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Atom is an indivisible and tiny particle.According to RUTHERFORD, an atom mainly contains two parts.

1. Nucleus

2.Energy levels

1.  Nucleus:

  • The central part of the atom is known as nucleus.
  • It possesses net  positive charge.
  • Its radius is 10-8 cm.
  • It consists of two charged particles.They are 1) Postively charged particle called proton.2)Neutrally charged particle called neutron.

2.  Energy levels :

  • The levels which are arranged around the nucleus are known as energy levels.
  • The energy levels are also called as shells or orbits.
  • These levels or shells are denoted as  K-shell,L-shell,M-shell,N-shell and so on...
  • The maximum number of electrons in a shell is determined by 2n2

Maximum number of electrons present in first shell or K-shell = 2n2 =2*12 =2

Maximum number of electrons present in second shell or L-shell =2*22 =8

Maximum number of electrons present in third  shell or M-shell  =2 * 32 =18

Maximum number of electrons present in fourth shell or N-shell  =2* 42 =32


For example:

Lithium atomic number is 3.It consists of 3 protons and 3 neutons inside the nucleus of an atom.and 3 electons are present around the nucleus which is said to be energy levels.The maximum number of elecctrons present in lithium is Two electrons are present in K-shell and one electron is present in L-shell.

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