The first 20 elements
The periodic table arranges the elements in order of atomic number, the number of protons in the nucleus. Rows are called periods and columns are called groups. Elements in the same group have similar properties because they have the same number of electrons in their outer shell.
Each atom has as many electrons as protons. The electrons are in shells: the first holds up to 2, the second up to 8 and the third up to 8 (for these elements). The electrons are written as an electron arrangement, for example sodium (11 electrons) is 2,8,1. The number of filled-or-part-filled shells is the period number, and the number of electrons in the outer shell gives the group (group 1 has 1 outer electron, group 2 has 2, group 7 has 7 and group 8 has 8).
Choose an atomic number from 1 to 20. See how many electrons are in each shell and where the element sits.
Metals and non-metals
Most elements are metals. They lie on the left and in the middle of the table. Non-metals lie on the right, with hydrogen. A few elements near the dividing line, such as silicon, are semi-metals (metalloids). Group 8 contains the noble gases, which are very unreactive because their outer shells are full.
| Property | Metals | Non-metals |
|---|---|---|
| Appearance | Shiny | Dull (solids) |
| Conduct heat and electricity | Yes | No (graphite is an exception) |
| Hardness and strength | Usually hard and strong | Soft or brittle |
| Malleable and ductile | Yes | No, brittle |
| State at room temperature | Solid (except mercury) | Solids, liquid (bromine) or gases |
| Oxides in water | Basic (alkaline) | Acidic |
An element has atomic number 16. Write its electron arrangement and say which period and group it is in, and whether it is a metal.
- 16 electrons: 2 in the first shell, 8 in the second, so 6 are left for the third.
- Arrangement 2,8,6. Three shells so period 3; six outer electrons so group 6.
- It is on the right of the table: a non-metal (sulphur).
Answer: Electron arrangement 2,8,6, period 3, group 6; a non-metal (sulphur, S).
Key terms
- Atomic number
- The number of protons in an atom.
- Period
- A horizontal row of the periodic table.
- Group
- A vertical column of the periodic table.
- Noble gas
- An unreactive element of group 8 with a full outer shell.
- Semi-metal
- An element with properties between those of metals and non-metals.
Summary
- The periodic table orders elements by atomic number.
- Period number = number of occupied shells; group number = outer electrons (for the first 20 elements).
- Metals are on the left and in the middle; non-metals are on the right; group 8 holds the noble gases.
- Metals conduct and are malleable; non-metals are mostly dull, brittle and insulators.