why the atomic radius decrease within a period when moving from left to right in a Periodic Table?

Answer 1

As you move from left to right across a period, the number of protons increases. They pull harder on the electrons and drag them in towards the nucleus. So, Potassium K has just 19 protons, but Bromine Br at the end of that period has 35 - almost twice as many, so the outer electrons are pulled harder and shrink in towards the nucleus....

Answer 2

Moving left to right across a period, atomic radius usually decreases. This occurs because each successive element has an added proton and electron which causes the electron to be drawn closer to the nucleus. This decrease in atomic radius also causes the ionization energy to increase when moving from left to right across a period. The more tightly bound an element is, the more energy is required to remove an electron. Similarly, electronegativity will increase in the same manner as ionization energy because of the amount of pull that is exerted on the electrons by the nucleus. Electron affinity also shows a slight trend across a period. Metals (left side of a period) generally have a lower electron affinity than nonmetals (right side of a period) with the exception of the noble gases.

Answer 3

This trend is caused by increasing positive charge in the nucleus and the fact that the principal energy level within a period remains the same.