The atomic size in a period from left to right.

As you move from left to right across a period in the periodic table, the atomic size decreases. Here's why:

1. Addition of Protons: As we move from left to right, we are adding more protons to the nucleus of the atoms. This increases the positive charge of the nucleus which attracts the electrons in the outer shells more strongly.

2. Same Energy Level: The electrons being added as we move from left to right in a period are in the same energy level. This means they are roughly the same distance from the nucleus and are not adding much shielding effect.

3. Increased Nuclear Charge: Because of the increased number of protons, the nuclear charge is greater. This pulls the electron cloud closer to the nucleus, making the atomic size smaller.

4. No New Energy Levels: No new energy levels are being added as we move from left to right in a period. This means the outermost electrons are not getting further from the nucleus, which would increase atomic size.

So, in summary, the atomic size decreases from left to right in a period due to an increase in the number of protons and no new energy levels being added.

As you move from left to right across a period in the periodic table, the atomic size decreases. Here's why:

1. Addition of Protons: As you move from left to right, each element has one more proton than the last. This increases the positive charge of the nucleus which attracts the electrons in the electron cloud closer to the nucleus, decreasing the atomic size.

2. Constant Energy Level: The electrons are added to the same energy level. This means that they are roughly the same distance from the nucleus, so the increase in nuclear charge has a greater effect on the atomic size than the addition of electrons.

3. Poor Shielding Effect: The shielding effect remains constant across a period. The inner electrons do not increase as you move across a period, so they do not provide additional shielding for the outer electrons from the increased nuclear charge. This means the outer electrons are pulled closer to the nucleus, decreasing the atomic size.

So, in summary, the atomic size decreases from left to right across a period due to the increase in nuclear charge which pulls the electrons closer to the nucleus.