Chemistry · Fundamentals of chemistry · Grade 7

Chemical formula and valency

Why is water H₂O but aluminium oxide Al₂O₃? How does valency “assemble” a formula?

The situation

Twelve different substances were put on the table in the chemistry room: iron nails, copper wire, sulfur powder, water, table salt, chalk, and cylinders filled with gas. Each has a chemical “name” — a formula: Fe, H₂O, NaCl, CaCO₃. What do these letters and numbers mean, and which substances are made of a single element? Why is water written H₂O but aluminium oxide Al₂O₃? How much of the urea fertiliser spread on the fields is nitrogen?

Topic

Chemical symbol, formula and valency. Molecule builder: assembling water, ammonia, methane, carbon dioxide and table salt by valency; what percentage of nitrogen is in urea fertiliser

Key ideas

Chemical symbol
Valency
Mass fraction

Formulas

M_{r} = \sum_{i} n_{i} A_{r} (E_{i})

Relative molecular mass — the sum of the relative atomic masses of all the atoms in the formula ( $n_{i}$ are the subscripts).

ω (E) = \frac{n \cdot A_{r} (E)}{M_{r}} \cdot 100 %

Mass fraction of an element: what part of the substance is that element.

A_{x} B_{y} : x \cdot a = y \cdot b

The valency rule: $a$ and $b$ are the valencies of the elements; the numbers of valency units of the two elements are equal.

{Al}^{III} O^{II} : LCM (3, 2) = 6 \Rightarrow Al_{2} O_{3}

Writing a formula from valencies: divide the lowest common multiple (LCM) of the valencies by each valency — Al: 6 : 3 = 2, O: 6 : 2 = 3.

ω (N) = \frac{2 \cdot 14.007}{60.06} \approx 46.6 %

Urea $CO (NH_{2})_{2}$ — the most widely used nitrogen fertiliser.

Theory

Chemical element and symbol

A chemical element is a kind of atom of one type (the same nuclear charge). Each element is written with a chemical symbol: in 1813–1814 the Swedish chemist J. Berzelius proposed taking the first letter of the element's Latin name, or its first letter together with one of the following letters: Fe — *Ferrum* (iron), Cu — *Cuprum* (copper), S — *Sulfur*, Na — *Natrium*, C — *Carboneum*, O — *Oxygenium*, H — *Hydrogenium*. A symbol denotes both the element and one atom of it: 2O — two oxygen atoms. More about the elements — in the “Elements and the periodic table” experiment.

Simple and compound substances, the molecule

A simple substance is made of atoms of one element: iron Fe, copper Cu, sulfur S, oxygen O₂. A compound consists of atoms of two or more elements: water H₂O, table salt NaCl, chalk CaCO₃. One element can form several simple substances: oxygen O₂ and ozone O₃, graphite and diamond — this is allotropy. A molecule is the smallest particle of a substance that keeps its chemical properties (H₂O, CO₂, S₈). But not all substances are made of molecules: in metals atoms, and in table salt Na⁺ and Cl⁻ ions, form a crystal lattice — for these the formula shows the ratio of atoms (ions).

What a chemical formula tells you

A formula shows the qualitative composition of a substance (which elements) and its quantitative composition (how many atoms of each). The small number at the lower right of a symbol is the subscript: H₂O has two H and one O (1 is not written). The large number in front of a formula is the coefficient: 3H₂O is three water molecules, containing 6 H atoms and 3 O atoms. A group in brackets is multiplied by the subscript: CO(NH₂)₂ has 2 N and 4 H. The dot in a hydrate adds water molecules: CuSO₄·5H₂O.

Valency

Valency is the number of bonds an atom forms with other atoms; the valency of hydrogen (I) is taken as the unit, and oxygen is almost always divalent (II) in its compounds. Constant valency: H, Na, K — I; O, Mg, Ca — II; Al — III. Variable: Fe — II, III; Cu — I, II; S — II, IV, VI; N — I to V; P — III, V; C — II, IV. In a compound of two elements $A_{x} B_{y}$ the valency units are equal: $x \cdot a = y \cdot b$ . Writing a formula: find the LCM of the valencies and divide it by each — Al(III) and O(II): LCM = 6, $x = 2$ , $y = 3$ → Al₂O₃. The other way round: in Fe₂O₃ oxygen provides 3 · II = 6 units, so each Fe atom has valency 6 : 2 = III. The lines in the scene form the structural formula: each line is one valency unit.

$M_r$ and mass fraction (practical)

The relative molecular mass $M_{r}$ is the sum of the relative atomic masses of the atoms in the formula: $M_{r} (H_{2} O) = 2 \cdot 1.008 + 15.999 \approx 18.02$ . The mass fraction $ω$ of an element is what part of the substance is that element: in water $ω (H) = 2.016 : 18.02 \approx 11.2 %$ — two-thirds of the atoms are hydrogen, but only 11 per cent of the mass. The mass of an element in a sample: $m (E) = ω \cdot m$ — 50 kg of urea contains $\approx 23.3$ kg of nitrogen. The inverse problem — a formula from mass fractions: the ratio of atoms $x : y = \frac{ω_{A}}{A_{r} (A)} : \frac{ω_{B}}{A_{r} (B)}$ , then reduced to the smallest whole numbers.

Model and sources

The relative atomic masses are IUPAC/CIAAW (2024) values ( $A_{r}$ (H) = 1.008, $A_{r}$ (O) = 15.999). The “School values” button switches on whole numbers (Cl = 35.5) — look at which value the problem gives. The structural formula shows only valency units, not the shape of the molecule: the water molecule is actually bent (104.5°); ionic compounds such as NaCl and Al₂O₃ have no separate bond lines; the molecule of phosphorus(V) oxide is actually P₄O₁₀ — the formula gives the simplest ratio. The molecules and crystals in the hologram are built from measured bond lengths and lattice constants, the sizes of the atoms are schematic; the structure of copper sulfate pentahydrate is simplified.

Did you know?

The chemical symbols in their present form were proposed in 1813–1814 by the Swedish chemist Jöns Jacob Berzelius. Before that John Dalton had represented elements by small circles with different marks drawn inside.
In 1828 the German chemist Friedrich Wöhler obtained urea by heating ammonium cyanate — one of the famous early examples of the synthesis of an organic substance from an inorganic one.

Inside the experiment

The experiment starts from a real-life situation: you predict the result, change the controls, run the experiment and draw a conclusion. Inside are missions, theory, a quiz and a guided tour in which the camera moves and explains the key points.

More experiments in this section

The chemistry lab and safety — Grade 7
Pure substances and mixtures — Grade 7
Physical and chemical changes — Grade 7
Mole, molar mass and volume — Grade 7
From alchemy to the periodic law — Year 1