Question Video: Identifying Why the Atomic Radius Increases upon Descending a Group on the Periodic Table | Nagwa Question Video: Identifying Why the Atomic Radius Increases upon Descending a Group on the Periodic Table | Nagwa

Question Video: Identifying Why the Atomic Radius Increases upon Descending a Group on the Periodic Table Chemistry • Second Year of Secondary School

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Why does the atomic radius of atoms get bigger as you move down a group in the periodic table?

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Video Transcript

Why does the atomic radius of atoms get bigger as you move down a group in the periodic table? (A) Because the masses of the atoms increase. (B) Because the number of neutrons in the atoms increases. (C) Because the electronegativity of the atoms increases. (D) Because the ionization energy of the atoms increases. Or (E) because the energy levels of the atoms increase.

As we descend a group in the periodic table, the atomic number and mass number increase significantly, as does the atomic radius. However, going across a period also means an increase in both the atomic number and the mass number, but in this case the atomic radius decreases. Therefore, it is not a simple matter of more protons, neutrons, and electrons always leading to a larger atom.

The number of protons increases along a period. The protons attract the electrons towards the nucleus. The number of electrons increases as the number of protons increases. But the electrons fill the same energy level. So the effective nuclear charge is larger, which attracts the electrons to the nucleus and causes the atomic radius to decrease. The increase in atomic radius down a group is therefore due to fitting in the increasing numbers of electrons into different shells.

Along a period, the additional electrons go into the same energy level shell. This means no change in energy level for the extra electrons. Down a group, however, the extra electrons need an extra shell at a higher energy level. This is what makes the atomic radii increase down a group. So the atomic radius of atoms get bigger as you move down a group in the periodic table because the energy levels of the atoms increase, or answer choice (E).

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