Different elements have different numbers of electrons at different distances from the center of the atom. One way to charge an atom is to change this distribution of electrons. Only a precise amount of energy can change the distribution of electrons for any given element. If an atom receives too much energy, the atom is not charged. If an atom receives too little energy, the atom is not charged. The atom is only affected when it receives exactly the right amount of energy. This amount depends on the type of element; every element requires different amounts of energy to get charged, because every element has different numbers and distributions of electrons.
Once the atom is charged, it will eventually become uncharged. Since only a very specific amount of energy can cause a particular atom to become charged, when the atom becomes uncharged and releases energy, only a very specific amount of energy is released. Exactly as much energy is released as initially went into the atom.
Atoms can be charged in different ways, meaning electromagnetic radiation can be produced in different ways.
Background Information for Activity #2: Spectroscopy: Electromagnetic Radiation | Electromagnetic Frequency | Light | Generating Electromagnetic Radiation | Sources of Electromagnetic Radiation | Reflection and Absorption | Atomic Spectroscopy | Molecular Spectrscopy