Cambridge IGCSE Chemistry · Syllabus 0620 · The Periodic Table
Proton Number
What is Proton Number?
The number of protons in the nucleus of an atom of an element, also called the atomic number. It defines which element the atom is, equals the number of electrons in a neutral atom, and is the quantity in whose increasing order the elements of the Periodic Table are arranged.
This definition is part of the The Periodic Table chapter in Cambridge IGCSE Chemistry.
Common mistakes with Proton Number
- 2 · “Arranged in order of increasing atomic mass” Defect Writing that the elements are ordered by relative atomic mass. Fix They are ordered by increasing proton number. The two orders agree most of the time, which is why the error survives, but they are not the same statement and only one of them is the syllabus wording. Tellurium sits before iodine despite having the larger relative atomic mass.
- Error 2 · “The elements are arranged in order of increasing relative atomic mass.” Defect Wrong ordering quantity. Chemistry Proton number defines the element and fixes the electron arrangement; mass does not. The two orders disagree in a few places, and the table follows proton number every time. Repair Replace “relative atomic mass” with “proton number”. Transfer Tellurium has a greater relative atomic mass than iodine. Which comes first in the table? Tellurium, because \(52 < 53\).
Examiner tips on Proton Number
- A question style worth recognising. Questions on this topic often use letters — element \(\mathrm{A}\), element \(\mathrm{Q}\) — rather than real symbols, so that you cannot answer from memory. Treat that as a gift rather than a trap: when a proton number, a configuration or a group is supplied, the answer is derivable from it — and when none of those is supplied, saying what is missing is the answer, as cases G and H in the position studio show. If you find yourself trying to remember which element \(\mathrm{Q}\) “really is”, you have left the method.