IGCSE Chemistry 0620: Atomic Structure, Ions and Isotopes - Study Guide PDF with Answers

Atomic number identifies the element

The atomic number, Z, is the number of protons. Changing the number of neutrons makes a different isotope, not a different element. Changing the number of electrons makes an ion. Mass number, A, counts protons plus neutrons in one atom's nucleus, so neutron number is A - Z. A neutral atom contains the same number of electrons as protons because the charges balance. Electrons have much less mass than protons and neutrons.

Count electrons in ions carefully

A positive ion has lost electrons; a negative ion has gained electrons. A 2+ ion therefore has two fewer electrons than the neutral atom, not two more protons. Chemical ion formation does not change the nucleus. For the first 20 elements, use the shell model appropriate to this course: the first shell contains up to 2 electrons, the second up to 8, and the familiar arrangements through calcium use 2,8,8,2. This simplified pattern is not a universal shell-capacity rule for all elements.

Use shell arrangements to read the table

For main-group elements in the course's Groups I to VII, the outer-shell electron count matches the group number. The number of occupied shells gives the period. Noble gases have a full outer shell; helium's full shell contains two electrons, so do not automatically assign eight electrons to every full shell. A neutral sodium atom has arrangement 2,8,1, placing it in Group I and Period 3. A sodium ion has 2,8, but it is still the element sodium.

Isotopes and average mass

Isotopes share an atomic number but have different neutron numbers. Neutral isotopes of an element have the same electron arrangement and therefore very similar chemical behaviour in this syllabus model. Relative atomic mass is an abundance-weighted average measured relative to one-twelfth of a carbon-12 atom's mass. It need not be a whole number and has no unit. A mass number, by contrast, counts nucleons in a particular isotope. Do not substitute an average relative atomic mass when calculating an isotope's neutron number.

1. Count the particles in an ion

An aluminium ion has atomic number 13, mass number 27 and charge 3+. Find its numbers of protons, neutrons and electrons.

1. Protons = Z = 13. Ion formation has not changed this number.
2. Neutrons = A - Z = 27 - 13 = 14.
3. Electrons = 13 - 3 = 10 because the ion has lost three electrons. Its shell arrangement is 2,8.

2. Find an isotope average

An invented sample of element X contains 60% atoms of mass 10 and 40% atoms of mass 11. Calculate its relative atomic mass.

1. Use fractional abundances: 60% = 0.60 and 40% = 0.40.
2. Weighted average = (10 x 0.60) + (11 x 0.40) = 6.0 + 4.4 = 10.4.
3. The answer lies between 10 and 11 and closer to 10, the more abundant isotope. This is an invented data set, not a statement about a real element's natural abundances.

3. Distinguish an element from a compound

Compare oxygen gas, pure water and a mixture of oxygen and nitrogen.

1. Oxygen gas contains only oxygen atoms, even though its particles are molecules containing two atoms. It is an element.
2. Pure water contains hydrogen and oxygen chemically combined in a fixed ratio, so it is a compound.
3. Oxygen mixed with nitrogen is a mixture: the substances are together without becoming one new compound, and their proportions can vary.