Zeff Periodic Trend Across The Period And Down The Group

Z_eff, effective nuclear charge, is a periodic trend that measures the net positive charge experienced by an electron in the outermost shell. It determines the chemical behavior and reactivity of the element.

Z_eff is sometimes mistaken for the nuclear charge of an element. But it is not. Although both properties measure attraction charges, they are different. Z_eff considers the attraction between electrons and the nucleus, and nuclear charge measures only the charge in the nucleus.

There is a lot more about the effective nuclear charge that you should understand beyond this difference. Understanding the periodic trend of Z_eff provides information on other periodic trends like ionization energy and electronegativity.

What is Z_eff?

Electrons in an atom are arranged in shells, and each shell has an energy level. All electrons revolve around the nucleus but experience different levels of pull from it.

Electrons in higher energy levels will experience less pull from the nucleus, and those in lower energy levels will experience more pull.

The amount of nuclear charge on an electron in a multi-electron atom is the effective nuclear charge, Z_eff, of that electron.

The Z_eff of an element depends on the nuclear charge and electron shielding or screening effect or shielding constant. It can be determined by the formula, Z_eff = Z – S. Z is the nuclear charge and S is the shielding effect or shielding constant. 

Z_eff also depends on the number of electron shells in the atom in consideration. The energy each of the outer electrons feels after the screening effect of the inner electrons is the effective nuclear charge.

What is the periodic trend for Z_eff across the period?

Z_eff increases as you move from left to right across the period. Across the period, the number of electrons increases, but the number of electron shells remains the same. There is a constant amount of shielding electrons but an increasing number of protons (because of the increasing atomic number).

Therefore, the distance remains the same, but the nuclear charge increases as the valence electrons are more drawn to the nucleus.

What is the periodic trend for Z_eff down the group?

As you go down the group, Z_eff decreases. Down the group, there are more electrons and more shells too. Since the number of shells increases, the shielding effect also increases.

The screening effect will naturally reduce the nuclear charge on the valence electrons, resulting in decreased effective nuclear charge.

In addition, as more electrons are added to new orbitals, atomic size will also increase, and the effective nuclear charge will decrease.

What is the relationship between the trend for Z_eff and electron affinity?

Z_eff and electron affinity are directly proportional to each other. The higher the Z_eff, the greater the electron affinity. Electron affinity is the amount of energy released when an electron is added to a neutral atom to form an anion.

The more electrons that are added to the nucleus, the shorter the distance and the stronger the attraction between the electrons and the nucleus.

What is the relationship between Z_eff and ionization energy?

The greater the effective nuclear charge, the higher the ionization energy. A higher Z_eff implies that screening is constant, the number of electron shells remains constant, and nuclear charge increases.

A higher nuclear charge means that the electrons are tightly bound to the nucleus, and it will require more energy to remove one electron from the atom of an element.

This energy needed to remove the most loosely bound electron of an isolated atom, a molecule, or a positive ion is the ionization energy. A higher Z_eff means a higher energy since all electrons will be tightly bound to the nucleus.

FAQs

Is there a difference between the Z_eff and the shielding effect?

Z_eff is the nuclear charge the nucleus has on an electron in an atom. The shielding effect or electron shielding is a reduction in the attraction force between electrons and the nucleus in an atom.

The shielding effect of an atom reduces the effective nuclear charge because it is reduced from the overall nuclear charge.

What is the difference between Z_eff and nuclear charge?

Zeff is the positive charge the nucleus exerts on the outermost electron shell, while the nuclear charge is the electrostatic force that binds the electrons to the nucleus.

The effective nuclear charge is also the total charge on the nucleus and the inner electrons, while the nuclear charge is the total positive charge on the protons in the nucleus of an atom. In addition, Z_eff is usually lower than the nuclear charge.

Does Z_eff increase or decrease with atomic radius?

A higher Z_eff means a smaller atomic radius. The higher the Z_eff, the greater the attraction between the electron cloud and the nucleus. Therefore, the smaller the atomic radius.

Conclusion

Z_eff, effective nuclear charge, increases across a period and decreases down the group. The higher the atomic number, the higher the Z_eff. But, down the group, the number of shells and shielding electrons increase, resulting in decreasing Z_eff.

Periodic trends have a relationship with each other. Z_eff increases across the period with increasing ionization energies and electron affinity. But, down the group, Z_eff decreases with increasing atomic radius.

Learn how to find the Z_eff of an electron using Slater’s rule.

Thanks for reading.