- State the periodic law
- Describe an atom's structure from its period and group number
- Explain how properties change across a period and down a group
In 1869 the Russian chemist Dmitri Mendeleev arranged the 63 elements known at the time in one table and noticed an amazing pattern: the properties of the elements repeat at regular intervals. Today the periodic table has 118 elements, but it is still built on Mendeleev's idea.
The periodic law
Mendeleev ordered the elements by increasing atomic mass. Later, once the structure of the atom was understood, it became clear that the real cause is the charge of the nucleus: as the atomic number increases, the arrangement of electrons in the outer shell repeats periodically, and so do the properties.
The properties of chemical elements and their compounds depend periodically on the charge of their atomic nuclei (the atomic number).
Periods and groups
The horizontal rows of the table are called periods; there are 7 of them. The vertical columns are groups: the modern table has 18 columns. The main groups are often labelled IA–VIIIA (groups 1, 2 and 13–18 in the modern numbering). Remember two simple rules: the period number equals the number of electron shells, and for main-group elements the A-group number equals the number of electrons in the outer shell.
Each cell of the table shows the element's symbol, name, atomic number and relative atomic mass. For example, the oxygen cell shows the symbol O, atomic number 8 and a relative atomic mass of ≈ 16. The table below compares the position of several elements with their electron arrangement.
| Element | Z | Electron shells | Period | Group |
|---|---|---|---|---|
| Lithium (Li) | 3 | 2, 1 | 2 | IA (1) |
| Oxygen (O) | 8 | 2, 6 | 2 | VIA (16) |
| Sodium (Na) | 11 | 2, 8, 1 | 3 | IA (1) |
| Chlorine (Cl) | 17 | 2, 8, 7 | 3 | VIIA (17) |
| Argon (Ar) | 18 | 2, 8, 8 | 3 | VIIIA (18) |
| Potassium (K) | 19 | 2, 8, 8, 1 | 4 | IA (1) |
An element has atomic number 16. Arrange its electrons in shells, find its period and group, and name the element.
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3 shells → period 3.
6 outer electrons → group VIA (16).
The element is sulfur (S), a non-metal.
How do properties change?
- Across a period, left to right: the nuclear charge grows, the atomic radius shrinks, metallic properties weaken and non-metallic properties strengthen. Each period (except the first) starts with an active metal and ends with a noble gas.
- Down a main group: the number of shells and the atomic radius grow, and metallic properties strengthen. For example, potassium is more reactive than sodium, and sodium more reactive than lithium.
Families of elements
| Family | Group | Examples | Typical behaviour |
|---|---|---|---|
| Alkali metals | IA (1) | Li, Na, K | soft, very reactive, react vigorously with water |
| Halogens | VIIA (17) | F, Cl, Br, I | reactive non-metals, form salts with metals |
| Noble gases | VIIIA (18) | He, Ne, Ar | full outer shell, hardly react at all |
Why are the elements of one group so alike? Because their atoms have the same number of electrons in the outer shell. All alkali metals have 1 outer electron and give it away easily, while halogens are 1 electron short of a full shell. It is the outer electrons that decide how an element behaves in reactions.
Sodium reacts with water, forming an alkali (sodium hydroxide) and hydrogen gas.
Key points
- Dmitri Mendeleev discovered the periodic law in 1869.
- The properties of elements depend periodically on the nuclear charge (atomic number).
- Period number = number of electron shells; A-group number = number of outer electrons.
- Metallic properties weaken across a period and strengthen down a group.
- Alkali metals, halogens and noble gases are families of elements with similar properties.
Check yourself
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