Answer:
The least electronegative atom is (B) Rb, which is rubidium
Step-by-step explanation:
Electronegativity is a measure of an atom's ability to attract electrons towards itself when it forms a chemical bond. The electronegativity of an atom depends on several factors such as the number of protons in the nucleus, the distance between the nucleus and the valence electrons, and the shielding effect of inner electrons.
Rubidium has the lowest electronegativity of the four options because it has a larger atomic radius and a lower effective nuclear charge than the other atoms. The larger atomic radius of rubidium means that the valence electrons are farther away from the nucleus and are therefore less strongly attracted to it. Additionally, the lower effective nuclear charge of rubidium (i.e., the net positive charge experienced by valence electrons) makes it less attractive to incoming electrons.
In contrast, option C, F (Fluorine) has the highest electronegativity of all elements because it has a smaller atomic radius and a higher effective nuclear charge due to its high atomic number and number of protons in the nucleus. Calcium (option D) has a higher electronegativity than rubidium because it has a smaller atomic radius and a higher effective nuclear charge than rubidium. Silicon (option A) has a moderately high electronegativity due to its intermediate atomic radius and effective nuclear charge.