For which of the following reactions are both reactants and products likely to be found when the reaction appears to be complete? how can you tell?
- mg + o2 --> 2mgo
- hf + h2o < --> h3o(+) +

1) elements in the same group have the same number of valence electrons.

for example halogens have 7 valence electrons.

group 17 (halogens) elements are: fluorine (f), chlorine (cl), bromine (br) and iodine (i). they are very reactive and easily form many compounds.

2) elements in the same period have the same number of electron shells.

for example all elements in period 2 have two electron shells.

3) metallic elements become less reactive as you move left to right in a period.

for example, magnesium (right) is less reactive than sodium (left).

4) metallic elements become more reactive as you move top to bottom in a group.

for example, potassium (down in group 1) is more reactive than sodium (up in group 1).