# T.1(a).5–6 Periodic Table · Electronic Arrangement

F.3 Chemistry · Topic I(a) Nature of Matter · Lesson 3

How the Periodic Table is arranged, what group and period numbers tell you, and how to write the electronic arrangement of an atom.

## Figure

Group I electron diagrams: one outermost shell electron each; one more occupied shell, and one period lower, at each step down.

## Key points

- In the **Periodic Table (週期表)** the elements are arranged in order of **increasing atomic number**.
- **Groups** are the vertical columns, I to VII, then Group 0 (or VIII). Group number = number of **outermost shell electrons** in an atom.
- The same group means the same number of outermost shell electrons, so **similar chemical properties**; reactivity changes gradually down the group.
- **Periods** are the horizontal rows. Period number = number of **occupied electron shells** in an atom.
- Group I **alkali metals**; Group II **alkaline earth metals**; Group VII **halogens (鹵素)**; Group 0 **noble gases (貴氣體)**.
- **Atomic size decreases across a period**: the added electrons go into the same shell and the larger nuclear charge pulls them closer. It **increases down a group**: more electron shells.
- **Electronic arrangement (電子排佈)** is the way electrons are arranged in electron shells (energy levels). Shell n holds at most $2n^2$ electrons: 2, 8, 18, 32.
- Fill the shell **closest to the nucleus** first; start the next shell when one is full (or has 8). Show it by numbering (Na: 2, 8, 1) or by an **electron diagram**.

## Must know

| Idea | Rule | Example |
|---|---|---|
| Periodic Table | increasing atomic number | Ar (Z = 18) before K (Z = 19) |
| Group (column) | = outermost shell electrons | Cl 2, 8, 7 → Group VII |
| Period (row) | = occupied electron shells | Cl 2, 8, 7 → Period 3 |
| Same group | similar chemical properties | Li, Na, K (alkali metals) |
| Atomic size | ↓ across a period, ↑ down a group | Na > Cl; Li < Na < K |
| Electron shells | shell n holds at most $2n^2$ | first 20: 2, 8, 8, 2 |

From element 21 the third shell takes up to 18 electrons: bromine is 2, 8, 18, 7.

## Common mistakes

- ✗ "Arranged by relative atomic mass." ✓ By **atomic number**: argon (39.9) comes before potassium (39.1).
- ✗ "A group has the same chemical properties." ✓ **Similar**, not identical: reactivity changes gradually down the group.
- ✗ "Sulphur, 2, 8, 6, is in Period 6, Group III." ✓ Swapped: **period = occupied shells**, **group = outermost electrons**: Period 3, Group VI.
- ✗ "Potassium is 2, 8, 9." ✓ Fill 2, 8, 8 first: potassium is **2, 8, 8, 1**.
- ✗ "Atoms get bigger across a period." ✓ The added electrons join the **same shell** and the larger nuclear charge pulls them closer: atoms get **smaller**.

## Quick check

1. Magnesium has atomic number 12. Give its electronic arrangement, group and period.
2. What is the maximum number of electrons the third electron shell can hold?
3. Arrange lithium, sodium and potassium in order of increasing atomic size. Explain.
4. An atom has the electronic arrangement 2, 8, 18, 5. What is its atomic number?
5. Why do sodium and potassium have similar chemical properties?

## Answers

1. 2, 8, 2; Group II; Period 3.
2. $2 \times 3^2 = 18$.
3. Lithium < sodium < potassium: each has one more occupied electron shell.
4. 2 + 8 + 18 + 5 = 33.
5. Their atoms have the same number of outermost shell electrons (one); both are in Group I.

## Did you know?

The name "halogen" comes from Greek words meaning "salt-former": the halogens react with metals to form salts such as sodium chloride.