Chemical formulae

1. Every element has a chemical symbol of one or two letters, such as $H$ for hydrogen and $Na$ for sodium.
2. The first letter of a symbol is always a capital letter, and any second letter is always lower case.
   1. This matters because $Co$ is the element cobalt, whereas $CO$ is the compound carbon monoxide.
3. A chemical formula puts these symbols together to show which elements a substance contains and in what ratio their atoms are combined.
4. A small number written below and to the right of a symbol is a subscript, and it gives the number of atoms of the element just before it.
   1. $H_{2}O$ contains two hydrogen atoms and one oxygen atom.
   2. $H_2{SO}_4$ contains two hydrogen atoms, one sulfur atom and four oxygen atoms.
5. When a symbol has no subscript, there is one atom of that element, so the number 1 is never written.
6. A subscript written after a bracket multiplies every atom inside the bracket.
   1. ${Ca(OH)}_2$ contains one calcium atom, two oxygen atoms and two hydrogen atoms.
   2. $({NH}_4{)}_2{SO}_4$ contains two nitrogen atoms, eight hydrogen atoms, one sulfur atom and four oxygen atoms.

Formulae of elements

1. Metals are written as their symbol alone, such as $Na$, $Mg$, $Al$, $K$, $Ca$, $Fe$, $Cu$, $Zn$, $Ag$ and $Pb$.
2. Carbon and sulfur are also written as their symbols alone, $C$ and $S$.
3. The noble gases exist as single atoms, so their formulae are just their symbols, such as $He$, $Ne$ and $Ar$.
4. Seven non-metal elements exist as diatomic molecules, so their formulae all carry the subscript 2.
   1. Hydrogen is $H_2$, nitrogen is $N_2$ and oxygen is $O_2$.
   2. The halogens are $F_2$, ${Cl}_2$, ${Br}_2$ and $I_2$.
5. Whenever one of these elements appears in an equation, its diatomic formula must be used, so oxygen gas is always $O_2$ and never $O$.

Formulae of compounds

1. A compound contains two or more elements chemically combined, and its formula shows the fixed ratio of their atoms.
2. You need to recall the formulae of the common compounds that appear throughout chemistry.
3. Simple molecules:
   1. Water is $H_{2}O$, ammonia is ${NH}_3$ and methane is ${CH}_4$.
   2. Carbon dioxide is ${CO}_2$, carbon monoxide is $CO$ and sulfur dioxide is ${SO}_2$.
   3. Ethene is $C_2{H}_4$ and ethanol is $C_2{H}_{5}OH$.
4. Acids:
   1. Hydrochloric acid is $HCl$, sulfuric acid is $H_2{SO}_4$ and nitric acid is ${HNO}_3$.
5. Ionic compounds:
   1. Sodium chloride is $NaCl$, sodium hydroxide is $NaOH$ and calcium hydroxide is ${Ca(OH)}_2$.
   2. Magnesium oxide is $MgO$, calcium oxide is $CaO$ and calcium carbonate is ${CaCO}_3$.
   3. Copper(II) oxide is $CuO$, copper(II) sulfate is ${CuSO}_4$ and iron(III) oxide is ${Fe}_2{O}_3$.
6. The ending of a name gives a clue to the formula.
   1. A name ending in -ide usually means the compound contains just two elements, such as magnesium oxide, $MgO$.
   2. Hydroxides are the exception, because the hydroxide group, $OH$, contains both oxygen and hydrogen.
   3. A name ending in -ate means the compound also contains oxygen, such as calcium carbonate, ${CaCO}_3$, and copper(II) sulfate, ${CuSO}_4$.

Molecular formulae

1. A molecule is a group of atoms held together by covalent bonds, and the molecular formula counts every atom in one molecule.
2. To deduce a molecular formula from a model or diagram, count the atoms of each element in one molecule.
3. In a ball-and-stick model, each ball is an atom, each stick is a bond, and a key shows which colour stands for which element.
4. Write the symbol of each element once, followed by a subscript giving its number of atoms.
5. Carbon compounds are usually written with carbon first and hydrogen second, as in $C_2{H}_6$.
6. Keep the actual numbers from the model, even when they share a common factor, because the molecular formula describes one real molecule.
   1. A model of ethane shows two carbon atoms and six hydrogen atoms, so its molecular formula is $C_2{H}_6$ and not ${CH}_3$.

Word equations

1. In a chemical reaction, the atoms in the reactants are rearranged to make new substances, the products.
2. A word equation names the reactants on the left and the products on the right.
3. The arrow means "react to form" and always points from the reactants towards the products.
4. A plus sign separates two or more reactants, or two or more products.
5. An equals sign is never used, because the substances on the two sides are different.
6. A condition such as heating can be written above the arrow, because it is needed for the reaction but is not a reactant.
7. Burning magnesium in air is summarised as $magnesium+oxygen\to magnesium\; oxide$.
8. Burning methane in a gas cooker is summarised as $methane+oxygen\to carbon\; dioxide+water$.
9. Heating limestone is summarised as $calcium\; carbonate\to calcium\; oxide+carbon\; dioxide$.
10. Neutralising an acid with an alkali is summarised as $hydrochloric\; acid+sodium\; hydroxide\to sodium\; chloride+water$.

Symbol equations

1. A symbol equation replaces each name in a word equation with its chemical formula.
2. Atoms are not created or destroyed in a reaction, only rearranged, so there must be the same number of atoms of each element on both sides.
3. An equation that meets this condition is called a balanced equation.
4. To balance an equation, you write a coefficient, which is a whole number, in front of a formula.
5. A coefficient multiplies every atom in the formula that follows it, so $2H_{2}O$ contains four hydrogen atoms and two oxygen atoms.
6. You must never change a subscript to balance an equation, because that changes the substance itself.
   1. Changing $H_{2}O$ to $H_2{O}_2$ would turn water into a different compound, hydrogen peroxide.
7. Balance an equation by following these steps in order:
   1. Write the word equation.
   2. Write the correct formula for every reactant and product.
   3. Count the atoms of each element on both sides of the arrow.
   4. Add coefficients one element at a time, leaving any element that appears on its own, such as $O_2$, until last.
   5. Recount every element and check that the coefficients are the smallest whole numbers possible.

State symbols

1. A state symbol is written in brackets straight after a formula to show the physical state of that substance.
2. $(s)$ means solid, such as a metal, a powder or a precipitate.
3. $(l)$ means liquid, such as water or molten bromine.
4. $(g)$ means gas, such as oxygen, carbon dioxide or steam.
5. $(aq)$ means aqueous, which describes a substance dissolved in water.
   1. Solutions of acids, alkalis and soluble salts are all $(aq)$.
6. Pure water is a liquid, so it is written $H_{2}O(l)$ and never $H_{2}O(aq)$.
7. Choose each state symbol from the conditions of the reaction, because one substance can be ice, $H_{2}O(s)$, water, $H_{2}O(l)$, or steam, $H_{2}O(g)$.
8. Marble chips reacting with dilute hydrochloric acid are written ${CaCO}_3(s)+2HCl(aq)\to {CaCl}_2(aq)+H_{2}O(l)+{CO}_2(g)$.