Valence electrons are easier to remove from an atom than core electrons because they are located in the outermost energy level of the atom. This means that they are furthest from the nucleus and therefore experience less of the nucleus's positive charge. The positively charged nucleus attracts the negatively charged electrons, and the valence electrons are farther away from this force of attraction than the inner-core electrons.

Additionally, valence electrons are involved in chemical bonding. When atoms form chemical bonds, they share or transfer valence electrons. This means that valence electrons are more loosely held to the atom than core electrons, which are not involved in chemical bonding.

The energy required to remove an electron from an atom is called the ionization energy. The ionization energy of an electron increases as the electron gets closer to the nucleus. This means that it takes more energy to remove a core electron than a valence electron.

In summary, valence electrons are easier to remove from an atom than core electrons because they are located further from the nucleus and are less involved in chemical bonding.