It always takes energy to remove an electron from a neutral atom. Explain why, and name the minimum amount of energy it takes to do this.

Ionization energy is the energy required to remove the least tightly bound electron from a neutral atom in its gaseous state. In other words, it's the energy needed to transform a neutral atom into a positively charged ion, also called a cation. Electrons are attracted to the positively charged nucleus, and this attraction creates a potential energy, which keeps the atom in a stable state. To remove an electron, we need to overcome this attractive force by providing energy.

To understand why it always takes energy to remove an electron from a neutral atom, let's consider the attractive forces between the protons in the nucleus and the negatively charged electrons. Electrons in an atom are located in electron clouds, which are different regions around the nucleus with specific energy levels. They are attracted to the protons because of the opposite charges. When an external energy is applied to the atom, the electrons tend to gain that energy. If the energy supplied is greater than the attractive force between the electron and the nucleus, the electron can overcome that force and be removed from the atom. Essentially, the binding energy holding the electron close to the nucleus must be overcome. Therefore, removing an electron from a neutral atom always requires energy.

The minimum amount of energy needed to remove an electron from a neutral atom in its gaseous state is called the first ionization energy. This energy is usually expressed in units of electron volts (eV) or kilojoules per mole (kJ/mol). The first ionization energy varies for different elements in the periodic table and follows periodic trends, such as generally increasing across a period (row) and decreasing down a group (column).