iGCSE Chemistry (0620) Core:2.2 Atomic structure and the Periodic Table: Exam Style Questions Paper 1- New Syllabus
▶️ Answer/Explanation
Detailed solution:
In the notation ${}^{167}_{68}\text{Er}$, the bottom number (68) is the atomic number, which tells us the number of protons. A neutral atom has the same number of electrons as protons, so electrons are also 68. The top number (167) is the mass number, which is protons + neutrons. So, neutrons = mass number − atomic number = 167 − 68 = 99. This matches row A perfectly.
▶️ Answer/Explanation
Detailed solution:
The atomic number is the basic identity card of an element, and it is defined solely by the count of protons in the nucleus. Option D states this exactly. Option C says “number of particles in the nucleus,” but that would include neutrons too, which actually gives you the mass number, not the atomic number. The group number and the mass of an atom have nothing to do with defining atomic number, so D is the clear winner.
▶️ Answer/Explanation
Detailed solution:
Sulfur has the atomic number 16, so its electronic configuration is 2,8,6. This means it has three occupied electron shells, placing it in the third period, and it has six electrons in its outer shell, which matches its group (Group VI). Aluminium is in period 3, not period 2. Helium is in Group VIII but has only 2 outer electrons (its first shell is full). Lithium has two occupied shells (2,1), not one.