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Difficulty: MediumAtomic Structure and Periodic Table

A neutral phosphorus atom contains 1515 protons and 1616 neutrons. If this atom gains 33 electrons to form a stable phosphide ion, what is the mass number of the resulting ion?

  1. A
    18
  2. 31Answer
  3. C
    34
  4. D
    16

Answer

The mass number of the resulting phosphide ion is 3131.
The mass number (AA) is defined as the total number of protons and neutrons located in the nucleus (15+16=3115 + 16 = 31). When an atom gains or loses electrons, it forms an ion with an electric charge, but the contents of the nucleus remain unchanged.

Step-by-Step Solution

1
Identify the components that determine the mass number of an atom or ion.
Mass number (AA) is calculated as the sum of protons (ZZ) and neutrons (NN).
Electrons have negligible mass relative to nucleons and reside outside the nucleus, so they are not included in the mass number.
2
Calculate the mass number using the given proton and neutron counts.
A=15+16=31A = 15 + 16 = 31
Adding 1515 protons and 1616 neutrons yields the total number of nucleons in the nucleus.
3
Evaluate the effect of gaining 33 electrons on the mass number.
The mass number remains 3131.
Gaining electrons changes only the net electric charge from 00 to −3-3, leaving the nucleus and mass number unaltered.

Key Concept

The mass number of an atom or ion is strictly the sum of its protons and neutrons (A=Z+NA = Z + N) and does not change during ionization.
Estimated Time:45s
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Topics

Atomic Structure and Periodic Table

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