Atomic mass of an element is equal to the sum of number of
- (a)electrons and protons only
- (b)protons and neutrons only
- (c)electrons and neutrons only
- (d)electrons, protons and neutrons
Answer
Why
Correct — B, protons and neutrons only. Almost all of an atom's mass is concentrated in its nucleus, which contains protons and neutrons (together called nucleons). Electrons are only about 1/1836 the mass of a proton, so their contribution is negligible. The mass number therefore equals the number of protons plus the number of neutrons.
Why the others are wrong
- (a)electrons and protons only — This leaves out the neutrons, which are nucleons contributing nearly half the mass, and wrongly counts the almost-massless electrons.
- (c)electrons and neutrons only — This omits the protons, which carry mass and define the element, and includes the negligible-mass electrons.
- (d)electrons, protons and neutrons — Electrons are so light (about 1/1836 of a proton) that they are not counted in the mass number; only the nucleons - protons and neutrons - are.
Concept
The mass number (A) of an atom equals the number of protons plus the number of neutrons - the two heavy particles found in the nucleus and jointly called nucleons. Electrons, orbiting outside the nucleus, are so light they are ignored in the mass.
Recall that an electron's mass is about 1/1836 that of a proton, so any option that counts electrons is wrong. The mass is carried by the nucleons, giving 'protons and neutrons only'.
Key facts
- Mass number (A) = number of protons + number of neutrons; both are called nucleons.
- An electron's mass is roughly 1/1836 that of a proton, so electrons barely affect atomic mass.
- Number of neutrons = mass number - atomic number (A - Z).
- A proton and a neutron each have a mass close to one atomic mass unit (u).
The mass sits in the nucleus (protons + neutrons); electrons are too light to include.
Study next
Common traps
- Including electrons in the mass, when they are negligible.
- Confusing mass number (protons + neutrons) with atomic number (protons only).
- Forgetting the neutrons when a formula lists only the atomic number.
NDA asks what the atomic mass / mass number is made of - the sum of protons and neutrons (nucleons), not electrons.
Related PYQs
For Aluminium (Al) (atomic number: 13, mass number: 27), which one among the following statements is NOT correct?
- (a) Number of electrons present in Al is 13
- (b) Number of protons present in Al is 13
- (c) Number of neutrons present in Al is 14
- (d) The valency of Al is 2
Answer(d) The valency of Al is 2
Same nucleon bookkeeping - aluminium's mass number 27 minus atomic number 13 gives 14 neutrons, using mass number = protons + neutrons.
The relative atomic mass of boron (which exists in two isotopic forms 10B and 11B) is 10·81. What will be the abundance of 10B and 11B, respectively (consider a sample of 100 atoms) ?
- (a) 19% and 81%
- (b) 81% and 19%
- (c) 38% and 62%
- (d) 62% and 38%
Answer(a) 19% and 81%
Extends the mass idea - relative atomic mass is the abundance-weighted average of isotopes, each of whose mass numbers is protons + neutrons.
Practice
- practice — not a real PYQ
The number of neutrons in an atom is equal to
- (a)Z
- (b)A + Z
- (c)A - Z
- (d)A x Z
Answer(c) A - Z - mass number minus atomic number. - practice — not a real PYQ
Which particle contributes the least to the mass of an atom?
- (a)proton
- (b)neutron
- (c)electron
- (d)nucleon
Answer(c) electron - about 1/1836 the mass of a proton.