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Why does atomic number increase as atomic radius decreases?

Why does atomic number increase as atomic radius decreases? This is for chemistry class.

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Rizul N. | UNC-CH Grad For Math and Science TutorUNC-CH Grad For Math and Science Tutor
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Ahh a wonderful question. 

Remember that as atomic numbers increase, the number of protons increase as well in the nucleus. This concentration of positive charge in the middle of an atom makes all the electrons come closer to the nucleus. Note that opposite charges attract (electrons are negative and protons are positive). Thus, the radius of the atom decreases because the electron cloud is less spread out. 

As we go down the table, the number of shells increase. Think of a chocolate truffle. Instead of just 1 harder chocolate shell, as you go down the table, you could have 3 or more shells. thickness of these shells and the addition of orbitals that come with it makes the radius even larger.

Bonnie A. | The Physical Sciences ExpertThe Physical Sciences Expert
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The increase in atomic number causesf an increase in the strong positive charge of the nucleus.  Thus, the positive charge increases due to the number of positive protons increasing in the nucleus.

The strong positive charge of the nucleus has a strong pull on the negatively charged electron cloud.  This causes the atom's radius to decrease in size as it is pulled inward.  However, the atom will never close in on itself or implode.  The neutrons assist with this.

AS you across the periodic table the atomic number increases, but the atomic radius decreases.

Ray H. | Senior Scientist & Adjunct Professor of ChemistrySenior Scientist & Adjunct Professor of ...
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The atomic radius increases as one goes down a group in the periodic table because more electrons are around the atom and more neutons and protons are present. As one goes across a period the atomic radius decreases because more protons are in the atoms as one goes across the period and the electrons are in the same shell. Ray L. Hanson, PhD.