How many more electrons can fit within the valence shell of a oxygen atom?

How have the greenhouse gasses increased from the year 2000 to 2018

Ants live on acacia trees in south america. the ants feed on sugars secreted by the trees. the trees provide room for the ants to live. the ants sting any other insect or animal that comes to eat the trees. what type of relationship is this?

What methods could you use to solubilize calcium carbonate