In the periodic table, a group is a vertical column of elements. Elements in the same group share similar chemical and physical properties because they have the same number of electrons in their outermost shell (valence electrons).
Key points about groups:
- There are 18 groups in the modern periodic table.
- Elements in a group often show a gradual change in properties as you move down the column (for example, increasing reactivity in alkali metals).
- The group number usually indicates the number of valence electrons.
- Example: Elements in Group 1 (alkali metals) all have 1 valence electron.
- Elements in Group 17 (halogens) all have 7 valence electrons.
- Elements in the same group form compounds with similar formulas.
- Example: Sodium (Na) and potassium (K) both form chlorides like NaCl and KCl.
So, in short, a group is a column of elements with similar outer electron configurations, which gives them similar chemical behavior.