Chemistry · Structure Of Atom · NEET
Electron: J.J. Thomson (1897) from cathode ray tube experiments. Proton: E. Goldstein (1886) first saw the rays (anode/canal rays), and the positive particle in hydrogen was later named the proton. Neutron: James Chadwick (1932). For NEET, remember Thomson = electron, Goldstein = proton (anode ray), Chadwick = neutron.
Cathode rays travel from the cathode (negative plate) to the anode and are made of electrons. They are always the same no matter which gas is inside the tube. Anode rays (canal rays) travel from the anode side toward the cathode and are made of positive ions. Anode rays DO depend on the gas used, because they come from the gas atoms losing electrons.
Because cathode rays are made only of electrons, and every electron is identical. The electron does not come from the gas atoms in a way that changes it. So e/m is a constant (1.758 x 10^11 C/kg) for any gas or any electrode metal. This is the key proof that the electron is a fundamental particle present in all matter. Anode rays are different: their e/m changes with the gas because they are gas ions.
R.A. Millikan measured the charge of a single electron using his famous oil drop experiment. He found the charge e = 1.602 x 10^-19 coulomb (negative). Thomson had only found the e/m ratio; once Millikan gave the charge, the mass of the electron could be calculated (about 9.1 x 10^-31 kg).
The neutron has no electric charge, so it is not bent by electric or magnetic fields and leaves no easy track. That made it very hard to detect. Chadwick found it in 1932 by hitting beryllium with alpha particles, which knocked out a neutral particle with mass almost equal to the proton. For NEET, remember: neutron is neutral and its mass is slightly more than a proton.
Electron: charge -1 (1.602 x 10^-19 C), mass about 9.1 x 10^-31 kg (very light). Proton: charge +1, mass about 1.672 x 10^-27 kg. Neutron: charge 0, mass about 1.674 x 10^-27 kg (a bit heavier than a proton). A proton is roughly 1836 times heavier than an electron.
The number of protons, neutrons and electrons in Lu-175 (atomic number 71), respectively, are:
Try the real previous-year questions from this chapter — each with the answer and a full solution.
Yes. The electron is about 1836 times lighter than the proton. The electron mass is about 9.1 x 10^-31 kg, while the proton mass is about 1.672 x 10^-27 kg. Almost all the mass of an atom is in the nucleus (protons and neutrons).
Electron has a negative charge (-1.602 x 10^-19 C). Proton has an equal positive charge (+1.602 x 10^-19 C). Neutron has no charge (it is neutral). Because proton and electron charges are equal and opposite, a neutral atom has equal numbers of both.
Thomson discovered the electron and measured its charge-to-mass ratio (e/m). Millikan later measured the charge (e) of a single electron with the oil drop experiment. Together, e and e/m gave the mass of the electron.
Atoms of the same element always have the same number of protons but can have different numbers of neutrons. These are called isotopes. So the neutron is what makes isotopes of an element differ in mass. This links directly to atomic number and mass number, the next topic.
