2. The radius of an atom is a measure of its size and is measured by the distance between the centers of two of the atoms bonded together and dividing this by 2.

5. Question: Why does the radius decrease as you go across a period?
Answer: The radius is dependent on the position of the electrons; if the electrons are pulled closer to the nucleus, then the radius is smaller.
As you go across a period, the nuclear charge of positive particles (protons) increases. This increase takes place in the very small concentrated space of the nucleus.
Even though the negative charge (due to additional electrons) also increases, the electrons are added in the very large volume of the electron cloud.
SO, the attractive influence of the added proton is much greater than the repulsive influence of the added electron and the result is that there is increased attraction between the nucleus and the electron cloud, making the atom smaller.

7. What happens to ionization energies as you: a) go down a group
What happens to ionization energies as you: a) go down a group? b) go across a period?

10. Question: why does the ionization energy get bigger as you go across the period?
Answer: the explanation is essentially the same as the one for why the atomic radius gets smaller. The amount of energy needed to pull an electron off of the atom (Ionization Energy) depends on how strongly the electron is being attracted to the nucleus. Adding a proton as you go across will increase the attraction between the nucleus and the electron, making it more difficult to remove, which translates to higher ionization energy.

11. Many other properties vary in a periodic fashion, too
Many other properties vary in a periodic fashion, too. Example: melting points

14. Hydrogen and helium were the first two elements in the universe.

15. The reaction of hydrogen with oxygen is used to propel the space shuttle.

16. The hydrogen in the balloon reacts with oxygen from the air

17. The alkali metals are very active: they react with water and oxygen
The alkali metals are very active: they react with water and oxygen. Lithium and water Potassium and water