Final answer:
Potassium (K) is an example of an element whose valence electrons feel a smaller effective nuclear charge than the valence electrons of a calcium (Ca) atom due to potassium's lower number of protons in the nucleus and similar shielding effect.
Step-by-step explanation:
The concept of effective nuclear charge (Zeff) is important to understand why valence electrons in some elements feel a weaker nuclear pull compared to calcium (Ca). Effective nuclear charge essentially means the net positive charge experienced by valence electrons, after accounting for the shielding effect of the inner electrons. Calcium is a Group 2 element with a 4s2 valence electron configuration. A good example of an element whose valence electrons would feel a smaller effective nuclear charge than those of calcium would be potassium (K).
Potassium, being in the same period but belonging to Group 1, has one valence electron in its 4s orbital. Due to the lower nuclear charge (less protons in the nucleus) and similar shielding by inner electrons, potassium's single valence electron feels less pull from its nucleus compared to a calcium's two valence electrons. Moreover, when potassium forms a cation by losing its valence electron, the remaining electrons are still shielded by the same number of inner electrons, but now there is one less proton in the nucleus to exert a pull. Consequently, the valence electron in a potassium atom would experience a lower effective nuclear charge compared to the valence electrons in a calcium atom.