Atoms, Elements and Compounds

• One sentence to carry the whole chapter: identify the particles, follow the outer electrons, name the structure, then read the property off the structure. If you can say which of those four you are doing, you can start any Topic 2 question.
• A test you can apply to any conduction question in this chapter: name the charged particle, then say whether it can move. If you cannot name one, the substance does not conduct.
• If you can reproduce table 6 from memory, you can answer almost every structure-and-property question in Topic 2. It is the one worth writing out from scratch until it is automatic.

• 2.1.1 — I can describe the differences between elements, compounds and mixtures, at the level of the particles as well as in words. Taught in: Lesson 2.1.
• 2.2.1 — I can describe the atom as a central nucleus containing protons and neutrons, surrounded by electrons in shells. Lesson 2.2A.
• 2.2.2 — I can state the relative charges and relative masses of the proton, the neutron and the electron. Lesson 2.2A.
• 2.2.3 — I can define proton number (atomic number) as the number of protons in the nucleus of an atom. Lesson 2.2B.
• 2.2.4 — I can define mass number (nucleon number) as the total number of protons and neutrons in the nucleus. Lesson 2.2B.
• 2.2.5 — I can determine the electronic configuration of an element or of its ion for proton numbers 1 to 20, for example 2,8,3. Lesson 2.2C.
• 2.2.6a — I can state that the Group VIII noble gases have a full outer electron shell. Lesson 2.2C.
• 2.2.6b — I can state that, in Groups I to VII, the number of outer-shell electrons equals the group number. Lesson 2.2C.
• 2.2.6c — I can state that the number of occupied electron shells equals the period number. Lesson 2.2C.
• 2.3.1 — I can define isotopes as different atoms of the same element that have the same number of protons but different numbers of neutrons. Lesson 2.3A.
• 2.3.2 — I can state that isotopes of an element have the same chemical properties because they have the same number of electrons and therefore the same electronic configuration. Lesson 2.3A.
• 2.3.3 — I can interpret and use nuclide symbols for atoms and for ions. Lesson 2.2B and Lesson 2.3A.
• 2.3.4 — I can calculate a relative atomic mass from isotopic relative masses and their abundances. Lesson 2.3B.
• 2.4.1 — I can describe the formation of positive ions, called cations, and negative ions, called anions. Lesson 2.4A.
• 2.4.2 — I can describe the giant lattice structure of ionic compounds as a regular arrangement of alternating positive and negative ions. Lesson 2.4B.
• 2.4.3 — I can state that an ionic bond is a strong electrostatic attraction between oppositely charged ions. Lesson 2.4B.
• 2.4.4 — I can describe the formation of ionic bonds between ions of metallic and non-metallic elements, including dot-and-cross diagrams. Lesson 2.4B.
• 2.4.5a — I can explain the high melting points and boiling points of ionic compounds in terms of structure and bonding. Lesson 2.4C.
• 2.4.5b — I can explain why ionic compounds conduct electricity well when aqueous or molten but poorly when solid. Lesson 2.4C.
• 2.5.1 — I can state that a covalent bond is formed when a pair of electrons is shared between two atoms, leading to noble-gas electronic configurations. Lesson 2.5A.
• 2.5.2 — I can describe the formation of covalent bonds using dot-and-cross diagrams for all eleven named molecules. Lesson 2.5A.
• 2.5.3a — I can explain the low melting points and boiling points of simple molecular compounds in terms of weak intermolecular forces. Lesson 2.5B.
• 2.5.3b — I can explain the poor electrical conductivity of simple molecular compounds. Lesson 2.5B.
• 2.6.1 — I can describe the giant covalent structures of graphite, diamond and silicon(IV) oxide. Lesson 2.6A.
• 2.6.2a — I can relate the structure and bonding of graphite to its use as a lubricant and as an electrode. Lesson 2.6B.
• 2.6.2b — I can relate the structure and bonding of diamond to its use in cutting tools. Lesson 2.6B.
• 2.6.3 — I can describe the similarities in properties between diamond and silicon(IV) oxide and relate them to their structures. Lesson 2.6A and Lesson 2.6B.
• 2.7.1 — I can describe metallic bonding as the electrostatic attraction between the positive ions in a giant metallic lattice and a sea of delocalised electrons. Lesson 2.7.
• 2.7.2a — I can explain the good electrical conductivity of metals in terms of structure and bonding. Lesson 2.7.
• 2.7.2b — I can explain the malleability and ductility of metals in terms of structure and bonding. Lesson 2.7.

Where Topic 2 appears. Atomic structure and bonding are theory, so they are examined mainly on Paper 1 (multiple choice) and Paper 2 (theory). They also underpin the practical papers indirectly: a question that gives you a melting point and a conductivity result and asks what kind of structure a substance has is a Topic 2 question wearing practical clothing. For the qualification as a whole, the weightings are Paper 1 = 30%, Paper 2 = 50% and either Paper 3 or Paper 4 = 20%. Those are weightings for the whole course, not for this chapter, and no reliable claim can be made about how many Topic 2 marks appear in any particular series.

Element

A substance that contains only one type of atom, meaning every atom in it has the same proton number. An element cannot be broken down into simpler substances by chemical means. Its atoms may exist separately, as in argon, joined in molecules, as in oxygen, or joined in a giant structure, as in diamond, and in every one of those cases the substance is still an element.

Mass number

The total number of protons and neutrons in the nucleus of an atom, also called the nucleon number and given the symbol A. It is always a whole number and belongs to one specific atom, which distinguishes it from relative atomic mass, an abundance-weighted average that belongs to the element as a whole and is usually not a whole number.

Mixture

Two or more substances that are together in the same place but are not chemically combined. The proportions of a mixture can vary, each component keeps its own chemical properties, and the components can be separated by physical methods rather than by a chemical reaction.

Cation

A positively charged ion, formed when an atom loses one or more electrons. Metal atoms commonly form cations by losing their outer-shell electrons. The nucleus is unchanged, so the proton number and therefore the element stay the same; the positive charge arises simply because there are now more protons than electrons.

Mobile ions

Charged particles that are free to move through a substance and so can carry an electric current. An ionic compound contains ions in every state, but they are locked in fixed positions in the solid lattice; melting or dissolving breaks the lattice apart and allows the ions to move, which is why an ionic compound conducts only when molten or in aqueous solution. The charge carriers in these cases are ions, never electrons.

Covalent bond

A covalent bond is formed when a pair of electrons is shared between two atoms. The shared pair counts towards the outer shell of both atoms at once, so both reach a noble-gas electronic configuration in the molecules required at this level, with hydrogen reaching two electrons and the other atoms reaching eight. Covalent bonding occurs between non-metal atoms.

Simple molecular structure

A structure made of small, separate molecules. Inside each molecule the atoms are joined by strong covalent bonds, but the forces between one molecule and the next are weak. Melting and boiling only have to overcome those weak forces between molecules, so simple molecular substances have low melting and boiling points, and because the molecules carry no overall charge and have no delocalised electrons, they conduct electricity poorly.

Giant ionic lattice

The structure of an ionic compound: a regular three-dimensional arrangement in which positive and negative ions alternate, each ion surrounded by ions of the opposite charge. The electrostatic attractions act in all directions and continue throughout the whole crystal, so an ionic compound has no molecules, only a repeating pattern of ions in a fixed ratio.

Compound

A substance in which atoms of two or more different elements are chemically bonded together in a fixed ratio. The chemical properties of a compound are completely different from those of the elements that formed it, and it can only be separated back into those elements by a chemical reaction, not by a physical method.

Metallic bonding

Metallic bonding is the electrostatic attraction between the positive ions in a giant metallic lattice and a sea of delocalised electrons. The metal atoms have lost their outer-shell electrons into this shared sea, so the lattice consists of positive ions rather than neutral atoms, and the attraction acts in all directions between those ions and the mobile electrons around them.

Relative atomic mass

The relative atomic mass of an element is the average mass of the isotopes of that element, weighted by the abundance of each isotope, compared with one twelfth of the mass of an atom of carbon-12. It is calculated by multiplying each isotopic mass by its percentage abundance, adding the results and dividing by the total abundance, normally 100. Because it is an average across isotopes it is usually not a whole number, and because it is a ratio of masses it has no unit.

Giant covalent structure

A structure in which a very large number of atoms are joined by strong covalent bonds into one continuous network, rather than into separate small molecules. Melting such a substance requires many strong covalent bonds to be broken, so giant covalent substances have very high melting points and are usually hard. Diamond, graphite and silicon(IV) oxide are the three examples required at this level.

Proton number

The number of protons in the nucleus of an atom, also called the atomic number and given the symbol Z. It is what defines the element: every atom with the same proton number is the same element, and changing it would change the element. In a neutral atom the proton number also equals the number of electrons.

Electronic configuration

The arrangement of the electrons of an atom or ion in its shells, written as a list of numbers separated by commas, such as 2,8,3. Shells are filled from the innermost outwards, holding up to 2 electrons in the first shell and up to 8 in the second and third for proton numbers up to 20. The number of electrons in the outer shell gives the group number for Groups I to VII, and the number of occupied shells gives the period number.

Ionic bond

An ionic bond is the strong electrostatic attraction between oppositely charged ions. It is not the transfer of electrons: transfer is the process that creates the ions, and the bond is the attraction between the ions that result. The attraction acts in all directions, so it holds together not just one pair of ions but a whole giant lattice.

Giant metallic lattice

The structure of a metal: a regular three-dimensional arrangement of positive metal ions with delocalised electrons moving freely between them. Because the electrons are mobile, a metal conducts electricity in the solid state, and because layers of ions can slide over one another while the attraction to the electrons is maintained, a metal can be hammered into shape or drawn into a wire without shattering.

Isotope

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 unchanged, they are the same element and, as neutral atoms, they have the same number of electrons and the same electronic configuration, which is why they have the same chemical properties. They differ in mass number, and therefore in properties that depend on mass.

Atom

The smallest particle of an element that still carries the identity of that element. It has a very small central nucleus containing protons and neutrons, surrounded by electrons arranged in shells. Almost all of the mass is in the nucleus because protons and neutrons have a relative mass of 1 while an electron has a relative mass of about one eighteen-hundredth, and an atom is electrically neutral because it contains equal numbers of protons and electrons.

Intermolecular forces

The weak forces of attraction that act between one molecule and the next in a simple molecular substance. They are much weaker than the covalent bonds within a molecule, so only a small amount of energy is needed to overcome them, which is why simple molecular substances have low melting and boiling points. When such a substance melts or boils, these forces are overcome while the covalent bonds inside each molecule remain intact.

Anion

A negatively charged ion, formed when an atom gains one or more electrons. Non-metal atoms commonly form anions by gaining enough electrons to complete their outer shell. The nucleus is unchanged, so the element is unchanged; the negative charge arises because there are now more electrons than protons.

Delocalised electron

An electron that is not held in a bond between two particular atoms and is free to move through the structure. In graphite each carbon atom is bonded to only three others, so one of its outer electrons is delocalised and can move through the layers, which is why graphite conducts electricity. Metals contain delocalised electrons for the same reason: mobile charge carriers make a substance conduct in the solid state.

• 1. “An element is a substance made of single, separate atoms.” Repair An element contains only one type of atom. Those atoms may be separate (argon), joined in molecules (\(\mathrm{O_2}\), \(\mathrm{S_8}\)) or joined in a giant structure (diamond). It is still an element. Taught fully in Lesson 2.1.
• 2. “Mass number and relative atomic mass are the same thing.” Repair Mass number belongs to one atom and is always a whole number. Relative atomic mass belongs to the element and is an abundance-weighted mean, so it is usually not a whole number. Taught fully in Lesson 2.3B.
• 3. “An ion forms when an atom gains or loses protons.” Repair Only electrons move. The nucleus is untouched, so the proton number — and therefore the element — never changes when an ion forms. Taught fully in Lesson 2.4A.
• 4. “Change the number of neutrons and you get a different element.” Repair You get a different isotope of the same element. Identity is set by the proton number alone. Taught fully in Lesson 2.3A.
• 5. “The transfer of the electron is the ionic bond.” Repair Transfer makes the ions. The bond is the strong electrostatic attraction between the oppositely charged ions that result. Two separate events, in that order. Taught fully in Lesson 2.4B.
• 6. “Covalent bonds break when a molecular substance boils.” Repair Only the weak forces between molecules are overcome. Steam is still made of \(\mathrm{H_2O}\) molecules, each with its covalent bonds intact. Taught fully in Lesson 2.5B.
• 7. “All covalent substances have low melting points.” Repair Only simple molecular covalent substances do. Diamond, graphite and silicon(IV) oxide are covalent and have very high melting points, because melting them means breaking strong covalent bonds themselves. Taught fully in Lesson 2.6A.
• 8. “Anything with a lattice structure conducts electricity.” Repair Conduction needs a mobile charged particle. A metallic lattice has mobile delocalised electrons, so it conducts. An ionic lattice has ions that are fixed until it is molten or dissolved. Diamond has neither, so it does not conduct at all. Taught fully in Lesson 2.4C and Lesson 2.7.
• An inconsistent key. Repair If the key says dots are the metal's electrons, then every dot in the diagram is a metal electron. Switching part-way through — or leaving the key off altogether — makes the diagram unreadable, because the whole point of the two symbols is to show provenance. Where three atoms contribute electrons, add a third symbol and say so.
• Extra electrons added to make a shell “look full”. Repair Fill shells by drawing the correct bonds, never by adding marks. If an atom will not reach eight, the bonding is wrong, not the electron count — go back and check whether a double or triple bond is needed.

• It is possible to know the chemistry in this topic and still score badly, by giving a state answer when the question said explain. The command word tells you the shape of the answer before you know any chemistry at all, so read it first and let it set how much you write.
• This is not wrong, and that is exactly why it is dangerous. It names no structure, no particles and no process. There is nothing for the second and third marks to attach to.
• Magnesium oxide has a giant ionic lattice of \(\mathrm{Mg^{2+}}\) and \(\mathrm{O^{2-}}\) ions.
• There are strong electrostatic attractions between the oppositely charged ions, acting in all directions throughout the lattice.
• A large amount of energy is therefore needed to overcome these many strong attractions, so the melting point is high.
• Indicative marking points (guidance, not an official mark scheme): giant ionic lattice / regular arrangement of oppositely charged ions (1); strong electrostatic attraction between oppositely charged ions (1); much energy required to overcome the attractions (1).