The ionization energy of an atom is the amount of energy it takes to remove an electron from the atom. The process of removing an electron leaves the atom with an extra proton, relative to the number of electrons, and thus creates a positively charged ion, known as a cation. The ionization energy can be thought of as a measure of how strongly an atom holds on to its electrons. In general, ionization energies increase from left to right across a period (though there are exceptions) due to an increasing number of protons to attract electrons in the valence shell. Ionization energies decrease going down a group in the periodic table, due to the valence electrons being farther from the nucleus, and thus more shielded from its positive charge. Note that the trends in atomic radii and ionization energy go in the same direction—larger atoms tend to have lower ionization energies.