (the number of protons), written with a + sign. In an atom it equals the number of electrons; in an ion it doesn’t.
Mistake 3: “Atoms with the same number of valence electrons always behave alike.” Helium and magnesium both have 2. But helium’s 2 electrons fill its only shell, making it very stable, while magnesium readily loses its 2.
Quick check
An atom has a nuclear charge of +13. How are its electrons arranged?
2 in the first shell, 8 in the second, and the remaining 3 in the third: 2, 8, 3. This is aluminum.
Which of these particles is an anion?
An anion has more electrons than protons: 17 protons and 18 electrons is a chloride ion. A and D are cations, and C is an argon atom.
Summary
- Shell rules: fill inner shells first; the first shell holds at most 2, the second at most 8, and the outer shell at most 8 (2 if there’s only one shell).
- A Bohr diagram shows the nucleus with its charge (+Z), a ring for each shell, and a dot for each electron.
- An outer shell of 8 (helium: 2) is stable. Metals tend to lose valence electrons and form cations; nonmetals tend to gain electrons and form anions. Equal protons and electrons means an atom; unequal means an ion.
On to AP (for later)
AP numbers the shells n = 1, 2, 3, 4 and splits each one into subshells, writing electron configurations such as for sodium (topic 1.5). Photoelectron spectroscopy (topic 1.6) is the experimental evidence that electrons really are arranged in shells. In Stage 2, lesson 2.2.3 covers the Bohr model and lesson 2.2.4 covers subshells.