Why does atomic radius increase across a period

Nuclear charge dominates whilst adding electrons to the same shell. The net result is atomic contraction across a period. This is a very important periodic phenomenon: the contraction of atomic radii across the period.

While as we add to Z the number of protons in the nucleus , we also add another electron and charge is therefore kept neutral , the increased nuclear charge acts disproportionately on the valence electrons , and contracts this shell.

Atomic radius decreases across a period because valence electrons are being added to the same energy level at the same time the nucleus is increasing in protons. The increase in nuclear charge attracts the electrons more strongly, pulling them closer to the nucleus. The following diagram shows the periodic trends of atomic radius of the representative elements main group elements for the first six periods.

Why does atomic radius decrease as you go across a period? The atomic radius r of an atom can be defined as one half the distance d between two nuclei in a diatomic molecule. Atomic radii have been measured for elements.

As an example, the internuclear distance between the two hydrogen atoms in an H 2 molecule is measured to be 74 pm. Figure 2. Atomic radii of the representative elements measured in picometers.

The atomic radius of atoms generally decreases from left to right across a period. There are some small exceptions, such as the oxygen radius being slightly greater than the nitrogen radius. Within a period, protons are added to the nucleus as electrons are being added to the same principal energy level.

These electrons are gradually pulled closer to the nucleus because of its increased positive charge. Since the force of attraction between nuclei and electrons increases, the size of the atoms decreases. The atomic radius of atoms generally increases from top to bottom within a group. As the atomic number increases down a group, there is again an increase in the positive nuclear charge. However, there is also an increase in the number of occupied principle energy levels. Higher principal energy levels consist of orbitals which are larger in size than the orbitals from lower energy levels.

The effect of the greater number of principal energy levels outweighs the increase in nuclear charge and so atomic radius increases down a group. Figure 3. Active 1 year, 1 month ago.

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