Final answer:
Ionization energy is the energy required to remove an electron from an atom because of the electrostatic forces between the positively charged nucleus and the negatively charged electrons. The first ionization energy specifically refers to removing the first electron and increases for successive electrons removed. Ionization energies vary across the periodic table, influencing element reactivity.
Step-by-step explanation:
Ionization energy (IE) refers to the energy required to remove an electron from an isolated atom in the gas phase to form a cation. This is because the positively charged nucleus of an atom attracts and holds onto the negatively charged electrons through electrostatic forces. To overcome this attraction, energy must be inputted to the system, and this energy is what we call the ionization energy. The first ionization energy is specific to removing the first electron, and it increases for successive electrons due to the increased attraction between the now more positively charged nucleus and the remaining electrons.
Equation ahead describes the process for an atom A in its ground state: A(g) → A+ (g) + e¯, where energy required is equal to IE, and this is always a positive value, denoting that energy input is necessary. Typical units for ionization energies are kilojoules per mole (kJ/mol) or electron volts (eV).
Ionization energies tend to increase across a period from left to right on the periodic table and decrease down a group. This is due to the fact that atoms possess different levels of nuclear charge and shielding, affecting how tightly electrons are held. The pattern of ionization energy helps explain the reactivity of elements and their tendency to form certain types of compounds based on their ability to lose electrons.