Imagine counting grains of sand. Not easy but not impossible. Now imagine trying to count grains of sand that are 2,000 miles away. Quite impossible even with the worlds best telescope. Well this is the challenge in counting atoms. Counting atoms from 1 foot away is the same as counting grains of sand 2,000 miles away.
Chemical Equations are simple. 2H 2 + O 2 2H 2 O Chemical equations are quite easy. Here we count out 4 hydrogens and 2 oxygen atoms and we can make two water molecules. But how many hydrogens and oxygens would we count out to make enough water to fill a one liter bottle of water?
Imagine the number of grains of sand in a handful of sand: thousands or maybe a million? It would be difficult to count that many. Now imagine the number of grains of sand that would be present if we covered all of the Earth (land and sea) 10 miles deep of sand. That number of grains of sands is the same number of water molecules needed to fill a 1 liter bottle.