The chemical and physical properties of elements recur in a systematic fashion because they are arranged in increasing atomic number

The size of atom is measured by examining the distance between two nuclei of the same atom and dividing by two… that is because the “edge” of an electron cloud is difficult to determine. Going across a period the atomic radius decreases. In a period each atom has the same number of shells, but an increasing number of protons. The greater nuclear charge holds the electrons closer to the nucleus.

As you go down a group the number of shells increases and therefore the electrons are further away from the nucleus. Each period is larger than the period of atoms before it, so atomic radii increases as period number increases.

Which atom is larger? Al or Cl? Mg or Ba? Al, B, or C?

Compare an ion’s size to the size of its parent atom. Cations are smaller than the parent atom. Cations have lost electrons and therefore dropped an outer shell. Thus their size is smaller. Anions are larger than the parent atom. They gain electrons and so the nuclear charge is spread out over more electrons and each electron is held with less force, therefore the ion expands a little.

For instance, Calcium makes a 2+ charge. Calcium ion is smaller than calcium atom Phosphorus makes a 3- charge. Phosphorus ion is larger than phosphorus atom. Which is smaller? Potassium or its ion? Oxygen or its ion? Na, Na +, or Li?

Energy needed to remove the outer most electron from an atom Across a period: IE increases When atoms are smaller their electrons are closer to the nucleus. The closer they are to the nucleus the more difficult it is to remove them. Therefore, IE increases as atomic size decreases Down a Group IE decreases down a group because the atomic size is increasing. Electrons are further away from nucleus and easier to remove.

Examples: Which atom has the lowest ionization energy? Rb, As, Kr?

How much an atom “wants” an electron. Atoms need a full outer shell. EN is a measure of how much it wants an additional electron to complete its outer shell. Across a period: EN increases until the Noble Gases. Group 18. Group 18 elements have zero EN. Down a group: EN decreases

Examples: Which atom is the lowest EN? Al, P, or Ar?

Across a period: Metallic Properties decrease Down a Group Metallic Properties increase