Each energy level is like a step on a stair. Electrons move up or down energy levels like going up or down stairs.

Each energy level is like a step on a stair. Electrons move up or down energy levels like going up or down a step. If electrons gain enough energy, they can move up to a higher energy level or step.

Each energy level is like a step on a stair. Electrons move up or down energy levels like going up or down a step. If electrons gain enough energy, they can move up to a higher energy level or step. If electrons lose enough energy, they go down to a lower energy level or step.

 Inside each energy level, electrons can only travel in certain areas called orbitals.  Each orbital can only hold two electrons.  Each energy level has a fixed number of orbitals.

 Energy level 1 has 1 orbital, with 2 electrons.  Energy level 2 has 4 orbitals, with 2 electrons in each orbital or 8 total electrons.  Energy level 3 has 9 orbitals, with 2 electrons in each orbital or 18 total electrons.  Energy level 4 has 16 orbitals, with 2 electrons in each orbital or 32 total electrons.

 An atom is most stable when all electrons are in the lowest possible orbitals and energy levels. Protons are always pulling them back in. This is called the ground state.

 An atom is most stable when all electrons are in the lowest possible orbitals and energy levels. This is called the ground state.  When electrons gain energy and move up to higher orbitals or energy levels, the atom is in an excited state.

 An atom is most stable when all electrons are in the lowest possible orbitals and energy levels. This is called the ground state.  When electrons gain energy and move up to higher orbitals or energy levels, the atom is in an excited state.  When the electrons drop back down to ground state, this energy is released as light or photons.

 We can never know both the velocity and the exact location of an electron.

 Therefore we show the nucleus surrounded by an electron cloud, a model that shows all possible locations of electrons.

 We can never know both the velocity and the exact location of an electron.  Therefore we show the nucleus surrounded by an electron cloud, a model that shows all possible locations of electrons.  The actual configuration of electrons depends on both the available orbitals and the energy levels the electrons occupy.