ydinvaraus

Ydinvaraus, symbol Z, is the total positive electric charge of the atomic nucleus. It equals the number of protons in the nucleus and is measured in elementary charges. The nucleus carries a positive charge of Z e, but this charge is screened by the surrounding electrons, so the effective attraction felt by an electron is not simply Z e.

In atoms, Z sets the baseline strength of the Coulomb attraction between the nucleus and electrons. However, inner electrons partially shield outer electrons, leading to the concept of the effective nuclear charge, Z_eff. Z_eff is roughly Z minus a shielding term and is a useful way to understand periodic trends and atomic properties.

Trends in the periodic table show that across a period Z increases while shielding changes less dramatically, resulting in smaller atomic radii and higher ionization energies. Down a group, Z increases but shielding grows as well, so outer electrons experience a weaker net attraction and atomic radii tend to expand while ionization energies fall.

Nuclear charge is a fundamental parameter in atomic and quantum chemical calculations. It is not measured directly but inferred from the atomic number and other observables such as spectral lines and binding energies. In ions, Z remains the same as for the neutral atom, but the loss or gain of electrons changes the electrons’ effective nuclear charge and related properties.

Examples: hydrogen has Z = 1, carbon Z = 6, uranium Z = 92.