Railway Science (Chemistry-Structure of Atom)

(lightest and simplest element). Isotopes differ from one another in respect of the presence of neutrons. Carbon contains two stable isotopes: ¹²C and ¹³C and a third isotope ¹⁴C is a radioactive isotope with half life 5770 years and used for radiocarbon dating.

An atom has a central nucleus which consists of neutrons and protons. The exception is hydrogen, where there is only one proton in the nucleus. Electrons revolve around the nucleus. Neutrons have a neutral electric charge and have slightly more mass than positively charged protons. A proton is a small, positively charged particle; Electron is a negatively charged particle. The positive charge on a proton is equal in magnitude to the negative charge on an electron.

Rutherford atomic model features - There is a positively charged centre in an atom called the nucleus. Nearly all the mass of an atom resides in the nucleus, The size of the nucleus is very small compared to the size of the atom. Drawbacks - The revolution of the electron in a circular orbit is not expected to be stable. Any particle in a circular orbit would undergo acceleration. During acceleration, charged particles would radiate energy. Thus, the revolving electron would lose energy and finally fall into the nucleus.

Isotopes: Atoms with the same number of protons but different numbers of neutrons. They share almost the same chemical properties, but differ in mass and therefore in physical properties. There are stable isotopes, which do not emit radiation, and there are unstable isotopes, which do emit radiation. The difference in the masses of isotopes of an element is due to the different number of neutrons in their nuclei. Names: Protium ₁H¹ , Deuterium ₁H², Tritium ₁H³ . When the number of Neutrons exceeds the number of Protons in the nucleus then it becomes unstable.

Valency - The combining capacity of an atom or ion. It is determined by the number of valence electrons (the electrons in the outermost shell). The higher the valency, the stronger the bond. Electron configuration - An arrangement of electrons in various shells (K, L, M, N) , sub-shells and orbitals in an atom. Penultimate shell is the shell which is filled just before the valence shell.

Rutherford proposed that an atom is composed of empty space mostly with electrons orbiting in a set, predictable paths around fixed, positively charged nucleus. Dalton in 1808 stated that all matter was made up of small, indivisible particles known as ‘atoms’. J.J Thomson stated that an atom is electrically neutral. Niel Bohr's atomic model was successfully explaining the stability of the atom.

Bohr's model of the atom was proposed by Neils Bohr. Bohr explained that a nucleus (positively charged) is surrounded by negatively charged electrons. According to Bohr, Electron which is moving in an orbital does not lose or radiate energy. Bohr was awarded the Nobel Prize in Physics in 1922. The Bohr model of an atom came as a modification to Rutherford’s model of an atom.

It is equal to 6.022 × 10²³ mol⁻¹ and is expressed as the symbol Nₐ. Normality (N) is described as the number of gram or molar equivalents of solute present in one litre of solution. Mass number (A) is defined as the total number of protons and neutrons in an atom’s nucleus. Atomic number (Z) is defined as the total number of protons in an atom's nucleus.

A coulomb is the amount of electricity that a 1-ampere (A) current carries in one second (s). Electron - The lightest stable subatomic particle known till now. It carries a negative charge of 1.6021 × 10⁻¹⁹ coulomb, which is considered the basic unit of electric charge. Electron mass - 9.11 × 10⁻³¹kg.