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Difficulty: MediumSubatomic Particles, Atomic Number, Mass Number, and Isotopy

An atom of an element XX forms a stable monoatomic anion X2−X^{2-} containing 18 electrons and 18 neutrons. What is the mass number of element XX?

  1. A
    16
  2. 34Answer
  3. C
    36
  4. D
    38

Answer

34
The monoatomic anion X2−X^{2-} carries a negative charge of 2, indicating that it has gained 2 electrons compared to its neutral atomic state. Since the ion has 18 electrons, the neutral atom of element XX has 18−2=1618 - 2 = 16 electrons, which corresponds to an atomic number of 16 (16 protons). The mass number (AA) is defined as the total number of protons and neutrons in the nucleus. Thus, mass number A=16 protons+18 neutrons=34A = 16\text{ protons} + 18\text{ neutrons} = 34.

Step-by-Step Solution

1
Determine the atomic number (number of protons) of element XX from its anion X2−X^{2-}.
Number of protons Z=18−2=16Z = 18 - 2 = 16.
An anion with a 2−2- charge has gained 2 electrons. Therefore, the neutral atom has 2 fewer electrons than the ion, which equals its atomic number.
2
Calculate the mass number (AA) of element XX by summing the number of protons and neutrons.
Mass number A=16+18=34A = 16 + 18 = 34.
Mass number is the total sum of protons and neutrons present in the nucleus of an atom (A=Z+NA = Z + N).

Key Concept

Relationship between ion charge, subatomic particles, atomic number, and mass number
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Topics

Subatomic Particles, Atomic Number, Mass Number, and Isotopy

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