Chemistry · Structure Of Atom · NEET
| Nature | Alpha: helium nucleus (2p + 2n) | Beta: fast electron |
| Charge | Alpha: +2 | Beta: -1 ; Gamma: 0 |
| Mass | Alpha: 4 u (heavy) | Beta: ~0 ; Gamma: 0 |
| Bending in field | Alpha: small bend to - plate | Beta: large bend to + plate ; Gamma: none |
| Effect on nucleus | Alpha: A -4, Z -2 | Beta: A same, Z +1 ; Gamma: no change |
An alpha ray (alpha particle) is a helium nucleus. It has 2 protons and 2 neutrons, so its mass number is 4 and its charge is +2. We write it as He-4 (2 protons, 2 neutrons) or the symbol 2-4-He. Because it has 2 protons and no electrons, it carries a +2 charge. It is heavy and moves slowly compared to beta and gamma.
A beta ray (beta-minus) is an electron, not a proton. It is written as 0/-1 e. Its mass is almost zero (about 1/1836 of a proton) and its charge is -1. It forms inside the nucleus when a neutron changes into a proton and throws out an electron. So the atomic number goes up by 1, but the mass number stays the same.
No. A gamma ray has zero mass and zero charge. It is not a particle at all. It is a high-energy electromagnetic wave (like light or X-rays but with even more energy). Because it has no charge, an electric field or magnetic field cannot bend it. It is emitted when a nucleus has extra energy after alpha or beta decay.
Beta rays bend the most because they are very light (tiny electrons). They bend toward the positive plate because they are negative. Alpha rays bend the opposite way, toward the negative plate, because they are positive, but they bend only a little since they are heavy. Gamma rays do not bend at all because they have no charge. This is a common NEET diagram question.
Tritium (H-3, hydrogen with 1 proton and 2 neutrons) emits a beta-minus particle. Inside it, one neutron turns into a proton and releases an electron, so tritium becomes He-3. The reaction is 1/3 H turns into 2/3 He plus 0/-1 e. This is why the answer is beta, not alpha or gamma. This matters for NEET because it tests whether you know each isotope's decay type.
Alpha particles are big and heavy, so they lose energy fast and cannot go through skin or even paper. Outside the body they are quite safe. But if you swallow or breathe in an alpha source, it releases all its energy inside your tissue and causes heavy damage. This ties into the next topic, penetrating power.
Tritium, a radioactive isotope of hydrogen, emits which of the following particles?
Try the real previous-year questions from this chapter — each with the answer and a full solution.
Ernest Rutherford named and studied alpha and beta rays around 1899, and Paul Villard discovered gamma rays in 1900. Rutherford showed they behave differently in electric and magnetic fields.
Pass them through an electric field. The ray that bends toward the negative plate is alpha (positive). The ray that bends the other way and by a large amount is beta (negative, light). The ray that goes straight is gamma (no charge).
Gamma rays carry the most energy per ray because they are high-frequency electromagnetic waves. But their energy does not come from mass or charge, so they pass through matter easily without pushing atoms aside.
Both are electrons, so they are the same type of particle. The difference is the source: a cathode ray electron comes from outside the nucleus (from an electrode), while a beta particle is created inside the nucleus during decay.
