Question

An atom of silicon has a mass number of 29. How many neutrons does it have?
(A) 14
(B) 15
(C) 16
(D) 28
(E) 29

Answer 1

Hint: Atomic number is the number of protons in the nucleus of an atom. Identity of an element is defined by the number of protons it has. Atomic mass is the total weight of an atom that is the total number of neutrons and protons. We can calculate the number of protons, neutrons and electrons if we know atomic number and atomic mass, and vice versa.

Complete Step by step solution
Proton is the particle with positive charge. It is found in an atom’s nucleus. The total number of protons in an atom is indicative of its atomic number. The property of atomic numbers is that it uniquely identifies chemical elements. In an uncharged atom, the number of electrons is also equal to atomic number. Electrons are used to take part in a chemical reaction by donating them or accepting them. The electrons are negatively charged particles whereas the neutrons are not charged at all, that is they are neutral particles. Neutrons are situated in the nucleus.
The atomic number of silicon is 14 and hence the number of protons and electrons in an uncharged silicon atom is also 14. The atomic mass is the total number of protons and neutrons in an atom. While calculating atomic mass, electrons are not counted as their weight is negligible. The atomic mass number of silicon is 29. The number of protons in a silicon atom is 14. Hence the number of neutrons will be the difference between the atomic mass and number of protons.
Atomic mass = number of protons + number of neutrons
Substituting the values,
 $29=14+\text{number of neutrons}$
Number of neutrons = $29-14=15$
Hence, the number of neutrons in silicon atoms with atomic mass number 29 is 15.
Therefore, the correct answer is option B.

Note
Protons as positively charged particles were discovered by Goldstein, neutrons as neutral particles were discovered by James Chadwick and electrons were discovered by Thomson.
Usually, the atomic mass number of silicon is considered 28. Silicon-29 is an isotope of silicon-28.