What is an  atom?

  • Atoms are the building blocks of universe.

  • They are very tiny particles.

  • Its very hard to imagine even its size.

  • The centre of atom is called nucleus.


Atom in detail

  • Atom is made of tiny particles called protons and neutrons.

  • Electrons circle around the nucleus in clouds, or shells, far from the nucleus.

  • When an atom is in balance, it has the same number of protons and electrons.

  • Atom can have a different number of neutrons.

  • Electrons stay in their shells because a special force holds them.

  • Protons and electrons of atom are attracted to each other.

  • We say protons have a positive charge (+) and the electrons have a negative charge (-).

The electrons near the nucleus are held tight to the atom while sometimes the ones further out are not. These electrons can be made to move out of their orbits.



Energy Levels of Atom

  • An atom consists of electrons orbiting around a nucleus. But, the electrons cannot choose any orbit they wish.

  • The orbits of atom are restricted to certain energies.

  • Electrons can jump from one energy level to another, but they can never have orbits with energies other than the allowed energy levels.




  • Consider a neutral hydrogen atom.

  • Its energy levels are given in the image shown below.

  • The x-axis shows the allowed energy levels of electrons in a hydrogen atom, numbered from 1 to 5. The y-axis shows each level's energy in electron volts (eV).

  • One electron volt is the energy that an electron gains when it travels through a potential difference of one volt (1 eV = 1.6 x 10-19 Joules).



  • Electrons in a hydrogen atom must be in one of the allowed energy levels.

  • If an electron is in the first energy level, it must have exactly -13.6 eV of energy. If it is in the second energy level, it must have -3.4 eV of energy.

  • An electron in a hydrogen atom cannot have -9 eV, -8 eV or any other value in between.

  • Let's say the electron in atom wants to jump from the first energy level, n = 1, to the second energy level n = 2.

  • The second energy level has higher energy than the first, so to move from n = 1 to n = 2, the electron needs to gain energy. It needs to gain (-3.4) - (-13.6) = 10.2 eV of energy to make it up to the second energy level.



  • The electron of an atom can gain the energy it needs by absorbing light.

  • If the electron in a atom jumps from the second energy level down to the first energy level, it must give off some energy by emitting light.

  • The atom absorbs or emits light in discrete packets called photons, and each photon has a definite energy.

  • Only a photon with an energy of exactly 10.2 eV can be absorbed or emitted when the electron jumps between the n = 1 and n = 2 energy levels.




  • The energy of the photon depends on its wavelength.

  • The wavelength, ? can be found by the following equation,

  ?=hc/E, where

  • E is the energy of the photon (in eV), h

  • is Planck's constant (4.14 x 10-15 eV s)

  • c is the speed of light (3 x 108 m/s)


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