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

WJEC Chemistry for AS Level

40 19 K +  18 Ar The product is one place to the left in the periodic table. 0 –1 e – → 40

3 Knowledge check Give the mass number and symbol of the isotope formed when 234 Th decays by β emission.

Another type of beta decay is positron emission or b + decay . In this process a proton is converted into a neutron while releasing a positron (and an electron neutrino). The positron is a type of beta particle ( β + ).

23 12 Mg →

23 11 Na + 

0 +1 β

+

Knowledge check

The product is one place to the left in the periodic table.

4

Strontium-83 is an unstable radioactive isotope that decays by positron emission. Write an equation to show this decay.

Half-life The rate at which a radioactive isotope decays cannot be speeded up or slowed down, it is proportional to the number of radioactive atoms present. The nature of radioactive decay is shown below.

N

N /2

128

N /4

N /8 N /16 Number of radioactive atoms t – 1 2

t – 1 2

t – 1 2

t – 1 2

Time

64

▲ Radioactive decay

01.02.04 AS Chemistry Eduqas Barking Dog Art DRAFT The time taken for N atoms to decay to N /2 atoms is the same as the time taken for N /2 atoms to decay to N /4 atoms and for N /4 atoms to decay to N /8 atoms. The time taken to decay to half the number of radioactive atoms is known as the half-life . The process resembles a knock-out competition such as Wimbledon where one half of the competitors (atoms) disappears over each round (half-life). The number of competitors disappearing during each round (number of atoms decaying each half-life) gets smaller and smaller but is always one half of those remaining. 32 16 8 6 5 4 3 Round Number of competitors left 2 1 Key term Half-life  is the time taken for half the atoms in a radioisotope to decay or the time taken for the radioactivity of a radioisotope to fall to half its initial value. 20