An atom is very small. Its mass is between 10-21 and 10-23g. A row of 107 atoms (10,000,000 atoms) extends only 1.0 mm. We know that atoms contain many different subatomic particles such as electrons, protons, and neutrons, as well as mesons, neutrinos, and quarks.
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The atomic model used by chemists requires knowledge of only electrons, protons, and neutrons, so our discussion is limited to them.
An electron is a tiny particle with a mass of 9.108 X 10-28g and a negative charge. All neutral atoms contain electrons. The electron was discovered and its properties defined during the last quarter of the nineteenth century. The experiments that proved its existence were studies of the properties of matter in gas-discharge or cathode-ray tubes.
This apparatus consists of a glass tube sealed at both ends. Within the tube are two metal plates called electrodes, which are connected to an outside power supply. If the tube is full of air or some other gas, no current flows between the electrodes, regardless of how large a voltage is applied from the power source. If the tube has been partially evacuated before sealing (that is, almost all the gas has been pumped out of it), the application of a high voltage from the power source across the two electrodes gives rise to a glow inside the tube, and simultaneously, a current begins to flow between the electrodes. We need not discuss in detail the various experiments performed with this apparatus; we will only state the conclusions drawn from them. The current is carried by streams of tiny particles given off by the negative electrode, called the cathode. The positive electrode is called the anode. The tiny particles are called electrons.
In these experiments, the presence of these electrons and their properties did not change if the metal of the electrode was changed, nor were any changes observed in their properties when different gases were used in the tube. Eventually, the experimenters became convinced that all matter contains electrons.
Each electron carries a single, negative electric charge and has a mass of 9.108 X 10-28g. Because the mass of an atom is approximately 10-23g, the mass of an electron is negligible compared to that of an atom.