Practise proton, neutron and electron counts, shell arrangements and isotope averages with three worked examples and 12 original questions with explained answers.
Readnary original study guide
Practise proton, neutron and electron counts, shell arrangements and isotope averages with three worked examples and 12 original questions with explained answers.
Original AI-assisted Readnary material. Not independently academically reviewed; answers may contain errors. Check important results against your course materials.
Selected content from 0620 sections 2.1-2.3 for exams in 2026-2028. Shell arrangements are limited to elements and simple ions with proton numbers up to 20. Weighted isotope averages and their chemical-property explanation include Supplement material.
| Particle | Relative charge | Approximate relative mass | Location |
|---|---|---|---|
| Proton | +1 | 1 | Nucleus |
| Neutron | 0 | 1 | Nucleus |
| Electron | -1 | 1/1836 | Shells outside nucleus |
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.
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.
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 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.
An aluminium ion has atomic number 13, mass number 27 and charge 3+. Find its numbers of protons, neutrons and electrons.
An invented sample of element X contains 60% atoms of mass 10 and 40% atoms of mass 11. Calculate its relative atomic mass.
Compare oxygen gas, pure water and a mixture of oxygen and nitrogen.
Try each question before opening its answer. Numerical answers should include your working.
Protons 12; neutrons 25 - 12 = 13; electrons 12. Neutrality means equal numbers of protons and electrons.
It loses two electrons, giving 10 electrons. Proton number remains 12 and neutron number remains 13.
Mass number = 17 + 18 = 35. There is one more electron than proton, so the charge is 1-.
They have the same six protons but different neutron numbers: six and eight. They are atoms of the same element with different masses.
2,8,8,1. The counts add to 19. It has four occupied shells and one electron in its outer shell.
2,8,8. Neutral calcium has 20 electrons; Ca2+ has lost two and therefore contains 18.
Group VI and Period 3. Six outer-shell electrons give the main-group number; three occupied shells give the period.
Its first and only occupied shell is full with two electrons. A full outer shell is the relevant feature; eight is not required for helium.
(20 x 0.75) + (22 x 0.25) = 15 + 5.5 = 20.5. It is an average, so do not round it to a mass number.
Let f be the fraction of mass-52 atoms. 50(1-f) + 52f = 50.6, so 2f = 0.6 and f = 0.30. The mass-52 isotope is 30%.
Copper is an element. Carbon dioxide is a compound with chemically combined elements in a fixed ratio. Air is a mixture of gases.
35.5 is an abundance-weighted average for a sample. Each isotope has its own whole-number mass number; use the stated isotope's mass number minus 17.
Do not alter the proton count when making an ion. Do not use a decimal average mass as a single isotope's nucleon count. Check that isotope percentages add to 100. Distinguish an element's group from the electron arrangement of one of its ions.
Chemistry · IGCSE · STUDY GUIDE
Practise proton, neutron and electron counts, shell arrangements and isotope averages with three worked examples and 12 original questions with explained answers.
Read the original Readnary guide in PDF view or use the web lesson above. Try the questions before revealing their explained answers.
