LEARNING OBJECTIVES
By the end of this chapter, you should be able to:
- distinguish elements, compounds and mixtures
- describe atomic structure, electronic configuration and isotopes
- calculate relative atomic mass from isotope abundance
- explain ionic, covalent and metallic bonding
- relate simple and giant structures to their properties
THE BIG IDEA
Connect atomic structure and electron arrangement to ions, bonding, structure and material properties.
Atoms contain a tiny positive nucleus surrounded by electrons. Chemical reactions rearrange electrons and atoms without creating or destroying them.
Bonding determines structure, and structure determines properties. This chain of reasoning explains melting point, conductivity, hardness and common uses.
SECTION 01
Elements, compounds and mixtures
An element contains only one type of atom. A compound contains atoms of different elements chemically bonded in a fixed ratio. A mixture contains substances together without chemical bonding.
A compound has properties different from its elements and is separated chemically. Mixture components keep their properties and can be separated physically.
KEY IDEAS
- Symbols represent elements; formulae show the fixed composition of compounds.
- Air is a mixture, water is a compound and oxygen is an element.
SECTION 02
Atomic structure and electronic configuration
The nucleus contains protons with relative charge +1 and neutrons with charge 0. Electrons have relative charge −1 and occupy shells around the nucleus.
Protons and neutrons each have relative mass 1; an electron has a much smaller relative mass of about 1/1836.
Atomic number is the number of protons. Mass number is protons plus neutrons. A neutral atom has equal numbers of protons and electrons.
KEY IDEAS
- For the first 20 elements, shells fill in patterns such as 2,8,3.
- Group I–VII number equals outer-shell electrons; period number equals occupied shells.
- Group VIII atoms have a full outer shell and are very unreactive.
Finding particles in aluminium-27
- Aluminium has atomic number 13, so it has 13 protons.
- A neutral atom has 13 electrons.
- Neutrons = 27 − 13.
Answer: 13 protons, 14 neutrons and 13 electrons; electronic configuration 2,8,3.
SECTION 03
Isotopes and relative atomic mass
Isotopes are atoms of the same element with the same proton number but different neutron numbers. They have the same chemical properties because their electron arrangements are the same.
Relative atomic mass, Aᵣ, is a weighted average of isotope masses compared with one twelfth of a carbon-12 atom.
Relative atomic mass from two isotopes
- An element is 75% isotope-35 and 25% isotope-37.
- Aᵣ = (35 × 75 + 37 × 25) ÷ 100.
- Aᵣ = 3550 ÷ 100.
Answer: Aᵣ = 35.5.
SECTION 04
Ions and ionic bonding
Metals lose electrons to form positive ions called cations. Non-metals gain electrons to form negative ions called anions.
An ionic bond is the strong electrostatic attraction between oppositely charged ions. Ionic compounds form giant lattices of alternating ions.
Strong attractions require much energy to overcome, giving high melting and boiling points. Ions conduct only when molten or aqueous because they must be free to move.
KEY IDEAS
- Electron transfer gives ions noble-gas electron arrangements.
- Formulae balance total positive and negative charge, for example Mg²⁺ and Cl⁻ form MgCl₂.
- Dot-and-cross diagrams show outer electrons and their origins.
SECTION 05
Covalent molecules and giant structures
A covalent bond is a shared pair of electrons. Simple molecules such as H₂O, CH₄, NH₃, O₂, N₂ and CO₂ have strong covalent bonds inside each molecule but weak forces between molecules.
Weak intermolecular forces explain low melting and boiling points. Simple molecular substances usually do not conduct because they have no mobile charged particles.
Diamond and silicon(IV) oxide are giant covalent networks with many strong bonds, so they are hard and have high melting points. Graphite has strong bonded layers, weak forces between layers and one delocalised electron per carbon.
KEY IDEAS
- Diamond is used in cutting tools because it is hard.
- Graphite is a lubricant because layers slide and an electrode because delocalised electrons conduct.
- Do not call intermolecular forces covalent bonds.
SECTION 06
Metallic bonding
A metal is a giant lattice of positive ions attracted to a sea of delocalised electrons. The attraction between them is metallic bonding.
Mobile electrons carry charge and thermal energy. Layers of ions can slide while attraction is maintained, explaining malleability and ductility.
KEY IDEAS
- Malleable means can be hammered into shape.
- Ductile means can be drawn into wires.
- Metallic bonding is an attraction, not a mixture of positive atoms and free protons.
QUICK CHAPTER SUMMARY
The ideas to carry forward
- Atomic number identifies an element.
- Isotopes share proton number but differ in neutrons.
- Ionic lattices contain mobile ions only when molten or aqueous.
- Simple molecules have weak intermolecular forces.
- Giant covalent and metallic structures explain characteristic properties.