Alkali Metals Halogens and Noble Gases
This topic covers the characteristic physical and chemical properties of three key vertical columns in the periodic table: the alkali metals (Group 1), halogens (Group 17), and noble gases (Group 18). Knowing their properties and trends is essential for predicting chemical behaviour without needing calculations.
Part of the ESAT Chemistry syllabus — revision for the Engineering and Science Admissions Test (ESAT), the UAT-UK admissions test for Cambridge, Imperial, Oxford and UCL.
Key points
- Alkali metals (Group 1) are soft, low-density metals with one valence electron. They are highly reactive, losing their outer electron to form a +1 ion. Reactivity increases as you go down the group.
- Halogens (Group 17) are reactive non-metals with seven valence electrons, existing as diatomic molecules (e.g., F2, Cl2). They gain one electron to form a -1 halide ion. Reactivity decreases as you go down the group.
- Noble gases (Group 18) have a full outer shell of electrons, making them chemically inert (unreactive). They exist as individual, monatomic gases.
- The reaction of an alkali metal with water is vigorous, producing a metal hydroxide (an alkali) and hydrogen gas.
- Physical states at room temperature vary: alkali metals are solid (except Francium), halogens range from gas (F2, Cl2) to liquid (Br2) to solid (I2), and noble gases are all gases.
- Melting and boiling points increase down both Group 17 and Group 18 due to stronger intermolecular forces between larger atoms/molecules.
Diagram
Formulae
2M(s) + 2H2O(l) → 2MOH(aq) + H2(g) To show the general reaction of an alkali metal (M) with water. This reaction produces an alkaline solution (metal hydroxide) and flammable hydrogen gas.
Definitions
- Valence Electrons
- The electrons located in the outermost electron shell of an atom, which determine the atom's chemical reactivity.
- Alkali Metals
- The elements in Group 1 of the periodic table, such as Lithium, Sodium, and Potassium. They are highly reactive and form alkaline solutions.
- Halogens
- The elements in Group 17 of the periodic table, such as Fluorine, Chlorine, and Bromine. They are reactive non-metals that form salts with metals.
- Noble Gases
- The elements in Group 18 of the periodic table, such as Helium, Neon, and Argon. They are extremely unreactive due to their stable electron configuration.
Worked example
An element X is a soft, silvery solid that reacts vigorously with water. An element Y is a colourless, unreactive gas. An element Z is a green-yellow gas that exists as diatomic molecules. Identify the groups to which X, Y and Z most likely belong.
- 1
Analyse element X:
'Soft, silvery solid' and 'reacts vigorously with water' are classic descriptions of an alkali metal.
This points to Group 1.
- 2
Analyse element Y:
'Colourless, unreactive gas' is the defining characteristic of a noble gas, which has a stable, full outer shell of electrons.
This points to Group 18.
- 3
Analyse element Z:
'Green-yellow gas' and 'diatomic molecules' specifically describes chlorine (Cl2), which is a halogen.
This points to Group 17.
- 4
Combine the deductions to assign each element to its group.
Answer: X: Group 1, Y: Group 18, Z: Group 17
Common mistakes
- ×Confusing the reactivity trends: For metals (Group 1), reactivity INCREASES down the group. For non-metals (Group 17), reactivity DECREASES down the group.
- ×Forgetting the state changes down Group 17: The halogens become less volatile down the group, changing from gas (F2, Cl2) to liquid (Br2) to solid (I2) at room temperature.
- ×Assuming alkali metals are typical metals: Remember they are unusually soft and have low densities (e.g., Li, Na, and K float on water).
- ×Mixing up atomic structure: Halogens are diatomic (X2) in their elemental form, while noble gases are monatomic (X).
No-calculator tips
- ✓Use electron shielding to understand reactivity trends. As you go down a group, there are more electron shells. This shielding makes it easier for Group 1 metals to lose their outer electron (more reactive) and harder for Group 17 non-metals to attract an electron (less reactive).
- ✓Relate physical properties to intermolecular forces. For Groups 17 and 18, larger atoms down the group have more electrons, leading to stronger London dispersion forces. This requires more energy to overcome, resulting in higher boiling points.
- ✓Think in terms of stability. Groups 1 and 17 are one electron away from a stable noble gas configuration, making them highly reactive. Group 18 is already stable, making it inert.