The number of effective nuclear charge (Zeff) and the number of valence electrons are two distinct concepts related to the electronic structure of atoms.
- Effective Nuclear Charge (Zeff): The effective nuclear charge refers to the net positive charge experienced by an electron in an atom. It takes into account both the actual nuclear charge (the number of protons in the nucleus) and the shielding effect of inner electrons. The shielding effect occurs because the inner electrons repel the outer electrons, reducing the net attraction felt by the outermost electrons towards the nucleus.
The effective nuclear charge can be approximated by subtracting the shielding constant from the actual nuclear charge. The shielding constant represents the average contribution of inner electrons in reducing the attraction felt by the outermost electrons. The higher the effective nuclear charge, the stronger the attraction experienced by the outer electrons towards the nucleus.
- Valence Electrons: Valence electrons are the electrons in the outermost energy level or shell of an atom. These electrons are primarily responsible for the chemical behavior of the atom, as they are involved in bonding and interactions with other atoms. The number of valence electrons determines the atom's ability to form chemical bonds and influences its reactivity and chemical properties.
The valence electrons are usually those in the highest energy level or the outermost occupied energy sublevel of an atom. For example, in the case of elements in the s and p blocks of the periodic table, the valence electrons are found in the s and p orbitals of the outermost energy level.
In summary, the effective nuclear charge represents the net positive charge experienced by an electron, taking into account the shielding effect of inner electrons. On the other hand, the number of valence electrons refers to the electrons in the outermost energy level or shell of an atom, which play a crucial role in determining the atom's chemical behavior and reactivity.