Video Transcript
Which of the following physical
characteristics of period four transition metals remains essentially constant from
chromium to copper? (A) Melting point, (B) boiling
point, (C) density, (D) electrical conductivity, (E) atomic radius.
The period four transition metals
from chromium to copper include manganese, iron, cobalt, and nickel. The melting and boiling point of a
given metal are determined by the strength of the forces of attraction between the
atoms of that metal. The strength of these attractions
arises from the metallic bonding between atoms and depends on the number of free
delocalized electrons. Since the number of free valence
electrons differs as we move from chromium to copper, the melting points also
differ.
Although we’re not showing it here,
the graph of the boiling points of these six transition metals is similar. There is not a clear overall trend
between the melting and boiling points and the atomic numbers of these elements. Because these two properties do not
remain constant from chromium to copper, we can eliminate answer choices (A) and
(B).
The amount of free, delocalized
electrons impacts the electrical conductivity of a metal. The valence electrons in period
four transition metal atoms are found in the 3d and 4s subshells. Because these six transition metals
have different numbers of valence electrons and different electronic configurations,
we would also expect their electrical conductivities to be different. Since the electrical conductivity
does not remain constant from chromium to copper, we can eliminate answer choice
(D).
Now let’s discuss density. We can see in the provided graph
that density increases from chromium to copper. Density is related to the mass of
the atoms and the volume they occupy. The atoms of copper are more
massive and smaller in size than those of chromium. So copper is more dense than
chromium. Because density does not remain
constant, we can eliminate answer choice (C).
Unlike all the other properties
we’ve discussed so far, atomic radius is essentially constant for the metals
chromium through copper. However, if we started with the
first transition metal of period four, which is scandium, we’d see that the atomic
radius decreases at first and then remains constant after chromium. As we move across the transition
metals in period four from left to right, the number of protons increases, which
increases the effective nuclear charge of the atoms.
The number of valence electrons in
the 3d orbitals also increases. But because the number of protons
is increasing at the same time, the valence electrons are pulled closer toward the
nucleus, making the atomic radius smaller. However, from chromium through
copper, the electrons added to the 3d orbitals increase repulsions and also provide
greater shielding to the 4s electrons. Because the 4s electrons are not
pulled in closer to the nucleus as we might expect, the atomic radius remains
essentially constant from chromium to copper.
In conclusion, the physical
characteristic of period four transition metals that remains essentially constant
from chromium to copper is atomic radius, or answer choice (E).
