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Structure of Atom Summary
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- Atoms are made up of three fundamental particles: electrons, protons and neutrons.
- Discharge tube experiments by William Crookes and J.J. Thomson led to the discovery of electron.
- Goldstein's experiment with the discharge tube having a perforated cathode led to the discovery of anode rays.
- Ionisation of gases in the discharge tube is carried out at low pressure.
- The existence of the nucleus in the atom was established by Rutherford by his alpha-particle scattering experiment.
- The neutron was discovered by James Chadwick.
- The nucleus contains protons and neutrons. It is positively charged.
- Ordinary hydrogen is the only element that has no neutron in it.
- According to Bohr electrons revolve around the nucleus in fixed orbit called K, L, M, N.
- The number of electrons in each orbit can be calculated by the formula 2n2.
- The number of protons is the atomic number of an element. It is abbreviated Z.
- Mass number is defined as the sum of the number of protons and neutrons in the nucleus of the atom. It is denoted by 'A'.
- Atoms of the same element having same atomic number but different mass numbers are called isotopes.
- Isotopes show similar chemical properties but the physical properties are different due to the difference in the number of neutrons.
- Valency is the combining capacity of an element.
- X-rays are electromagnetic waves of very short wavelength. They are produced when fast moving cathode rays are stopped suddenly by some obstruction like tungsten metal.
- Radioactivity is the spontaneous emission of invisible high energy radiations from certain elements especially heavy ones.
- The radioactive emissions are alpha, beta and gamma rays.
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