Atoms, elements and compounds

AQA·Chemistry (Combined Science)·5.1.1.1·Trilogy — Higher

Chemical formulae

A chemical formula represents a substance using the symbols of the elements it contains. For a compound, it shows which elements are chemically combined and the fixed proportions of their atoms.

For example, water has the formula $$\ce{H2O}$$. It contains hydrogen and oxygen in a ratio of two hydrogen atoms to one oxygen atom. Carbon dioxide, $$\ce{CO2}$$, contains carbon and oxygen in a ratio of one carbon atom to two oxygen atoms.

Reading a chemical formula

Each element has its own symbol. The first letter is a capital; if there is a second letter, it is lower case. For example, $$\ce{Co}$$ is the symbol for cobalt, whereas $$\ce{CO}$$ contains carbon and oxygen.

A small number written after a symbol is a subscript. It shows how many atoms of that element are represented. If there is no subscript, the formula represents one atom of that element. In $$\ce{CO2}$$, there is one carbon atom for every two oxygen atoms.

A subscript after brackets applies to everything inside them. In $$\ce{Ca(OH)2}$$, the 2 applies to both oxygen and hydrogen. The formula therefore represents one calcium atom, two oxygen atoms and two hydrogen atoms.

Some formulae describe the atoms in a molecule. Others, such as $$\ce{NaCl}$$ for sodium chloride, show the simplest ratio of atoms in the compound: one sodium atom for each chlorine atom. They do not necessarily represent separate molecules.

Writing and using formulae

A formula provides a compact way to identify a substance and connect it with its name. To write the correct formula for a compound, use the correct element symbols and the known proportions in which its atoms combine. For example, carbon dioxide has one carbon atom for every two oxygen atoms, so its formula is $$\ce{CO2}$$, not $$\ce{CO}$$. The latter represents a different compound, carbon monoxide.

When writing formulae for a reaction, use the correct formula for each substance. For example, hydrogen and oxygen react to form water: their formulae are $$\ce{H2}$$, $$\ce{O2}$$ and $$\ce{H2O}$$ respectively. A number placed before a formula counts whole units of that substance; it is not part of the formula. Changing a subscript instead changes the substance represented.

Chemical symbols and the periodic table

Each element has a chemical symbol: a standard shorthand used in chemistry. For example, oxygen is represented by $$\ce{O}$$ and sodium by $$\ce{Na}$$.

A symbol has one or two letters. Its first letter is always a capital; if there is a second letter, it is lowercase. The symbol does not always match the beginning of the element’s English name, as $$\ce{Na}$$ for sodium shows.

The periodic table organises the elements. You can use it to look up an element’s name from its symbol, or its symbol from its name. The elements below are listed in the order in which they appear in the table.

The first 20 elements

  • Hydrogen $$\ce{H}$$, helium $$\ce{He}$$
  • Lithium $$\ce{Li}$$, beryllium $$\ce{Be}$$, boron $$\ce{B}$$, carbon $$\ce{C}$$, nitrogen $$\ce{N}$$, oxygen $$\ce{O}$$, fluorine $$\ce{F}$$, neon $$\ce{Ne}$$
  • Sodium $$\ce{Na}$$, magnesium $$\ce{Mg}$$, aluminium $$\ce{Al}$$, silicon $$\ce{Si}$$, phosphorus $$\ce{P}$$, sulfur $$\ce{S}$$, chlorine $$\ce{Cl}$$, argon $$\ce{Ar}$$
  • Potassium $$\ce{K}$$, calcium $$\ce{Ca}$$

Elements in Groups 1 and 7

A group is a vertical column in the periodic table. Some elements in Groups 1 and 7 are among the first 20; others appear further down the table.

  • Group 1: hydrogen $$\ce{H}$$, lithium $$\ce{Li}$$, sodium $$\ce{Na}$$, potassium $$\ce{K}$$, rubidium $$\ce{Rb}$$, caesium $$\ce{Cs}$$ and francium $$\ce{Fr}$$.
  • Group 7: fluorine $$\ce{F}$$, chlorine $$\ce{Cl}$$, bromine $$\ce{Br}$$, iodine $$\ce{I}$$, astatine $$\ce{At}$$ and tennessine $$\ce{Ts}$$. This column is also called Group 17 in some periodic tables.

Use the periodic table in the same way to find the names and symbols of other elements named in the course.

Chemical equations

A chemical equation represents a reaction by showing the reactants that are used and the products that are formed. An arrow points from the reactants to the products; a plus sign separates substances on the same side.

A word equation names the substances. For example, hydrogen reacts with oxygen to form water:

hydrogen+oxygen⟶water\text{hydrogen} + \text{oxygen} \longrightarrow \text{water}

A symbol equation represents the same reaction using chemical symbols and formulae. Formulae show which elements are present and how many atoms of each element are in a particle. The balanced symbol equation for this reaction is:

2 HX2+OX2→2 HX2O\ce{2H2 + O2 -> 2H2O}

Balancing chemical equations

In a balanced equation, the number of atoms of each element is the same on both sides. In the equation above, there are four hydrogen atoms and two oxygen atoms on each side. The atoms have been rearranged into water molecules, not created or destroyed.

The large numbers in front of formulae are coefficients. They tell us how many particles of a substance are represented. For example, $$\ce{2H2O}$$ represents two water molecules, containing four hydrogen atoms and two oxygen atoms in total.

To balance an equation, change the coefficients, not the small numbers within a formula. Changing a formula would change the substance being represented. For example, $$\ce{H2O}$$ identifies water; the coefficient in $$\ce{2H2O}$$ represents two molecules of water.

Ionic equations and half equations

An ionic equation shows the particles that change in a reaction involving ions. It leaves out ions that remain unchanged on both sides. For example, when magnesium reacts with copper(II) ions, magnesium ions and copper are formed:

Mg+CuX2+→MgX2++Cu\ce{Mg + Cu^2+ -> Mg^2+ + Cu}

A half equation shows the electron transfer for one part of such a reaction. Here, magnesium loses two electrons, while a copper(II) ion gains two electrons:

Mg→MgX2++2 eX−\ce{Mg -> Mg^2+ + 2e-}
CuX2++2 eX−→Cu\ce{Cu^2+ + 2e- -> Cu}

The electrons lost in the first half equation are gained in the second. Combining the two half equations gives the ionic equation. Both the number of each type of atom and the total electrical charge balance across each equation.

Balanced chemical equations

A symbol equation shows the substances that react and the substances they form. The substances on the left are the reactants; those on the right are the products.

Atoms are rearranged in a chemical reaction, not created or destroyed. A balanced chemical equation therefore has the same number of atoms of each element on both sides.

Coefficients and subscripts

A coefficient is a whole-number multiplier written in normal script before a chemical formula. It changes the number of whole particles represented. For example, $$\ce{2H2O}$$ represents two water molecules, containing four hydrogen atoms and two oxygen atoms in total.

A subscript is the small number within a formula. It shows how many atoms of an element are in one particle. In $$\ce{H2O}$$, the subscript 2 means that each water molecule contains two hydrogen atoms; there is one oxygen atom because no subscript is written after O.

To balance an equation, change the coefficients, not the subscripts. Changing a subscript changes the formula and represents a different substance.

Balancing an equation for water

Consider the equation for hydrogen reacting with oxygen:

HX2+OX2→HX2O\ce{H2 + O2 -> H2O}

The hydrogen atoms balance: there are two on each side. The oxygen atoms do not: there are two on the left but only one on the right.

Place a coefficient of 2 before $$\ce{H2O}$$. The product side now has two oxygen atoms, but it also has four hydrogen atoms. Place a coefficient of 2 before $$\ce{H2}$$ to give four hydrogen atoms on the reactant side:

2 HX2+OX2→2 HX2O\ce{2H2 + O2 -> 2H2O}

The equation is now balanced: each side has four hydrogen atoms and two oxygen atoms. The formulas have not changed. No coefficient is written before $$\ce{O2}$$ because its coefficient is 1.

Atoms and elements

All substances are made from atoms. An atom is the smallest part of an element that can exist as that element.

An element is a pure substance made from only one type of atom. For example, a sample of the element carbon contains only carbon atoms. The atoms in an element may be joined together, but they are still all the same kind of atom.

There are about 100 different elements. Their atoms are the basic building blocks from which other substances are made.

Compounds

A compound is a substance made when atoms of two or more different elements are chemically joined. The atoms are held together by chemical bonds, so the compound is a new substance, not simply a mixture of its elements.

The elements in a compound are joined in fixed proportions. For example, water has the chemical formula $$\ce{H2O}$$. The small 2 shows that there are two hydrogen atoms for every oxygen atom. A given compound always has the same composition; every sample of pure water has this same ratio of hydrogen to oxygen atoms.

In a physical mixture, substances are together but are not chemically joined to form a new substance, and their proportions can vary. By contrast, separating a compound back into its elements requires chemical reactions that break and rearrange chemical bonds. Physical separation alone cannot do this.

Chemical reactions

A chemical reaction changes one or more starting substances into one or more new substances. The starting substances are called reactants, and the new substances are called products.

During a reaction, atoms are rearranged: chemical bonds can break and new bonds can form. The atoms themselves are not created or destroyed, so the products contain the same numbers of each type of atom as the reactants.

Forming a compound

When elements react chemically, they can form a compound: a substance containing atoms of different elements chemically bonded together. For example, hydrogen and oxygen react to form water:

2 HX2+OX2→2 HX2O\ce{2H2 + O2 -> 2H2O}

Hydrogen and oxygen are the reactants; water is the product. There are four hydrogen atoms and two oxygen atoms on each side of the equation. They have been rearranged into water molecules.

Energy transfer in reactions

A reaction can transfer energy to or from its surroundings. Energy transferred to the surroundings may make their temperature rise. Energy transferred from the surroundings to the reacting substances may make their temperature fall. A temperature change can therefore be a detectable sign of a chemical reaction.

Writing ionic equations

An ionic equation shows a reaction in terms of the particles that undergo a chemical change. Ions that are present but remain unchanged are called spectator ions. They appear on both sides of the full equation and are omitted from the ionic equation.

Reacting or product particles that are not ions must still be included when they are part of the change. An ionic equation must be balanced for both atoms and electrical charge.

Removing spectator ions

When aqueous silver nitrate reacts with aqueous sodium chloride, solid silver chloride forms:

AgNOX3(aq)+NaCl(aq)→AgCl(s)+NaNOX3(aq)\ce{AgNO3(aq) + NaCl(aq) -> AgCl(s) + NaNO3(aq)}

The aqueous ionic substances are present as dissolved ions. Writing these ions separately gives:

AgX+(aq)+NOX3X−(aq)+NaX+(aq)+ClX−(aq)→AgCl(s)+NaX+(aq)+NOX3X−(aq)\ce{Ag+(aq) + NO3^-(aq) + Na+(aq) + Cl^-(aq) -> AgCl(s) + Na+(aq) + NO3^-(aq)}

The sodium and nitrate ions are unchanged on both sides, so they are spectator ions. Removing them gives the ionic equation:

AgX+(aq)+ClX−(aq)→AgCl(s)\ce{Ag+(aq) + Cl^-(aq) -> AgCl(s)}

The symbols (aq) and (s) mean dissolved in water and solid, respectively. Solid silver chloride remains written as $$\ce{AgCl(s)}$$ rather than as separate aqueous ions: it is the product formed by the change.

Balancing atoms and charge

Choose coefficients—the numbers placed in front of formulae—so that each element has the same number of atoms on both sides. Then check that the total charge is also the same on both sides. Do not change the formula of a particle to balance the equation.

For example, magnesium reacts with hydrogen ions to form magnesium ions and hydrogen gas:

Mg(s)+2 HX+(aq)→MgX2+(aq)+HX2(g)\ce{Mg(s) + 2H+(aq) -> Mg^2+(aq) + H2(g)}

The coefficient $$2$$ gives two hydrogen atoms on each side. There is also one magnesium atom on each side. The two hydrogen ions have a total charge of $$+2$$, matching the charge of the magnesium ion on the right. The neutral magnesium and hydrogen particles remain in the equation because they undergo the change.

Writing half-equations

A reaction involving electron transfer can be separated into two parts: one in which electrons are lost and one in which electrons are gained. A half-equation represents one of these parts and shows the electrons transferred.

  • In an oxidation half-equation, electrons are lost, so they appear among the products, on the right.
  • In a reduction half-equation, electrons are gained, so they appear among the reactants, on the left.

Balancing a half-equation

A balanced half-equation has the same number of each type of atom on both sides. It also has the same total electrical charge on both sides. Electrons account for the change in charge; each electron has a charge of $$-1$$.

To write a half-equation:

  1. Write the reactant and product for the part of the reaction you are describing.
  2. Balance the atoms by adding numbers in front of the species where needed.
  3. Add electrons to the appropriate side until the total charge is equal on both sides.
  4. Check both the atoms and the total charge.

Oxidation half-equation

A magnesium atom forms a magnesium ion by losing two electrons:

Mg→MgX2++2 eX−\ce{Mg -> Mg^{2+} + 2e-}

There is one magnesium atom on each side. The left side has a total charge of zero. On the right, the magnesium ion has charge $$+2$$ and the two electrons have a combined charge of $$-2$$, giving a total of zero. The electrons appear on the right because they are lost.

Reduction half-equation

A chlorine molecule contains two chlorine atoms. It gains two electrons to form two chloride ions:

ClX2+2 eX−→2 ClX−\ce{Cl2 + 2e- -> 2Cl-}

There are two chlorine atoms on each side. The two electrons give the left side a total charge of $$-2$$, matching the combined charge of the two chloride ions on the right. The electrons appear on the left because they are gained.