What is exited state of an element ?
What is ground state of an element ?
We have electrons orbiting the nucleus in the centre. The more energy an electron has, the further it is from the nucleus.
Let’s take a Lithium atom as a simple example.
When no energy is supplied to this atom, you have an electron configuration of 2,1 . The atom now has the lowest possible energy which we call the ground state.
Now a specific form of energy, let’s say, heat, is supplied to the atom. One of the electrons in the first shell absorbs that energy, and jumps to the second shell. Now you have a Lithium atom of electron configuration of 1,2 . This process is called excitation. The resulting Lithium atom we now have is thought to be excited, and this state is call excited state.
However, atoms in excited state are not stable, so they tend to give that extra energy away in terms of photon. The amount of energy given out, in this case, equals to the energy difference between the first and second shell.
After that you get a Lithium atom in its ground state again!