WJEC Chemistry for AS Level Student Book: 2nd Edition (Draft)

WJEC Chemistry for AS Level

Radio-dating ▪ Carbon-14 (half-life 5570 years) is used to calculate the age of plant and animal remains. All living organisms absorb carbon, which includes a small proportion of the radioactive carbon-14. When an organism dies there is no more absorption of carbon-14 and that which is already present decays. The rate of decay decreases over the years and the activity that remains can be used to calculate the age of organisms. ▪ Potassium-40 (half-life 1300 million years) is used to estimate the geological age of rocks. Potassium-40 can change into argon-40 by the nucleus gaining an inner electron. Measuring the ratio of potassium-40 to argon-40 in a rock gives an estimate of its age. Industry and analysis ▪ Dilution analysis. The use of isotopically labelled substances to find the mass of a substance in a mixture. This is useful when a component of a complex mixture can be isolated from the mixture in the pure state but cannot be extracted quantitatively. ▪ Measuring the thickness of metal strips or foil. The metal is placed between two rollers to get the right thickness. A radioactive source (a β emitter) is mounted on one side of the metal with a detector on the other. If the amount of radiation reaching the detector increases, the detector operates a mechanism for moving the rollers apart and vice versa.

Rollers

Source

Aluminium

Thin sheet of foil

Detector

Electronic structure Electrons hold the key to almost the whole of chemistry since only electrons are involved in the changes that happen during chemical reactions. Electrons within atoms occupy fixed energy levels or quantum shells. Shells are numbered 1, 2, 3, 4, etc. These numbers are known as principal quantum numbers, n. The lower the value of n, the closer the shell to the nucleus and the lower the energy level. In a quantum shell there are regions of space around the nucleus where there is a high probability of finding an electron of a given energy. These regions are called atomic orbitals . Orbitals of the same type are grouped together in a subshell. Each orbital can contain two electrons. Along with charge, electrons have a property called ‘spin’. In order for two electrons to exist in the same orbital they must have opposite spins: this reduces the effect of repulsion. Each orbital has its own three-dimensional shape. There is only one type of s orbital and it is spherical

Key term Atomic orbital is a region in an atom that can hold up to two electrons with opposite spins.

x DRAFT y z normally drawn as: Exam tip You need to know the electronic configuration for the first 36 elements. The configurations for chromium and copper are not as expected, they both end in 4s 1 . The 4s orbitals are filled before the 3d orbitals. 22