WASSCE SHS
The atomic number is the number of protons. The mass number is protons plus neutrons. In a neutral atom the number of electrons equals the number of protons.
Protons are positive and sit in the nucleus. Neutrons are neutral and sit there too. Electrons are negative and are arranged in shells around the nucleus. The atomic number fixes the element. Change the proton count and you have a different element. Isotopes are atoms of the same element with different numbers of neutrons, so they share an atomic number and differ in mass number. Chemically they behave alike, because chemistry is about the electrons.
The relative atomic mass on a periodic table is a weighted average of the isotopes, using their abundances. If a sample is 75 percent of an isotope of mass 35 and 25 percent of an isotope of mass 37, the relative atomic mass is (75 × 35 + 25 × 37) divided by 100, which is 35.5. Those abundances are the data of the question. You do not have to remember them. You do have to use the percentages as weights, not as a simple mean of 35 and 37.
Protons, neutrons and electrons are three answers. Do not give the mass number when you were asked for neutrons. Neutrons are mass number minus atomic number. For a relative atomic mass, the denominator is 100 if the abundances are percentages that add to 100. A simple average of the two mass numbers ignores the abundances and loses the method mark.
These are practice questions in the style of the paper, with a model answer and a mark scheme. They are not a promise of what your paper will ask.
4 marks
An atom of sodium has atomic number 11 and mass number 23.
(a) State the number of protons, neutrons and electrons. (3 marks)
(b) Why is the atom neutral? (1 mark)
(a) 11 protons, 12 neutrons, 11 electrons.
(b) The number of protons equals the number of electrons, so the charges cancel.
3 marks
A sample of an element is 75 percent isotope of mass 35 and 25 percent isotope of mass 37. Calculate the relative atomic mass. (3 marks)
(75 × 35 + 25 × 37) / 100 = (2625 + 925) / 100 = 3550 / 100 = 35.5.
2 marks
Define isotopes. (2 marks)
Isotopes are atoms of the same element, with the same number of protons, and different numbers of neutrons.
More questions on this topic are inside GLearn, marked as you answer, with the reason written out. The first ones need no account.
Practise Chemistry freeAtomic number is the number of protons and it identifies the element. Mass number is protons plus neutrons. In a neutral atom the electron count equals the proton count. Neutrons are mass number minus atomic number.
Multiply each isotope's mass by its percentage abundance, add the products, and divide by 100. A sample that is 75 percent mass 35 and 25 percent mass 37 has relative atomic mass 35.5. Do not take a plain average of 35 and 37 unless the abundances are equal. All WASSCE Chemistry past questions