ACS Chemistry Practice Exam 2025 – The All-in-One Guide to Exam Success!

As one moves across a period from left to right in the periodic table, both ionization energy and electron affinity generally increase.

Ionization energy is the energy required to remove an electron from a neutral atom. As you move across a period, the number of protons in the nucleus increases, resulting in a stronger positive charge that attracts the electrons more strongly. This increased nuclear charge means that it requires more energy to remove an electron, thus causing the ionization energy to rise.

Electron affinity, which is the amount of energy released when an electron is added to a neutral atom, also tends to increase across a period. As the nuclear charge increases, the atom's ability to attract additional electrons strengthens, leading to a more negative electron affinity value (more energy is released when an electron is added).

These trends are a result of the changes in atomic structure and the effective nuclear charge felt by the electrons as one moves from left to right across a period, highlighting the general behavior of elements in the periodic table regarding these two properties.