Question

4 protons, 5 neutrons, and 4 electrons are there in an atom of beryllium. Its mass number is:
A.10
B.5
C.8
D.9

Answer 1

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 answer:
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 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. So, for beryllium, there are 4 protons, 5 neutrons and 4 electrons but to calculate atomic mass number, four electrons won’t be counted.
Hence the atomic mass number of beryllium will be the sum of 4 protons and 5 neutrons and that will be equal to 9.

Therefore, the correct answer is option D.

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.