Question Video: Explaining the Difference in Electronegativity between Elements Chemistry

Why is the element potassium less electronegative than the element lithium? [A] Potassium has a greater number of protons and greater nuclear charge to attract the bonding electrons. [B] Lithium has less electrons than potassium. [C] Potassium does not follow the general trend of electronegativity. [D] Bonding electrons are further from the nucleus in an atom of potassium causing less attraction. [E] Electronegativity increases as you descend a group in the periodic table.

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Video Transcript

Why is the element potassium less electronegative than the element lithium? (A) Potassium has a greater number of protons and greater nuclear charge to attract the bonding electrons. (B) Lithium has less electrons than potassium. (C) Potassium does not follow the general trend of electronegativity. (D) Bonding electrons are further from the nucleus in atom of potassium causing less attraction. (E) Electronegativity increases as you descend a group in the periodic table.

In this question, we’re being asked to compare the electronegativity of two elements. Remember that electronegativity is the attraction that an atom has for the shared pair of electrons in a covalent bond. To answer this question, we’ll check the validity of each statement against the main factors that influence electronegativity. Electronegativity depends on the number of protons in the nucleus of an atom, sometimes referred to as the nuclear charge. It also depends on the number of shells occupied, which influences the size of an atom and also the amount of shielding or screening taking place in that atom.

As we move across a period in the periodic table, electronegativity values generally increase. This is due to increasing nuclear charge, the same number of shells being occupied, and therefore no additional screening or shielding. As we move down a group, electronegativity values decrease. Although there are more protons in the nucleus of these atoms, the number of shells being occupied is increasing. This increases the amount of shielding or screening. And this factor outweighs the increasing nuclear charge.

We’re comparing potassium, symbol K, with the element lithium, symbol Li. Both metals are located in group one of the periodic table. And since potassium is further down the group than lithium, the element potassium is certainly less electronegative than lithium. Statement (A) suggests that potassium has a greater number of protons and hence nuclear charge than lithium. This part of the statement is true. Lithium has three protons in its nucleus; potassium has 19. This statement could be a good argument to explain why potassium is more electronegative than lithium. This argument would be tempting because electronegativity does depend on the size of the nuclear charge. This statement does not provide any support, though, as to why potassium is less electronegative than lithium. It’s therefore not a correct answer.

Despite the greater nuclear charge in potassium, we find there’s more shielding in this atom, and this outweighs the factor of the increasing nuclear charge as far as electronegativity is concerned. Statement (B) suggests that lithium has less electrons than potassium. On the face of it, this statement is true. Lithium contains three electrons, and potassium contains 19. Simply stating how many electrons each atom has does not attempt to explain how many shells are occupied or the degree of shielding in each atom. There’s also no mention of the nuclear charge in each atom in this statement. And therefore, it doesn’t provide an explanation at all. Statement (B) is not the correct answer.

Statement (C) suggests that potassium doesn’t follow the general trend of electronegativity patterns in the periodic table. In fact, the elements in group one do follow the pattern for electronegativity. As we descend group one, electronegativity values steadily decrease. This pattern is not always observed throughout the transition elements, but that’s not what we’re dealing with here. Statement (C) is therefore not a correct answer.

Statement (D) suggests that the bonding electrons are further from the nucleus in an atom of potassium. This is a true statement so far as a potassium atom having more shells occupied is a larger atom. Since the valence electrons are further from the nucleus, there’ll be less attraction for them too. This will indeed reduce the electronegativity of the atom and it does form part of an explanation. Statement (D) could be the correct answer. Statement (E) offers a full statement. Electronegativity does not increase as you descend a group in the periodic table. Statement (E) does not offer an explanation either. So it’s not the correct answer. Statement (D) is the correct answer.

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