We now know that an atom is made of still smaller particles protons, neutrons and electrons. Protons and neutrons from the core of the atom called the nucleus. Protons have a positive electrical charge. Neutrons have no change at all.
Around the nucleus are much lighter atomic particles called electrons. Electrons have a negative electrical charge.
Electrons were discovered when electric currents were passed through tubes from which nearly all the air had been pumped out. The electrons flowed from a metal terminal called the cathode, to another one called the anode.
Scientists quickly realized that the glow was caused by rays of some kind raveling from the cathode, and called them ‘cathode rays’, They did not know at first whether they were waves of some kind, like light or whether they were particles. Sir William Crookes found evidence that cathode rays consisted of particles. He showed that when a magnet was placed near them, the rays were bent. Waves would not have behaved like this.
Crookes also showed that the rays could push round a light metal vane when it was placed in their path. This also helped to show that the rays were made of particles with weight.
In 1897 the question was settled by the famous scientist J.J. Thomson. He showed that the ‘rays’ could be bent by nearby electric changes and very strongly. This indicated that they were tiny negatively-charged particles. He called them electrons. Experiments showed that each electron was less than one thousandth of the mass of a hydrogen atom, the lightest atom known.
Further experiments with cathode-ray tube led to the discovery of the proton. An open cathode was placed in the middle of the tube, which contained a little hydrogen gas. As the steam of electrons moved from the cathode to the anode, it stripped electrons from the gas in the tube. The resulting particles were repelled from the anode. The particles were later identified as having a positive charge. These particles were called protons. The proton is about t1837 times heavier than electron. It was discovered by Goldstein.
Methods were found to calculate the mass of an atom and to calculate the number of electrons. It was discovered that the nucleus of an atom had a greater mass then could be accounted for by protons. This discovery led to the conclusion that there must be another particle in the nucleus. Moreover, this particle must be neither positive nor negative. This particle was called neutron. It was discovered by James Chadwick.
Then came the discovery of radioactive elements. These substances gave out three kind of radiation. The three types of radiation were named alpha, beta and gamma rays, after the first three letters of the Greek alphabet. Beta rays could penetrate thin sheets of metal. They were found to be ordinary electrons. Gamma rays were quickly found to be similar to the highly penetrating X-rays that had been discovered earlier. But alpha rays were quite new.