How is the periodic table arranged?
Short answer
Elements are arranged in order of increasing atomic number (the number of protons). Columns are called groups: elements in the same group have the same number of electrons in their outer shell, so they react in similar ways. Rows are called periods: the period number equals the number of electron shells. Metals are on the left, non-metals on the right.
Step by step
- Atomic number increases from left to right along each period.
- Group 1 (alkali metals) have one outer electron and react vigorously with water.
- Group 7 (halogens) have seven outer electrons and become less reactive down the group.
- Group 0 (noble gases) have full outer shells and are unreactive.
- The transition metals sit in the central block.
Worked example
Sodium, element 11
- Electron arrangement: 2, 8, 1
- One outer electron, so Group 1
- Three shells, so Period 3
Common mistakes
- Saying elements are ordered by atomic mass. Mendeleev used mass; the modern table uses atomic number.
- Mixing up groups (columns) and periods (rows).
- Saying Group 1 reactivity decreases down the group. It increases.
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