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
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.
- (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.
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'.
- 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.
- 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.
No directly related past PYQ was found.
- 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.