Atoms, elements and compounds

Elements, compounds and mixtures

An element is a substance that contains only one type of atom. It cannot be split into anything simpler by chemical means. Copper, oxygen and helium are elements, and each one has its own symbol and its own place in the Periodic Table.

A compound is formed when two or more different elements are chemically combined in fixed proportions. Water is always H2O - two hydrogen atoms for every one oxygen atom - and the elements in a compound lose their individual properties. A compound can be broken back down into its elements only by a chemical reaction.

A mixture contains two or more substances that are not chemically combined. The proportions can be varied, each substance keeps its own properties, and the mixture can be separated by a physical method such as filtration, distillation or chromatography. Air is a mixture of nitrogen, oxygen, argon, carbon dioxide and water vapour.

• Element: one type of atom only, and it cannot be broken down chemically.
• Compound: different elements chemically combined in fixed proportions, separated only by chemical reactions.
• Mixture: substances not chemically combined, in any proportions, separated by physical methods.
• In a compound the properties of the elements change; in a mixture they do not.

Inside the atom: protons, neutrons and electrons

Every atom has a tiny, dense nucleus at its centre containing protons and neutrons, surrounded by electrons arranged in shells. Almost all of the mass of an atom is in the nucleus, but almost all of its volume is the empty space through which the electrons move.

Protons are positively charged, electrons are negatively charged and neutrons carry no charge. An atom is electrically neutral because the number of protons equals the number of electrons. The relative charge and the relative mass of each particle are worth learning exactly, because examiners ask for them directly.

The proton number (also called the atomic number) is the number of protons in the nucleus. It is the fingerprint of an element: change the proton number and you have a different element. The mass number (also called the nucleon number) is the total number of protons plus neutrons, so the number of neutrons is the mass number minus the proton number.

Electrons fill the shells from the inside outwards. For elements up to proton number 20 the first shell holds a maximum of 2 electrons, the second 8, the third 8 and the fourth 2. The electronic configuration is written as a list of these numbers, so sodium (proton number 11) is 2,8,1. When an atom becomes an ion it gains or loses electrons, so the configuration changes: Na+ is 2,8 and Cl- is 2,8,8.

• Proton number = number of protons = the identity of the element.
• Mass number = protons + neutrons, so neutrons = mass number - proton number.
• Atoms are neutral because the number of protons equals the number of electrons.
• Shells fill 2, then 8, then 8, then 2 for the elements up to proton number 20.
• For Groups I to VII the group number equals the number of electrons in the outer shell, and the period number equals the number of occupied shells.
• Group VIII noble gases have a full outer shell, which is why they are so unreactive.

Isotopes and relative atomic mass

Isotopes are different atoms of the same element that have the same number of protons but different numbers of neutrons. Because the proton number is the same they are atoms of the same element, and because the number of neutrons is different their mass numbers are different.

The symbol for an atom or an ion shows both numbers. Carbon-12 is written with 12 at the top left and 6 at the bottom left, meaning 6 protons, 6 neutrons and 6 electrons. The chloride ion is written with 35 at the top left, 17 at the bottom left and a minus sign at the top right, so it contains 17 protons, 18 neutrons (35 - 17) and 18 electrons, because the chlorine atom gained one electron to become a negative ion.

Isotopes of an element have the same chemical properties because they have the same number of electrons, and therefore the same electronic configuration. The extra neutrons change only the mass, so physical properties such as density differ, but the way the atom reacts does not.

The relative atomic mass (Ar) of an element is the average mass of its atoms compared with 1/12 of the mass of a carbon-12 atom, taking the abundance of each isotope into account. To calculate it, multiply each isotope's mass number by its percentage abundance, add the results and divide by 100. Chlorine is 75% chlorine-35 and 25% chlorine-37, so Ar = (35 x 75 + 37 x 25) / 100 = 3550 / 100 = 35.5.

Ions and ionic bonding

An ion is a charged particle formed when an atom gains or loses electrons. Metals lose electrons to form positive ions (cations), because removing a negative electron leaves more protons than electrons. Non-metals gain electrons to form negative ions (anions).

Ionic bonds form between ions of metallic and non-metallic elements. Sodium (2,8,1) loses its single outer electron to chlorine (2,8,7). The sodium ion Na+ is 2,8 and the chloride ion Cl- is 2,8,8, so both ions now have a full outer shell like a noble gas. Magnesium (2,8,2) loses two electrons to form Mg2+ and oxygen (2,6) gains two to form O2-, so the compound formed is MgO.

An ionic bond is the strong electrostatic attraction between oppositely charged ions. In a solid ionic compound the ions do not sit in isolated pairs: they pack into a giant lattice, a regular arrangement of alternating positive and negative ions in which every ion is surrounded by ions of the opposite charge. This is why the formula of sodium chloride is NaCl even though there is no such thing as a single NaCl molecule.

The lattice explains the properties. Melting or boiling the solid means overcoming the strong electrostatic attractions between the ions throughout the lattice, which takes a great deal of energy, so the melting and boiling points are high. A solid ionic compound does not conduct electricity because its ions are held in fixed positions and cannot move. Once the compound is melted or dissolved in water the lattice breaks down, the ions become free to move, and the molten or aqueous compound conducts well.

• Metal atoms lose electrons to form positive ions; non-metal atoms gain electrons to form negative ions.
• Na+ is 2,8 and Cl- is 2,8,8 - both have full outer shells.
• The ionic bond is the attraction between the oppositely charged ions, not a shared pair of electrons.
• Ionic compounds form giant lattices, not separate molecules.

Simple molecules and covalent bonds

A covalent bond is formed when a pair of electrons is shared between two atoms. Both atoms count the shared pair as part of their outer shell, so each atom reaches a noble gas electronic configuration. Covalent bonds form between non-metal atoms, and the shared pair is attracted to both nuclei, which is what holds the atoms together.

In hydrogen, two hydrogen atoms each contribute one electron to a single shared pair, giving H2, and chlorine forms Cl2 in exactly the same way. In water each hydrogen shares one pair with the oxygen, giving two single bonds and two lone pairs on the oxygen. Methane has four single bonds, ammonia three and hydrogen chloride one. Some molecules need more than one shared pair: oxygen is O=O with two shared pairs, carbon dioxide is O=C=O with two double bonds, and nitrogen has one triple bond.

Dot-and-cross diagrams show this sharing. Draw the outer shells only, use a dot for the electrons of one atom and a cross for the electrons of the other, and put the shared pair in the overlap between the two shells so it is clearly counted by both atoms. Nothing is transferred in a covalent bond, so there are no charges to show.

Simple molecular compounds have low melting points and boiling points because the forces between one molecule and the next (the intermolecular forces) are weak, so only a little energy is needed to pull the molecules apart. The covalent bonds inside each molecule are strong, but they are not broken when the substance melts. Simple molecular compounds do not conduct electricity, because their molecules are uncharged and there are no free ions or electrons to carry the charge.

• Covalent bond = a pair of electrons shared between two atoms.
• Both atoms reach a noble gas configuration - that is why the sharing happens.
• The intermolecular forces between molecules are weak, but the covalent bonds inside each molecule are strong.
• Simple molecular compounds: low melting and boiling points, and no electrical conductivity.

Giant covalent structures: diamond, graphite and silicon(IV) oxide

Diamond is a giant covalent structure in which every carbon atom is joined to four other carbon atoms by strong covalent bonds. The bonds point to the corners of a tetrahedron, so the lattice extends in three dimensions. Every atom is locked in place by strong covalent bonds, so a very large amount of energy is needed to break the structure apart. Diamond therefore has a very high melting point, it is the hardest natural substance, and it is used in cutting tools such as drill bits and saw blades.

Graphite is also made only of carbon, but each carbon atom is joined to just three others, forming flat layers of hexagonal rings. The fourth electron of each carbon atom is delocalised, so it is free to move along the layer, which is why graphite conducts electricity and is used for electrodes. The layers themselves are held together only by weak forces, so they slide over one another easily, which makes graphite soft and slippery and useful as a lubricant.

Silicon(IV) oxide (SiO2) is another giant covalent structure. Each silicon atom is bonded to four oxygen atoms and each oxygen atom is bonded to two silicon atoms, so the whole structure is held together by a network of strong covalent bonds. Its structure is very like diamond's and so are its properties: it has a very high melting point, it is hard, and it does not conduct electricity. It is the main substance in sand and quartz and is used to make glass.

Metallic bonding

Metals have a giant metallic lattice of positive ions surrounded by a sea of delocalised electrons. The outer electrons of each metal atom leave the atom and become free to move throughout the whole structure, leaving behind a positive ion. Metallic bonding is the electrostatic attraction between these positive ions and the delocalised electrons.

This structure explains the properties of metals. The delocalised electrons are free to move through the lattice and carry charge, so metals are good conductors of electricity, which is why copper is used for cables. The same electrons can carry energy through the metal, so metals are also good conductors of heat.

Metals are malleable (they can be hammered or pressed into shape) and ductile (they can be drawn out into wires) because the layers of positive ions can slide over one another. As the layers move, the delocalised electrons move with them and the attraction is maintained, so the metal bends instead of shattering. Unlike an ionic lattice, the structure does not break apart when the layers shift.

• Metallic bonding = the attraction between positive ions and a sea of delocalised electrons.
• Delocalised electrons can move, so metals conduct electricity and heat.
• Layers of positive ions can slide, so metals are malleable and ductile.
• There are no separate molecules in a metal - it is one giant lattice.