# Question Video: Selecting the Element with the Largest Atomic Radius Chemistry

Which of the following elements has the largest atomic radius across the periodic table? [A] Copper [B] Cesium [C] Lithium [D] Neon [E] Magnesium

04:24

### 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.