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. Whether you must then explain the property, or only describe it, is what your route decides.
• 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. At Core the charged particles available to you are ions in an ionic compound and the delocalised electrons of graphite; Extended candidates add the delocalised electrons of a metal.
• Those six are the whole of Core 2.5.2. If you can draw all six from the group numbers alone and pass the count-to-eight check on every atom, the Core half of Route 2.5 is done.
• 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 — three columns if you are on the Core route, four if you are on the Extended route.

• Core 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.
• Core 2.2.1 — I can describe the structure of the atom as a central nucleus containing neutrons and protons, surrounded by electrons in shells. Lesson 2.2A.
• Core 2.2.2 — I can state the relative charges and relative masses of a proton, a neutron and an electron. Lesson 2.2A.
• Core 2.2.3 — I can define proton number / atomic number as the number of protons in the nucleus of an atom. Lesson 2.2B.
• Core 2.2.4 — I can define mass number / nucleon number as the total number of protons and neutrons in the nucleus of an atom. Lesson 2.2B.
• Core 2.2.5 — I can determine the electronic configuration of elements and their ions with proton number 1 to 20, for example 2,8,3. Lesson 2.2C.
• Core 2.2.6 — I can state that (a) Group VIII noble gases have a full outer electron shell, (b) in Groups I to VII the number of outer-shell electrons equals the group number, and (c) the number of occupied electron shells equals the period number. Lesson 2.2C.
• Core 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.
• Core 2.3.2 — I can interpret and use symbols for atoms, such as \({}^{12}_{6}\mathrm{C}\), and for ions, such as \({}^{35}_{17}\mathrm{Cl^{-}}\). Lesson 2.2B and Lesson 2.3A.
• Supplement 2.3.3 — I can state that isotopes of the same element have the same chemical properties because they have the same number of electrons and therefore the same electronic configuration. Lesson 2.3A, Supplement box.
• Supplement 2.3.4 — I can calculate the relative atomic mass of an element from the relative masses and abundances of its isotopes. Lesson 2.3B.
• Core 2.4.1 — I can describe the formation of positive ions, known as cations, and negative ions, known as anions. Lesson 2.4A.
• Core 2.4.2 — I can state that an ionic bond is a strong electrostatic attraction between oppositely charged ions. Lesson 2.4B.
• Core 2.4.3 — I can describe the formation of ionic bonds between elements from Group I and Group VII, including the use of dot-and-cross diagrams. Lesson 2.4B.
• Core 2.4.4 — I can describe the properties of ionic compounds: (a) high melting points and boiling points, (b) good electrical conductivity when aqueous or molten and poor when solid. Lesson 2.4C.
• Supplement 2.4.5 — I can describe the giant lattice structure of ionic compounds as a regular arrangement of alternating positive and negative ions. Lesson 2.4B, Supplement box.
• Supplement 2.4.6 — I can describe the formation of ionic bonds between ions of metallic and non-metallic elements generally, including the use of dot-and-cross diagrams. Lesson 2.4A and 2.4B, Supplement boxes.
• Supplement 2.4.7 — I can explain, in terms of structure and bonding, the high melting and boiling points of ionic compounds and their conductivity in each state. Lesson 2.4C, Supplement box.
• Core 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.
• Core 2.5.2 — I can describe the formation of covalent bonds in \(\mathrm{H_2}\), \(\mathrm{Cl_2}\), \(\mathrm{H_2O}\), \(\mathrm{CH_4}\), \(\mathrm{NH_3}\) and \(\mathrm{HCl}\), using dot-and-cross diagrams, and in similar molecules. Lesson 2.5A, Atlas A and Atlas B.
• Core 2.5.3 — I can describe, in terms of structure and bonding, the properties of simple molecular compounds: (a) low melting points and boiling points, (b) poor electrical conductivity. Lesson 2.5B.
• Supplement 2.5.4 — I can describe the formation of covalent bonds in \(\mathrm{CH_3OH}\), \(\mathrm{C_2H_4}\), \(\mathrm{O_2}\), \(\mathrm{CO_2}\) and \(\mathrm{N_2}\), using dot-and-cross diagrams, and in similar molecules. Lesson 2.5A, Atlas C and Atlas D.
• Supplement 2.5.5 — I can explain the low melting and boiling points of simple molecular compounds in terms of weak intermolecular forces, and explain their poor electrical conductivity. Lesson 2.5B, Supplement box.
• Core 2.6.1 — I can describe the giant covalent structures of graphite and diamond. Lesson 2.6A.
• Core 2.6.2 — I can relate the structures and bonding of graphite and diamond to their uses, limited to (a) graphite as a lubricant and as an electrode, and (b) diamond in cutting tools. Lesson 2.6B.
• Supplement 2.6.3 — I can describe the giant covalent structure of silicon(IV) oxide, \(\mathrm{SiO_2}\). Lesson 2.6A, Supplement box.
• Supplement 2.6.4 — I can describe the similarity in properties between diamond and silicon(IV) oxide, related to their structures. Lesson 2.6A and 2.6B, Supplement boxes.
• Supplement 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.
• Supplement 2.7.2 — I can explain, in terms of structure and bonding, (a) the good electrical conductivity of metals and (b) their malleability and ductility. Lesson 2.7.

Extended is Core plus Supplement, not a different course. An Extended candidate is examined on all 18 Core statements and all 11 Supplement statements. A Core candidate is examined on the 18 Core statements only. There is no Supplement-only paper. Practical work belongs to both routes. Every candidate takes Paper 5 or Paper 6, and the practical component is not Supplement material. Topic 2 is theory, so it is examined mainly on the multiple-choice and theory papers — but it also underpins 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. The weightings above are for the whole qualification, not for this chapter, and no reliable claim can be made about how many Topic 2 marks appear in any particular series.

Simple molecular structure

A structure made of small, separate molecules, in which the atoms inside each molecule are joined by strong covalent bonds. Simple molecular substances have low melting points and low boiling points, so many of them are liquids or gases at room temperature, and they conduct electricity poorly. Describing the structure and those two properties is Core 2.5.3; explaining them, through the weak forces between molecules and the absence of mobile charged particles, is Supplement 2.5.5.

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.

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.

Properties of ionic compounds

Ionic compounds have high melting points and high boiling points, so they are solids at room temperature. They conduct electricity well when molten or dissolved in water, and poorly when solid. These two statements are what Core 2.4.4 asks a candidate to describe; explaining them in terms of the lattice, the electrostatic attractions and the mobility of the ions is Supplement 2.4.7.

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.

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.

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.

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 and graphite are the two examples every candidate needs; Extended candidates add silicon(IV) oxide.

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.

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.

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 because the neutron number differs they have different mass numbers. Extended candidates also 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.

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.

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.

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.

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.

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.

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.

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 and can be used as an electrode. Extended candidates meet delocalised electrons again in metallic bonding, which is Supplement 2.7.1.

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.

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. Learning this sentence exactly is Core 2.4.2; describing the giant lattice those attractions build up is Supplement 2.4.5.

• 1. “An element is a substance made of single, separate atoms.” Core 2.1.1 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.” Supplement 2.3.4 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, which is a Supplement — Extended candidates section. Core candidates still need to know that the two quantities are different (mass number is Core 2.2.4); only the calculation is Supplement.
• 3. “An ion forms when an atom gains or loses protons.” Core 2.4.1 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.” Core 2.3.1 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.” Core 2.4.2 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.” Supplement 2.5.5 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. The weak intermolecular forces wording belongs to Supplement 2.5.5; at Core you describe the low melting point rather than explaining it this way.
• 7. “All covalent substances have low melting points.” Core 2.6.1 Repair Only simple molecular covalent substances do. Diamond and graphite are covalent and have very high melting points, because melting them means breaking strong covalent bonds themselves. Extended candidates: silicon(IV) oxide behaves the same way and for the same reason (Supplement 2.6.3). Taught fully in Lesson 2.6A.
• 8. “Anything with a lattice structure conducts electricity.” Core 2.4.4 Repair Conduction needs a mobile charged particle. An ionic lattice has ions that are fixed until it is molten or dissolved, which is why an ionic compound conducts only in those two states. Diamond has neither ions nor delocalised electrons, so it does not conduct at all. The ionic half is taught fully in Lesson 2.4C. Supplement — Extended candidates Supplement 2.7.2 The metallic case completes the set: a metallic lattice has mobile delocalised electrons, so a metal conducts even as a solid. Taught fully in 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. Extended candidates: where a third kind of atom contributes electrons, as in methanol, add a third symbol and say so in the key.
• 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. In Topic 2 the command word does something extra: for ionic and molecular properties it also tells you which tier you are being asked about.
• Extended is Core plus Supplement, not a different course. An Extended candidate is examined on all 18 Core statements and all 11 Supplement statements. A Core candidate is examined on the 18 Core statements only. There is no Supplement-only paper.
• Practical work belongs to both routes. Every candidate takes Paper 5 or Paper 6, and the practical component is not Supplement material. Topic 2 is theory, so it is examined mainly on the multiple-choice and theory papers — but it also underpins 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.
• The weightings above are for the whole qualification, not for this chapter, and no reliable claim can be made about how many Topic 2 marks appear in any particular series.
• Describe against explain is the tier boundary in this chapter. For the properties of ionic compounds and of simple molecular compounds, describe is the Core statement and explain is the Supplement one. Read the verb and you know which answer is being bought.
• It has a high melting point and a high boiling point. It conducts electricity well when molten or in aqueous solution, and poorly when solid.