Question

The ratio between the neutrons present in nitrogen atom and silicon atoms with mass numbers 14 and 28 is:
(A) 7:3
(B) 3:7
(C) 1:2
(D) 2:1

Answer 1

Hint: Together, the number of protons and the number of neutrons determine an element’s mass number. The small contribution of mass from electrons is disregarded in calculating the mass number. Atomic number is determined by number of protons only.

Complete step by step answer:
An element’s mass number (A) is the sum of the number of protons and the number of neutrons. This approximation of mass can be used to easily calculate how many neutrons an element has by simply subtracting the number of protons from the mass number. Protons and neutrons both weigh about one atomic mass unit or amu. Isotopes of the same element will have the same atomic number but different mass numbers.
For nitrogen: atomic number = 7 and mass number = 14
Therefore, Number of neutrons =14−7 = 7
For silicon: atomic number = 14 and mass number = 28
Therefore, number of neutrons =28 − 14 = 14
Hence, Ratio of number of neutrons in nitrogen and silicon =7:14 = 1:2
So, the correct answer is “Option C”.

Note: Neutral atoms of an element contain an equal number of protons and electrons. The number of protons determines an element’s atomic number (Z) and distinguishes one element from another. For example, carbon’s atomic number (Z) is 6 because it has 6 protons. The number of neutrons can vary to produce isotopes, which are atoms of the same element that have different numbers of neutrons.