0620 · 3.1Formulae

Reading chemical formulae

0:00Water and hydrogen peroxide contain the same two elements, but they are different substances. Their formulae show why, because the numbers of atoms are different. Chemical formulae describe composition, while equations show how substances react. By the end, you will understand how to read formulae, construct them from particles, and write balanced equations. An atom is a particle of an element, and each element has its own chemical symbol. A molecule contains atoms joined together by covalent bonds.

0:27An ion is a charged particle, which may contain one atom or several. We need those distinctions because a formula can describe either an individual molecule or a ratio within an ionic compound. Start by recognising the formulae of familiar elements. Hydrogen, nitrogen, oxygen and the halogens shown occur as molecules containing two atoms under ordinary conditions. Oxygen gas is therefore written as O₂, showing two atoms in each molecule.

0:54Metals such as magnesium are written using their element symbols, without adding a subscript of two. Capital letters matter because they identify where element symbols begin. Some symbols contain one capital letter, while others also contain a following lower case letter. A small number after a symbol is called a subscript. It counts atoms of that element, and an absent subscript means one, not zero. The molecular formula of a compound gives the number and type of different atoms in one molecule.

1:21Water has the molecular formula H₂O. This means one molecule contains two hydrogen atoms and one oxygen atom. The formula identifies composition, but it does not show the positions of those atoms or the arrangement of bonds. To deduce a molecular formula from a diagram, first identify one complete molecule. Use the labels or key to count each type of atom inside that particle. Each methane molecule shown contains one carbon atom and four hydrogen atoms, so its formula is CH₄.

1:50Three identical molecules in the diagram do not change that formula into a larger one. A number placed in front of a formula is called a coefficient. It counts complete particles or formula units, rather than changing the substance itself. Three water molecules contain six hydrogen atoms and three oxygen atoms altogether. By contrast, changing water’s oxygen subscript changes its composition and produces the formula of hydrogen peroxide. Familiar names need to become connected with their correct formulae.

2:19Carbon dioxide contains one carbon atom and two oxygen atoms, while carbon monoxide contains only one oxygen atom per molecule. Ammonia contains one nitrogen atom and three hydrogen atoms. Use these compositions to check what a formula means, rather than treating its letters and numbers as an unexplained code. You must also recognise formulae of substances named throughout the chemistry course. This reference panel includes common acids, an alkali, salts and organic compounds.

2:46Their reactions and structures belong in later videos, but their formulae use the same counting rules. Some written forms also group atoms to show structure, although the total composition still comes from counting every atom.

Empirical and molecular formulae

2:59This next part is for the Extended paper. The empirical formula gives the simplest whole number ratio of the different atoms or ions in a compound. Unlike a molecular formula, it does not necessarily give the actual numbers in one molecule. It expresses composition using the smallest possible whole numbers. The formula of an ionic compound is always an empirical formula. Ethene has the molecular formula C₂H₄, so one molecule contains two carbon atoms and four hydrogen atoms.

3:28Its carbon to hydrogen ratio is two to four. Dividing both numbers by two simplifies that ratio to one to two. The empirical formula is therefore CH₂, without suggesting that an ethene molecule contains only three atoms. Only reduce a ratio when all of its numbers share a common factor. Water’s ratio of hydrogen to oxygen is two to one, which cannot be simplified using whole numbers. Its molecular and empirical formulae are therefore the same.

3:54Reducing just one subscript would alter the ratio and describe a different composition.

Writing ionic formulae

3:58Ionic compounds do not consist of separate molecules like water. Their ions form an extended lattice, and the formula gives the simplest whole number ratio present. This diagram contains three sodium ions and three chloride ions. The ratio simplifies to one to one, giving sodium chloride, rather than a formula based on all six visible ions. Ion charges also let us deduce an ionic formula without a diagram. An ionic compound is electrically neutral overall, so its positive and negative charges must balance.

4:29A sodium ion has a 1+ charge, while a chloride ion has a 1− charge. One of each gives a total charge of zero. Magnesium oxide needs one magnesium ion with a 2+ charge for each oxide ion with a 2− charge. Their charges cancel in a one to one ratio. Calcium chloride is different because each chloride ion carries only 1− charge. Two chloride ions are needed to balance one calcium ion with a 2+ charge.

4:55For aluminium oxide, an aluminium ion has a 3+ charge and an oxide ion has a 2− charge. The smallest matching charge totals are six plus and six minus. Two aluminium ions provide the positive total, while three oxide ions provide the negative total. The formula therefore contains aluminium and oxygen in a two to three ratio. Crossing over charge numbers can be a shortcut, but neutrality explains why the method works.

5:20Always simplify the resulting ratio when possible. Magnesium oxide needs one ion of each type, even though both charges have a magnitude of two. A correct ionic formula represents the smallest neutral ratio, with the positive ion written first. Some ions contain a group of atoms that stays together when we construct the formula. The ammonium ion has a 1+ charge, and the sulfate ion has a 2− charge. Two ammonium ions are needed for each sulfate ion.

5:47Brackets surround ammonium because the outside subscript applies to the entire group, giving two nitrogen atoms and eight hydrogen atoms. The sulfate group has no outside subscript, so it supplies one sulfur atom and four oxygen atoms.

Balancing symbol equations

6:00Word equations describe reactions by naming the starting substances and the substances formed. Reactants belong on the left, with products on the right. The arrow means gives, and plus signs separate substances on the same side. Magnesium plus hydrochloric acid gives magnesium chloride plus hydrogen is one example. A symbol equation replaces those names with correct chemical formulae. We then balance it because atoms are conserved during a chemical reaction.

6:25The balanced symbol equation is on the board, including the physical state of each substance. One magnesium atom reacts with two units of hydrogen chloride, producing magnesium chloride and one hydrogen molecule. State symbols give information that the formula alone cannot provide. They identify a solid, a liquid, a gas, or an aqueous substance dissolved in water. Aqueous does not mean that the substance itself is a pure liquid. Liquid water and sodium chloride solution therefore use different state symbols, even though both can be poured.

6:56Balancing must change coefficients, never the subscripts inside a formula. Subscripts identify substances, so altering them changes the reaction rather than balancing it. Begin with correct formulae, count atoms on both sides, and adjust the numbers in front. Finish by checking every element and reducing the coefficients to the smallest whole number ratio. Consider hydrogen reacting with oxygen to form liquid water after cooling. The word equation is hydrogen plus oxygen gives water.

7:22Start with H₂ plus O₂ gives H₂O, but notice that oxygen is not balanced. The left side contains two oxygen atoms, while the right contains only one. Place a coefficient of two before water to give two oxygen atoms on the right. This also produces four hydrogen atoms there, so hydrogen now needs attention. Put a coefficient of two before hydrogen on the left. The final equation is 2H₂ plus O₂ gives 2H₂O.

7:48The final count is four hydrogen atoms and two oxygen atoms on each side. There are three reactant molecules but only two product molecules in this representation. That is acceptable because atoms must be conserved, not the number of molecules. Chemical reactions rearrange atoms into different combinations without creating or destroying them.

Writing ionic equations

8:07This next part is for the Extended paper. An ionic equation shows the particles that actually take part in a reaction. Ions that remain unchanged are called spectator ions and are omitted. To construct the equation, first write a balanced symbol equation, then identify which dissolved substances supply separate ions. Consider two aqueous solutions producing an insoluble solid. The word equation is silver nitrate plus sodium chloride gives silver chloride plus sodium nitrate.

8:36The balanced symbol equation is on the board. Silver chloride is a solid precipitate, while sodium nitrate remains dissolved in the water. Separate the soluble ionic compounds into their aqueous ions. Leave solid silver chloride intact because it is not present as separate dissolved ions. Sodium ions and nitrate ions appear unchanged on both sides, so cancel them. The ionic equation shows silver ions and chloride ions in solution combining to form solid silver chloride.

9:03Check charge as well as atoms when balancing an ionic equation. Here, one plus and one minus give zero overall charge on the left. The solid product is neutral, so charge is balanced too. Do not split every substance into ions, because solids, liquids, gases and dissolved molecular substances are not automatically represented that way.

Constructing equations from descriptions

9:22An Extended question may ask you to construct an equation from information rather than supply a starting equation. Here, solid calcium carbonate reacts with aqueous hydrochloric acid to form calcium chloride solution, carbon dioxide gas and liquid water. First identify the reactants and products from the description. Write their correct formulae and use the stated conditions to assign state symbols. The word equation is calcium carbonate plus hydrochloric acid gives calcium chloride plus carbon dioxide plus water.

9:51Calcium, carbon and oxygen already balance in the starting symbol equation. Calcium chloride requires two chlorine atoms, so place a coefficient of two before hydrochloric acid. This supplies two hydrogen atoms as well, matching the water on the right. The balanced symbol equation is shown, with one calcium carbonate unit reacting with two units of hydrogen chloride. It produces calcium chloride, carbon dioxide and water in a one to one to one ratio.

10:17A final count confirms that every element is conserved. All five state symbols follow directly from the information in the question. Examiners commonly see correct balancing attached to incorrect formulae, so establish composition before adjusting coefficients. Another mistake is counting every molecule in a diagram instead of identifying the contents of one. For ionic compounds, check both charge balance and the simplest ratio. When a question says state a definition, give the precise meaning rather than only an example.

10:45Write each state symbol in lower case, in brackets, directly after its formula. Formulae tell us which elements are present and how their atoms or ions are related. A molecular formula counts atoms in one molecule, while an empirical formula gives the simplest whole number ratio. Ionic formulae must also represent an electrically neutral composition. Balanced equations conserve atoms, ionic equations conserve charge as well, and state symbols complete the description of the reaction.