One of the fundamental rules of the universe is that systems change to reduce their energy. A hot object in a cooler environment will cool down over time. A river will flow downhill. This principle is also true for subatomic particles. Protons and electrons arrange their positions to minimize their potential energy. The total energy of bonded atoms is always less than the total energy of the atoms outside of the bond. In chemical bonds, electrons interact with more protons than in isolated atoms. Electrostatic interactions lower the energy of a system.
The electrons of an atom are arranged in shells. Electrons in the innermost shells are attracted strongly to the nucleus of the atom. Electrons in the outermost shell of an atom are farther away from the nucleus. The electrons between the outermost electrons and the nucleus also cause a shielding effect, resulting in outermost electrons being attracted with a weaker force. Electrons in the outermost shell are not attracted to the nucleus as strongly as electrons in the inner shells because of the distance and the shielding effect. The outermost shell of an atom is called the valence shell. An electron in the outermost shell of an atom is a valence electron. In almost all cases, only valence electrons participate in bonding. Inner electrons, also known as core electrons, are not involved in bonding.Electronegativity, electron affinity, and ionization energy are all measures of how strongly an atom attracts its valence electrons. Electronegativity is the tendency of an atom to attract electrons toward itself when forming bonds. It can be used to predict the type of bond that will form between two atoms. Electronegativity generally increases from left to right across the periodic table. Noble gases are an exception to this trend because their outermost electron shell is full, which is a stable, low-energy arrangement. Electronegativity values for noble gases are typically not listed because they generally have such low chemical reactivity. Electronegativity generally decreases from top to bottom down the periodic table. This trend is a result of elements lower in a group having a larger atomic radius and thus an increased distance between the valence electrons and the nucleus, resulting in less electrostatic interaction.