Characteristics of sub-atomic particles
The particles constituting an atom are called sub–atomic particles or elementary particles. There are three sub–atomic particles –protons, electrons and neutrons. Their characteristic properties are given below.
Characteristic properties of the sub-atomic particles
Property Electron −1e0 Proton +1p0 Neutron 0n1 Mass The mass of an electron is about 1/1836 that of a hydrogen atom. Absolute mass of an electron is 9.1x10−28g = 9.1 x10−31 kg Relative mass of an electron is 0.00054859 u The mass of a proton is equal to that of a hydro- gen atom. Absolute mass of a proton is 1.672 x 10−24g = 1.672 x10−27 kg Relative mass of a proton is 1.007277 u The mass of a neutron is almost equal to that of a proton. Absolute mass of a neutron is 1.676 x 10−24 g = 1.676 x10−27 kg Relative mass of a neutron is 1.009665 u Charge An electron has unit negative charge. Absolute charge on an electron is - 1.602 x 10−19 C It carries a charge equal in magnitude but opposite in sign to that carried by an electron = 1.602x 10−19 C It carries no charge i.e. it is a neutral particle.
Extra-nuclear part electrons and their distribution
Bohr’s model suggested that there is a nucleus at the center of the atom and electrons revolve around the nucleus in selected stationary orbits. The plane of the orbit can change. Hence, it was concluded that ’atom is a three dimensional body’. The surface generated by the changing plane of the orbit is called a ’shell’.
The shells or orbits in which electrons revolve are given numbers. The shell nearest to the nucleus is given number 1. As we go away from the nucleus, the numbers go on increasing as 2,3,4,5,....... . These shells are also denoted by capital letters K,L,M,N,O ......respectively.
The electrons occupy shells or orbits following certain rules :
- The maximum number of electrons in any shell is 2n2 where n is the number of the shell. So the first, second, third and fourth shell can accommodate 2, 8, 18 and 32 electrons respectively.
- The outermost shell can accommodate maximum 8 electrons. Therefore, the last shell of any atom is also called an octet
- The maximum capacity of the first or K shell is 2 electrons and is maintained in all the elements. When K shell is the last shell, it is called duplet.