IMO Practice Test — Structure of the Atom
14 Questions • 15 min • Olympiad level
15:00
Question 1 of 14
An ion X²⁺ has 10 electrons and 12 neutrons. The atomic number and mass number of X are:
Z = 10, A = 22
Z = 12, A = 24
Z = 12, A = 22
Z = 10, A = 20
Explanation: X²⁺ lost 2 electrons, so the neutral atom has 10 + 2 = 12 electrons; Z = 12. A = protons + neutrons = 12 + 12 = 24.
Question 2 of 14
Two atoms have configurations 2, 8, 1 and 2, 8, 7. The compound they form has the formula:
XY₂
X₂Y
XY
X₇Y
Explanation: The first has valency 1 and the second valency 1, so they combine in a 1 : 1 ratio, giving XY (like NaCl).
Question 3 of 14
An element has 2 electrons in K, 8 in L and 8 in M. Its valency is:
0
2
6
8
Explanation: Configuration 2, 8, 8 (Z = 18, argon) has a complete octet, so it is a noble gas with valency 0.
Question 4 of 14
An anion Y⁻ has 18 electrons and mass number 35. The number of neutrons in Y is:
16
17
18
20
Explanation: Y⁻ gained 1 electron, so the neutral atom has 17 electrons; Z = 17. Neutrons = A − Z = 35 − 17 = 18.
Question 5 of 14
If element M (Z = 12) and element N (Z = 17) combine, the formula of the compound is:
MN
MN₂
M₂N
M₂N₃
Explanation: M (2, 8, 2) has valency 2; N (2, 8, 7) has valency 1; cross-multiplying gives MN₂ (like MgCl₂).
Question 6 of 14
The number of times an alpha-particle bounced back in Rutherford's experiment was about:
1 in 10
1 in 100
1 in 1000
1 in 12,000
Explanation: Only about 1 in 12,000 alpha-particles rebounded, showing how tiny the nucleus is.
Question 7 of 14
Element X has isotopes of mass 10 (20%) and mass 11 (80%). Its average atomic mass is:
10.2
10.5
10.8
11.0
Explanation: Average = 0.20 × 10 + 0.80 × 11 = 2 + 8.8 = 10.8 u (this is boron).
Question 8 of 14
An atom has 17 protons, 18 neutrons and 17 electrons. Which statement is correct?
Z = 18
A = 18
It is a negatively charged ion
A = 35 and it is neutral
Explanation: Protons = electrons = 17 means neutral; A = 17 + 18 = 35. (This is a chlorine atom.)
Question 9 of 14
Which configuration → valency pair is correctly matched?
2, 8, 1 → 7
2, 8, 5 → 3
2, 6 → 6
2, 8, 8 → 8
Explanation: 2, 8, 5 has 5 valence electrons, so valency = 8 − 5 = 3. (2,8,1 → 1; 2,6 → 2; 2,8,8 → 0.)
Question 10 of 14
The species ⁴⁰Ca²⁺ and ⁴⁰Ar are best described as:
isotopes
isobars
the same atom
isomers
Explanation: Ca (Z = 20) and Ar (Z = 18) are different elements with the same mass number 40, so they are isobars.
Question 11 of 14
Why did Bohr's model improve upon Rutherford's?
It added neutrons
It allowed electrons in fixed orbits that do not radiate energy
It removed the nucleus
It made the atom solid
Explanation: Bohr postulated fixed energy shells in which electrons do not radiate energy, explaining the stability Rutherford could not.
Question 12 of 14
An atom of element P has 2, 8, 8, 1 configuration. The number of protons in P is:
18
19
20
17
Explanation: Total electrons = 2 + 8 + 8 + 1 = 19, so a neutral atom has 19 protons (potassium, K).
Question 13 of 14
Deuterium and tritium differ from protium only in their number of:
protons
electrons
neutrons
shells
Explanation: All hydrogen isotopes have 1 proton and 1 electron; they differ only in the number of neutrons (0, 1, 2).
Question 14 of 14
An element forms an oxide of formula X₂O₃. The valency of X is:
1
2
3
6
Explanation: Oxygen has valency 2; in X₂O₃ the total bonds from O are 3 × 2 = 6, shared over 2 X atoms, so each X has valency 3.