Alkali metals, halogens, transition elements, and noble gases are families of elements classified by position in the periodic table. At SL and HL, their positions help predict bonding, ion formation, and chemical reactivity.
Classification and Properties
Elements in the same group have similar chemical properties because they have the same number of valence electrons.
| Family | Position and electron structure | Characteristic properties |
|---|---|---|
| Alkali metals | Group 1, excluding hydrogen; one valence electron | Metals that lose one electron to form ions with a +1 charge. Reactivity generally increases down the group because the outer electron is farther from the nucleus and more shielded. |
| Halogens | Group 17; seven valence electrons | Non-metals that gain one electron to form ions with a −1 charge. They occur as diatomic molecules, such as chlorine, Cl₂. Reactivity generally decreases down the group. |
| Transition elements | Central d-block | Metals associated with incomplete d-sublevels. Typical properties include variable oxidation states, catalytic activity, and coloured complex ions; these characteristic properties are AHL content. |
| Noble gases | Group 18; complete outer electron shell | Unreactive, monatomic gases. Helium has two outer electrons, while the other noble gases usually have eight. |
In Structure 3.1, electron configuration explains reactivity. Alkali metals readily lose one electron, whereas halogens readily gain one; both processes produce more stable electron arrangements.
A common misconception is that every d-block element is automatically a transition element. A transition element has an incomplete d-sublevel in its atom or a relevant ion, so group 12 elements are not normally classified as transition elements.
Exam Technique
For a describe question, state the trend. For an explain question, connect the trend to nuclear attraction, distance, and electron shielding. Always use group numbers 1, 17, and 18 and give ion charges with the sign before the number: +1 and −1.
