Valency and Chemical Properties

Why valency matters in CDS Science

Roughly a third of CDS Science questions come from Chemistry, and a large slice of those test whether you understand how atoms combine. Valency is the master key: once you can read an atom’s combining power, you can predict chemical formulae, name compounds, balance simple equations and judge reactivity.

The good news is that this is a high-yield, low-effort topic. The rules are short, the exceptions are few, and the questions repeat year after year. Examiners love asking the valency of an element, the formula of a compound, or which element is most/least reactive in a group.

Most candidates lose marks here not because the chemistry is hard but because they try to memorise hundreds of formulae and valencies blindly. That approach collapses under exam pressure. The smarter route is to understand one core principle — that atoms react to achieve a stable electron arrangement — and then derive everything else on the spot. Spend an hour mastering the logic on this page and you will save weeks of rote revision later.

Inside the atom: electrons, shells and configuration

An atom has a tiny dense nucleus (protons + neutrons) surrounded by electrons arranged in shells or energy levels labelled K, L, M, N… outward from the nucleus.

The maximum number of electrons a shell can hold follows the Bohr–Bury rule 2n², where n is the shell number:

• K (n=1): 2 electrons
• L (n=2): 8 electrons
• M (n=3): 18 electrons
• N (n=4): 32 electrons

The shells fill from the inside out, but the outermost shell never holds more than 8 electrons at the stage CDS tests. The electrons in the outermost shell are called valence electrons, and they decide chemical behaviour.

To write any electronic configuration, you only need the atomic number (Z), which equals the number of electrons in a neutral atom. Fill K first (up to 2), then L (up to 8), then M, keeping the outermost shell at 8 or below. For example, calcium (Z = 20) is 2, 8, 8, 2 — the eighteenth and nineteenth electrons go into M but the twentieth opens the N shell rather than overfilling M, which is why calcium has 2 valence electrons. This small rule prevents most configuration errors in the exam.

What valency actually means

Valency is the combining capacity of an element — the number of electrons an atom loses, gains or shares to attain a stable, fully-filled outermost shell.

The driving force is the octet rule: atoms are most stable with 8 electrons in their outermost shell (or 2 for the smallest atoms like helium). Noble gases such as neon (2, 8) and argon (2, 8, 8) already have this, which is why they are chemically inert.

The two ways to count valency

• If valence electrons are 1 to 4, the atom usually loses them. Valency = number of valence electrons.
• If valence electrons are 4 to 7, the atom usually gains electrons. Valency = 8 − (number of valence electrons).

Working out valency from electronic configuration

Let us run through the common elements CDS asks about. Write the configuration, find the valence electrons, then apply the rule.

• Hydrogen (1): 1 valence electron → valency 1
• Carbon (2, 4): 4 valence electrons → valency 4
• Nitrogen (2, 5): 5 valence electrons → valency 8 − 5 = 3
• Oxygen (2, 6): 6 valence electrons → valency 8 − 6 = 2
• Sodium (2, 8, 1): 1 → valency 1
• Aluminium (2, 8, 3): 3 → valency 3
• Sulphur (2, 8, 6): 6 → valency 8 − 6 = 2
• Chlorine (2, 8, 7): 7 → valency 8 − 7 = 1
• Neon (2, 8): 8 → valency 0 (inert)

Notice carbon is special: with 4 valence electrons it neither cleanly loses nor gains, so it shares electrons, giving it a fixed valency of 4 and the ability to form millions of organic compounds.

Variable valency and radicals

Some elements, especially transition metals, show more than one valency because they can lose different numbers of electrons. Iron is the classic example.

• Iron(II) — ferrous — Fe²⁺, valency 2
• Iron(III) — ferric — Fe³⁺, valency 3
• Copper: cuprous Cu⁺ (1) and cupric Cu²⁺ (2)

You also need the valency of common radicals (groups of atoms that behave as a unit):

• Valency 1: hydroxide OH⁻, nitrate NO₃⁻, ammonium NH₄⁺
• Valency 2: sulphate SO₄²⁻, carbonate CO₃²⁻
• Valency 3: phosphate PO₄³⁻

How valency builds bonds: ionic vs covalent

Once you know valency you can predict the type of bond two elements form.

Ionic (electrovalent) bonds

A metal with low valency electrons transfers them to a non-metal. The metal becomes a positive ion (cation), the non-metal a negative ion (anion), and opposite charges attract. Example: sodium gives its 1 electron to chlorine → Na⁺ and Cl⁻ form NaCl.

Covalent bonds

Two non-metals share electrons to complete their octets. Example: two hydrogen atoms share a pair to form H₂; oxygen shares two pairs with two hydrogens to form H₂O.

Ionic compounds tend to have high melting points, conduct electricity when molten or dissolved, and are usually solids. Covalent compounds tend to have low melting points and are poor conductors.

These contrasting properties are themselves popular CDS questions. Because ionic solids are held together by strong electrostatic forces in a rigid lattice, they need a lot of heat to melt and they dissolve readily in water, splitting into free ions that carry current. Covalent molecules, held to each other only by weak intermolecular forces, melt and boil at low temperatures and stay electrically neutral, so they generally do not conduct. Knowing this lets you reason out the answer even when the exact compound is unfamiliar.

Writing chemical formulae using valency

The fastest tool is the criss-cross method: write the symbols, put each valency above its element, then swap the valencies to become subscripts.

For radicals, treat the whole group as one unit and enclose it in brackets when more than one is needed. Calcium (valency 2) with nitrate NO₃ (valency 1) gives Ca(NO₃)₂.

Valency and the periodic table

The modern periodic table arranges elements by increasing atomic number into 18 groups (columns) and 7 periods (rows). Position directly reveals valency.

• Group 1 (alkali metals): 1 valence electron → valency 1
• Group 2 (alkaline earth metals): valency 2
• Group 13: valency 3; Group 14: valency 4
• Group 17 (halogens): 7 valence electrons → valency 1
• Group 18 (noble gases): full shell → valency 0

Trends to remember

• Down a group: number of shells increases, so atomic size increases.
• Across a period (left→right): nuclear pull increases, so atomic size decreases.
• Metals are on the left, non-metals on the right; metallic character increases down a group and decreases across a period.

Chemical properties: reactivity and the activity series

Valency also explains reactivity — how readily an element reacts.

Metals react by losing electrons. The easier they lose them, the more reactive they are, so reactivity increases down Group 1: potassium reacts more violently with water than sodium, which reacts more than lithium.

Non-metals react by gaining electrons. The stronger the pull, the more reactive, so reactivity decreases down Group 17: fluorine is the most reactive halogen, iodine the least.

The reactivity (activity) series of metals

A useful order, most reactive first: K > Na > Ca > Mg > Al > Zn > Fe > Pb > (H) > Cu > Ag > Au. A more reactive metal can displace a less reactive one from its salt solution.

Solved illustration: predicting a compound

Here X behaves exactly like magnesium and Y like chlorine, so the compound XY₂ is the analogue of MgCl₂ — an ionic solid.

Common mistakes to avoid

• Forgetting to reduce formulae to the lowest ratio (Mg₂O₂ → MgO).
• Assuming valency equals the group number for groups 15–17 — use 8 − valence electrons instead (nitrogen, group 15, has valency 3, not 5, in hydrides).
• Treating noble gases as having valency 8 — their valency is 0 because the shell is already complete.
• Mixing up reactivity trends: metals get MORE reactive down a group, non-metals get LESS reactive down a group.

Previous-year question and quick recap