To understand the arrangement of electrons in atoms, Neils Bohor understood the spectra. When white light (eg. sunlight or light from an ordinary electric light bulb) is passed through a prism, the white light is broken up into many different colours. This called a continuous spectrum. But when Bohor carried out this experiment with a hydrogen discharge tube (a glass tube filed with hydrogen at low pressure through which an electric current is passed) instead of white light,  he found that instead of seeing a continuous spectrum, he saw a series of narrow lines. These lines are called line spectrum. Since the light emitted when an electric current is passed through hydrogen, the spectrum is more accurately called an emission line spectrum. Each elements produce a different line spectra unique to each other.

A line spectrum is a series of colored lines against a dark background.

Studying the spectra, scientists have also found that atoms can absorb light.The absorption spectrum consists of series of dark lines. These dark lines are exactly the same wavelengths as the coloured lines in the emission spectrum of the element.

 The absorption spectrum of an element is the spectrum that is observed after white light has been passed through the element.

Bohr’s model of the atom

Neils Bohr put forward a theory of the hydrogen atom. He stated that:

  1. Electrons revolve around the nucleus in fixed paths called orbits.
  2. Electrons in any one orbit have a fixed amount of energy values (energy levels). Associated with each energy level is an interger , n, called the principal quantum number.
  3. As long as an electron is in any one particular energy level, it doesn’t gain or lose energy.
  4. When an atom absorbs energy, electrons jump from a lower energy level to a higher energy level. Electrons are less stable in the higher energy levels and do not remain there for long.
  5. Energy is lost when an electron falls from a higher energy level to a lower energy level. Since an electron can only fall back to certains definite energy levels, only fixed amounts of light energy can be given off.

An energy level is a fixed energy value that an electron in an atom may have.