Question

What must happen for an electron to move to a higher energy level?

Answer 1

It must absorb energy.

Explanation:

According to Bohr's model; electrons revolve in shells and each has specific amount of energy. The lower shell has less energy and higher has greater energy. The electron in low shell also has low energy and when it move to higher shell it must absorb energy.

Answer 2

It must absorb energy.

Explanation:

An electron in the ground state, for example, can absorb energy in the form of a photon of light (carrying an energy `E=h*"frequency"`, where `h` is Plank's Constant) and jump to an higher level BUT the amount of energy it absorbs must exactly match the difference in energy of the gap between the two levels. It cannot, say, take a little bit less and stay in the middle!

Have a look what happens for Hydrogen:

The spectrum at the bottom tells us that white light (all colours=all energies) after passing through a cloud of hydrogen had some energies/colours removed (dark stripes at fixed values of wavelength such as `400nm` in the violet); these energies (corresponding to `E=h*"frequency"` for each specific colour) where absorbed by electrons of hydrogen and "used" to jump to higher levels!

Remember that frequency and wavelength are related through the speed of light `c` as:
`c="wavelength"*"frequency"`