If you had two quarters next to each other, one (weird) way to figure out their radius is by taking half the distance between the centers of the quarters

If the distance between the centers of two touching quarters is 1 inch..... then the radius of one quarter is half as much, 0.5 inches.

• The size of an individidual atom is hard to measure

• Instead scientists measure the distance between nuclei of atoms when theyre bonded to themselves and divide by 2 to get an atom's radius

• This is called the "bonding atomic radius", "atomic radius", or "covalent radius" or just "size"

• The sizes of atoms are really small and is measured in angstroms (Å) or picometers (pm)

• 1 Å = 10^-10 m = 0.000 000 000 01 m

• 1 Å = 100 pm (1 pm = 10^-12 m)

• can be looked up on a chart (check your reference paper)

**a meter and a yard are about the same length***

• Imagine that the distance between the center of two touching coins can be determined by adding together their radii

• Similarly you can predict the length of a bond between two atoms by adding their atomic radii

• Similarly you can predict the length of a bond between two atoms by adding their atomic radii

• As you move right across a period, electrons are added to the same energy level, and protons are added to the nucleus.

• added protons make more positive charge that pulls more strongly on the electrons and makes the atoms smaller

• Going to the right generally makes atoms smaller

• As you move down a group, electrons are added to higher energy levels / shells. Atoms get bigger.

• Going down generally makes atoms bigger

• Atoms get bigger to the left across a period

• Atoms get bigger downward across a group

• Atoms get smaller to the right across a group and

• bigger down a period

• ***there are exceptions but I wont test you on them