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.
