Question

Which of the following statements about effective nuclear charge for the outermost valence electron of an atom is incorrect? (i) The effective nuclear charge can be thought of as the true nuclear charge minus a screening constant due to the other electrons in the atom. (ii) Effective nuclear charge increases going left to right across a row of the periodic table. (iii) Valence electrons screen the nuclear charge more effectively than do core electrons. (iv) The effective nuclear charge shows a sudden decrease when we go from the end of one row to the beginning of the next row of the periodic table. (v) The change in effective nuclear charge going down a column of the periodic table is generally less than that going across a row of the periodic table.

Short answer

The incorrect statement is (iii): Valence electrons screen the nuclear charge more effectively than do core electrons. In reality, core electrons screen the nuclear charge more effectively than valence electrons, as they are closer to the nucleus and more effective at repelling valence electrons away from the nucleus.

Step-by-step solution

Statement (i)

The effective nuclear charge can be thought of as the true nuclear charge minus a screening constant due to the other electrons in the atom. This statement is correct. The effective nuclear charge is the net charge experienced by a valence electron after taking into account the "shielding" or "screening" effect of all other electrons present in the atom.

Statement (ii)

Effective nuclear charge increases going left to right across a row of the periodic table. This statement is correct. As we move from left to right across a period, the number of protons in the nucleus increases, and the number of core electrons remains constant. This results in an increased effective nuclear charge experienced by the valence electrons.

Statement (iii)

Valence electrons screen the nuclear charge more effectively than do core electrons. This statement is incorrect. In fact, core electrons screen the nuclear charge more effectively than valence electrons, as they are closer to the nucleus and are more effective at repelling valence electrons away from the nucleus.

Statement (iv)

The effective nuclear charge shows a sudden decrease when we go from the end of one row to the beginning of the next row of the periodic table. This statement is correct. As we move from the end of one row (noble gases) to the beginning of the next row (alkali metals) in the periodic table, the valence electrons occur in a new energy level and are further from the nucleus. This will result in lower effective nuclear charge experienced by those valence electrons even though there is an increase in the actual nuclear charge.

Statement (v)

The change in effective nuclear charge going down a column of the periodic table is generally less than that going across a row of the periodic table. This statement is correct. Going down a column, the number of core electrons increases due to the addition of a new energy level. This results in a greater shielding effect, but the actual nuclear charge also increases. Consequently, the overall change in effective nuclear charge going down a column is generally less pronounced compared to going across a row.