State the electric charge of the nucleus He 2 3

The nucleus of He 2 3 consists of 2 protons each having a charge of + 1 unit i.e total of two positive charges and 1 neutron which does not have any charge Thus the total charge on this nucleus is + 2 . However, the atom also has 2 electrons each with - 1 charge, so that the atom as a whole is neutral

State two ways in which the nuclei of isotopes O 8 16 and O 8 18 differs from each other( other than they have different neutrons.

The isotopes contain the same number of protons hence have similar chemical properties but a different numbers of neutrons. The different numbers of neutrons cause: (1) a difference in the atomic weight or mass of the atoms and some physical properties. (2) The difference in the ratio between the number of protons (the number of protons stays the same) and the number of neutrons creates a difference in the nuclear reactions of the isotopes.

The graph shows the variation with time of the activity of a radioactive sample, Use the graph to estimate half life of the sample.

Half - life (symbol t1&frasl;2) is the time required for a quantity to reduce to half of its initial value. From the graph, it is shown that The initial activity of the sample is 400 bq The time when the activity is reduced to half 200 b q is 4 min Since the Half-life is the time required for a quantity to reduce to half of its initial value, therefore the half-life of the sample is 4 min .