iGCSE Chemistry (0620) Theory (Core):2.3 Isotopes: Exam Style Questions Paper 3- New Syllabus
Question
This question is about nitrogen and its compounds.
Complete Table 4.1 to show the number of protons, neutrons and electrons in one atom of these isotopes.
(b) Oxides of nitrogen are air pollutants.
(b)(i) State one source of oxides of nitrogen in the air.
(b)(ii) Ammonia reacts with oxygen to form nitrogen dioxide and water.
Complete the symbol equation for this reaction.
$4\mathrm{NH}_3 + 7\mathrm{O}_2\rightarrow 4\mathrm{NO}_2 + \dots \mathrm{H}_2\mathrm{O}$ [1]
(b)(iii) State the type of bonding between the atoms in nitrogen dioxide.
(b)(iv) State whether nitrogen dioxide is an acidic or basic oxide.
Give a reason for your answer.
(c) A compound of nitrogen has the formula $\mathrm{Al(NO_3)_3}$.
Complete Table 4.2 to calculate the relative formula mass of $\mathrm{Al(NO_3)_3}$.
Most-appropriate topic codes (Cambridge IGCSE Chemistry 0620):
• Topic 8.1 — Arrangement of elements (Part (a)(i))
• Topic 2.3 — Isotopes (Part (a)(ii))
• Topic 10.3 — Air quality and climate (Parts (b)(i), (b)(ii), (b)(iv))
• Topic 2.5 — Simple molecules and covalent bonds (Part (b)(iii))
• Topic 3.2 — Relative masses of atoms and molecules (Part (c))
▶️ Answer/Explanation
(a)(i)
For the correct answer:
has 5 electrons in its outer shell
The Periodic Table is organized based on electronic configuration. The group number (for Groups I to VII) is directly equal to the number of electrons in the outermost shell (valence electrons) of an atom of that element. A nitrogen atom has the electronic configuration 2,5, so it belongs in Group V.
(a)(ii)
For the correct answer:
The atomic number of nitrogen is always 7, which defines the element and tells us it has 7 protons. In a neutral atom, the number of electrons exactly equals the number of protons, so each has 7 electrons. The mass number (14 or 15) is the total of protons and neutrons. Therefore, the neutrons are found by subtraction: $14-7=7$ and $15-7=8$.
(b)(i)
For the correct answer:
car engines
Oxides of nitrogen ($\mathrm{NO_x}$), such as nitrogen monoxide ($\mathrm{NO}$) and nitrogen dioxide ($\mathrm{NO_2}$), are formed inside the very hot combustion chambers of car engines. The high temperature causes the nitrogen and oxygen in the air to react together.
(b)(ii)
For the correct answer:
$6\mathrm{H_2O}$
To balance the equation $4\mathrm{NH_3} + 7\mathrm{O_2} \rightarrow 4\mathrm{NO_2} + \underline{6}\mathrm{H_2O}$, you check each atom. Left side: N=4, H=12, O=14. Right side without water: N=4, O=8. We need 12 H atoms and 6 extra O atoms. This requires 6 molecules of water ($6\mathrm{H_2O}$), which supplies $6 \times 2 = 12$ H atoms and 6 O atoms. The right side then has N=4, H=12, O=$8+6=14$, which fully balances.
(b)(iii)
For the correct answer:
covalent
Nitrogen dioxide ($\mathrm{NO_2}$) is a simple molecular compound formed from two non-metal atoms. Non-metal atoms bond together by sharing electrons to achieve a full outer shell. This type of strong electrostatic attraction between the shared electrons and the nuclei is a covalent bond.
(b)(iv)
For the correct answer:
acidic because nitrogen is a non-metal
Generally, metallic elements form basic oxides (e.g., calcium oxide), while non-metallic elements form acidic oxides (e.g., carbon dioxide, sulfur dioxide). Nitrogen is a non-metal, so its oxide, nitrogen dioxide, dissolves in water to form an acidic solution (nitric and nitrous acids).
(c)
For the correct answer:
Al = $1 \times 27 = 27$; N = $3 \times 14 = 42$; relative formula mass = $213$
The formula $\mathrm{Al(NO_3)_3}$ shows one aluminium atom and three nitrate groups. Each nitrate group has one nitrogen and three oxygens. So total atoms: 1 Al, 3 N, 9 O. Mass of Al = $1 \times 27 = 27$. Mass of N = $3 \times 14 = 42$. Mass of O = $9 \times 16 = 144$. Adding these together: $27 + 42 + 144 = 213$. The relative formula mass ($M_r$) is the sum of the relative atomic masses of all the atoms in the formula. No units are used for $M_r$.