Home / iGCSE Chemistry 0620 Extended – 2.2 Atomic structure and the Periodic Table- Exam Style Questions Paper 4
Most-appropriate topic codes (Cambridge IGCSE Chemistry 0620):
• Topic 2.2 — Atomic structure and the Periodic Table
• Topic 2.3 — Isotopes
• Topic 3.2 — Relative masses of atoms and molecules
(a)
For the correct answers:
Neutron: relative charge is $0$, relative mass is $1$.
Proton: relative charge is $+1$, relative mass is $1$.
Explanation: Protons and neutrons are nucleons found in the dense nucleus of an atom, each having a relative mass assigned as $1$. Protons carry a positive fundamental charge ($+1$), whereas neutrons are electrically neutral ($0$).
(b)
For the correct answers:
Row 1 (${}_{20}^{40}Ca^{2+}$): Number of electrons is $18$.
Row 2 (${}_{9}^{19}F$): Number of neutrons is $10$.
Row 3 (Unknown): Symbol is ${}_{8}^{17}O^{2-}$.
Explanation:
1. A neutral Calcium atom has $20$ electrons. The $2+$ charge indicates a loss of $2$ valence electrons, leaving $18$.
2. The number of neutrons is the mass number minus the atomic number: $19 – 9 = 10$.
3. The atomic number (protons) is $8$, which identifies the element as Oxygen (O). The mass number is protons + neutrons ($8 + 9 = 17$). It has $10$ electrons compared to $8$ protons, giving it a net charge of $2-$. Hence, ${}_{8}^{17}O^{2-}$.
(c)(i)
For the correct answer:
Isotope X is Carbon-12 (or ${}^{12}C$).
Explanation: By international IUPAC convention, the unified atomic mass unit is defined as exactly $\frac{1}{12}$ of the mass of one unbound neutral atom of carbon-12 at rest and in its ground state.
(c)(ii)
For the correct answer:
Percentage abundance = $85\%$.
Calculation:
Let the fractional abundance of Mg-24 be $x$ and the fractional abundance of Mg-26 be $(1 – x)$.
The relative atomic mass is the weighted average:
$$24x + 26(1 – x) = 24.3$$
$$24x + 26 – 26x = 24.3$$
$$-2x = -1.7$$
$$x = 0.85$$
Multiply by $100$ to get the percentage abundance: $85\%$.