Orbital energy is the energy required to remove an electron from an atom's outermost shell. The outermost shell of an atom is also called its valence shell. The valence shell contains electrons that are involved in chemical reactions, and the energy required to remove these electrons is a crucial factor in determining an atom's reactivity. The orbital energy of an atom decreases as its atomic number increases. This phenomenon is known as the Aufbau principle.



The Aufbau principle is a concept in chemistry that explains the order of electron filling in atoms. This principle states that electrons fill orbitals in order of increasing energy. The energy of an orbital is determined by its distance from the nucleus and the number of electrons it contains. The closer an electron is to the nucleus, the lower its energy. The more electrons an orbital contains, the higher its energy.



The shielding effect is another factor that contributes to the decrease in orbital energy with an increase in atomic number. The shielding effect occurs when the inner electrons in an atom shield the outer electrons from the full charge of the nucleus. The more inner electrons an atom has, the more shielding it provides to its outer electrons. As a result, the outer electrons are less strongly attracted to the nucleus, and their orbital energy is lower.



The nuclear charge is the positive charge of the nucleus that attracts electrons towards it. As the atomic number increases, the nuclear charge also increases. This increase in nuclear charge should lead to an increase in orbital energy. However, the shielding effect of the inner electrons is stronger than the attraction of the nucleus. As a result, the outer electrons are less strongly attracted to the nucleus, and their orbital energy decreases.



In conclusion, the orbital energy of an atom decreases as its atomic number increases. This phenomenon is due to the Aufbau principle, the shielding effect, and the nuclear charge. The Aufbau principle dictates the order of electron filling in atoms, while the shielding effect and the nuclear charge contribute to the decrease in orbital energy. Understanding the factors that determine an atom's orbital energy is crucial in understanding its reactivity and chemical behavior.