Video Transcript
Which of the following elements has
the largest atomic radius across the periodic table? (A) Copper, (B) cesium, (C)
lithium, (D) neon, or (E) magnesium.
Atomic radius is a quantity that
defines the size of an atom. The covalent atomic radius can be
determined from the length of a covalent bond between two atoms. But we don’t need to know the value
of the covalent atomic radius in order to answer this question. Instead, we can use the periodic
table to predict the relative atomic radius of atoms of each element. Let’s compare an atom of lithium
and an atom of neon, two elements that are found in the same period on the periodic
table.
An atom of lithium has three
protons and three electrons. The protons are found in the
nucleus of the lithium atom. Two electrons are found in the
first electron shell. And one electron is in the second
electron shell. An atom of neon has 10 protons and
10 electrons. The 10 protons are found in the
nucleus of the neon atom. Two electrons are found in the
first electron shell. And eight electrons are in the
second electron shell.
Looking at the two diagrams, we
might think that an atom of neon is larger than an atom of lithium because an atom
of neon has more electrons. We also see that both atoms have
the same number of electron shells. So we might think that the atoms
are the same size. But let’s consider the outermost
electrons. The outermost electrons in each
atom will experience a similar force of repulsion from the inner electrons. However, an outer electron of neon
experiences a stronger force of attraction to the 10-positive nucleus than an outer
electron of lithium, which is attracted to a three-positive nucleus. Thus, an atom of neon is smaller
than an atom of lithium because the outer electrons of neon are pulled closer to the
nucleus than the outer electrons of lithium. In general, we find that atomic
radius increases as we move from right to left across the periodic table.
Now, let’s compare an atom of
lithium to an atom of potassium, two elements that are found in the same group on
the periodic table. An atom of potassium has 19 protons
and 19 electrons. The 19 protons are found in the
nucleus of the potassium atom. Two electrons are found in the
first electron shell. Eight electrons are found in the
second and third electron shells. And one electron is in the fourth
electron shell.
Based on our previous reasoning, we
might assume that since the nuclear charge in a potassium atom is greater than the
nuclear charge in a lithium atom, that potassium atoms must be smaller. But a potassium atom has more
electron shells than a lithium atom. This means that the outermost
electron in a potassium atom is farther away from the nucleus and there are more
electrons in between the outermost electron and the nucleus.
The increased distance and the
electron shielding from the inner electrons outweighs the increase in nuclear
charge. So a potassium atom is larger than
a lithium atom. In general, we find that atomic
radius increases as we move down a group. We can now use the periodic table
to determine which of the elements listed has the largest atomic radius. We just need to find which of the
elements is the farthest to the left and down on the periodic table.
We can see that of the given answer
choices, cesium, chemical symbol CS, is the farthest to the left and down on the
periodic table. Thus, the answer choice that has
the largest atomic radius across the periodic table is (B) cesium.
