Which one of the following statements about the properties of neutrons is not correct?
- (a)Neutron mass is almost equal to proton mass.
- (b)Neutrons possess zero charge.
- (c)Neutrons are located inside the atomic nuclei.
- (d)Neutrons revolve around the atomic nuclei.
Correct — D, Neutrons revolve around the atomic nuclei. That is the false statement, and it is false because it describes electrons. In the nuclear model of the atom the protons and neutrons are packed together in the nucleus, and only the electrons occupy the space around it. Nothing revolves around the nucleus except electrons, and the nucleus is not orbited by any part of itself. The other three statements are all sound. A neutron's mass is very close to a proton's — 1.675 × 10⁻²⁷ kg against 1.673 × 10⁻²⁷ kg, both roughly one atomic mass unit and about 1,836 times the mass of an electron. The neutron is electrically neutral, which is where its name comes from. And it sits inside the nucleus, which is what makes the mass number the sum of protons and neutrons.
- (a)Neutron mass is almost equal to proton mass. — Correct as printed. Proton and neutron masses differ by well under one per cent, both close to one atomic mass unit, which is why the mass number counts them together.
- (b)Neutrons possess zero charge. — Correct. The neutron carries no charge at all — the atom's positive charge comes entirely from the protons, and it is exactly balanced by the electrons in a neutral atom.
- (c)Neutrons are located inside the atomic nuclei. — Correct. Protons and neutrons together are called nucleons because both reside in the nucleus; the neutron was discovered there by James Chadwick in 1932.
Rutherford's alpha-scattering experiment on gold foil showed that almost all the mass and all the positive charge of an atom sit in a tiny central nucleus, with the electrons outside it. Chadwick completed the picture in 1932 by identifying the neutron, which explained why atomic mass exceeds the mass accounted for by protons alone and made isotopes intelligible. The atomic number is the number of protons and fixes the element and its chemical behaviour; the mass number is protons plus neutrons; isotopes of an element share the atomic number and differ in neutron count.
This item is settled by one sentence of the nuclear model: electrons revolve, nucleons do not. Any statement that has a proton or a neutron orbiting is wrong on its face. The other three statements are the standard properties of the neutron and are worth holding as a triple — nearly the proton's mass, no charge, inside the nucleus — because a 'not correct' question on the neutron will nearly always attack one of these three or invent an orbit. The neutron's neutrality also explains its practical importance, since an uncharged particle feels no electrostatic repulsion and can therefore be used to probe and split nuclei.
- Neutron mass is about 1.675 × 10⁻²⁷ kg and proton mass about 1.673 × 10⁻²⁷ kg, both close to one atomic mass unit and roughly 1,836 times the electron mass.
- The neutron carries no electric charge; protons carry the positive charge and electrons the negative.
- Protons and neutrons together are called nucleons and reside in the nucleus; electrons occupy the space around it.
- Atomic number is the proton count and determines the element; mass number is protons plus neutrons.
- The neutron was discovered by James Chadwick in 1932; Rutherford's gold-foil alpha-scattering experiment had established the nucleus itself.
- Reading 'revolve around the nucleus' as applying to any particle other than the electron.
- Assuming the neutron is much heavier than the proton because it is often listed second; the two masses are nearly equal.
- Confusing atomic number with mass number when counting neutrons.
As a 'not correct' statements item on subatomic particles, or as a numerical on protons, neutrons and electrons in a given atom.
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(b) protons and neutrons only
The consequence of the neutron sitting in the nucleus. Mass number counts protons and neutrons precisely because those are the two heavy particles held there, while the far lighter electrons outside contribute almost nothing.
Which of the following particles are subatomic particles? 1. Electron 2. Proton 3. Neutron 4. Muon Select the correct answer using the code given below.
- (a) 1 and 4 only
- (b) 1, 2, 3 and 4
- (c) 2 and 3 only
- (d) 1, 2 and 3 only
Answer(b) 1, 2, 3 and 4
The same cast of particles, sorted rather than described. Knowing that the electron sits outside the nucleus while the proton and neutron sit inside it is the structural fact underneath both questions.
- practice — not a real PYQ
The mass number of an atom is equal to the number of
- (a)protons only
- (b)protons and neutrons
- (c)protons and electrons
- (d)neutrons only
Answer(b) protons and neutrons — together called nucleons; the number of protons alone is the atomic number.
- practice — not a real PYQ
The neutron was discovered by
- (a)J. J. Thomson
- (b)Ernest Rutherford
- (c)James Chadwick
- (d)Niels Bohr
Answer(c) James Chadwick, in 1932 — Thomson found the electron and Rutherford established the nucleus.