SSC
Which positively charged particle has a mass equal to that of an electron?
Answer (Detailed Solution Below)View answer ▾Hide ▴
Answer: Option A — Positron
Detailed Solution
The positron is the antiparticle of the electron. It has exactly the same mass as an electron, but its charge is positive (+1) instead of negative. It is written as e+ or β+.
The existence of the positron was predicted by the British physicist Paul Dirac in 1928 from his equation for the electron, which combined quantum mechanics with relativity. In 1932, the American physicist Carl D. Anderson discovered positrons in cosmic rays using a cloud chamber. Anderson received the Nobel Prize in Physics in 1936 for this discovery. The positron was the first antimatter particle to be found.
When a positron meets an electron, the two annihilate each other. Their mass is converted completely into energy, usually in the form of two gamma-ray photons that travel in opposite directions. This idea is used in medicine in PET scans (Positron Emission Tomography). A patient is given a substance containing a radioactive isotope that emits positrons. The gamma rays produced when these positrons meet electrons in the body are detected to form images of the working of organs, such as the brain and the heart, and to detect cancer.
Positrons are also produced in beta-plus decay, in which a proton inside a nucleus changes into a neutron and emits a positron and a neutrino.
Why not the other options?
- ✗The proton is positively charged, but its mass is about 1,836 times the mass of an electron.
- ✗The alpha particle is a helium nucleus (two protons and two neutrons) with a charge of +2; it is much heavier than an electron.
- ✗The neutron has no charge, and its mass is slightly more than that of a proton.
Quick tip
Remember: Positron = antiparticle of the electron, same mass but positive charge, predicted by Dirac (1928), discovered by Carl Anderson (1932), used in PET scans.
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Atomic Structure — Preparation
This question tests your understanding of Atomic Structure, a key topic in the General Knowledge section. The detailed solution above uses a step-by-step approach to show the reasoning, and where available, a shortcut that helps you solve similar questions in under 90 seconds during the actual exam.
This topic covers the structure of the atom: the discovery of subatomic particles, the atomic models of Dalton, Thomson, Rutherford and Bohr, atomic number and mass number, isotopes, isobars and isotones, electronic configuration and valency. Questions commonly ask which scientist proposed which model, what Rutherford's gold foil experiment showed (a small, dense, positively charged nucleus), and how isotopes differ from isobars. For example, isotopes have the same atomic number but different mass numbers. Practise working out the number of protons, neutrons and electrons from given numbers. Practise the atomic structure questions below, written in the style of the general awareness sections of exams such as SSC, banking, railways, CUET GAT, MAH CET and CLAT. Each question comes with a detailed explanation that adds related facts, so every question helps you revise more than one point.