Final answer:
Ionization energy is defined as the energy required to remove an electron from an atom in its gaseous state, always requiring a positive amount of energy and resulting in a positively charged ion.
Step-by-step explanation:
The correct phrase that describes ionization energy is the energy required to remove an electron from an atom. Ionization energy is a term used in chemistry to describe the amount of energy necessary to detach an electron from a neutral atom in its gaseous state. This process results in the formation of a cation, which is a positively charged ion. The ionization energy is always a positive value, indicating that energy must be input to remove the electron.
Moreover, the energy required for this reaction can be expressed as:
A(g) → A+(g) + e−
Where A represents an atom in its gaseous form, A+ is the resulting cation, and e− is the ejected electron. The ionization energies are often measured in units such as kilojoules per mole (kJ/mol) or electron volts (eV).
It is noteworthy that when multiple electrons are present in an atom, the energy required to remove successive electrons generally increases, leading to the concept of first, second, and higher order ionization energies.