Because electrons in an atom can only have certain fixed energies, the electron levels are said to be

Because the electron can only exist in certain orbits, and therefore can have only certain energies, then the electron levels or the energy of the electron are said to be Quantized. From Bohr's model each possible electron in his model has fixed energy. The fixed energies an electron can have are called energy levels. The fixed energy levels of electrons are like rungs of ladder, such that the lowest rung of the ladder corresponds to the lowest energy level. A person ca climb up or down a ladder by going from rung to rung. An electron can similarly jump from one energy level to another but they can not be between energy levels. 

All the elements are arranged by melting points on the periodic table. in row they go from metals to gasses. in the row from sodium to argon, sodium is more reactive than the one on its right, magnesium.