Take a piece of regular white chalk, like for writing on a blackboard. Make sure the chalk is calcium carbonate like the kind in the picture and not made of plaster of paris. Drop the chalk in a cup of vinegar or lemon juice. Vinegar is made of water and acetic acid, a kind of hydrocarbon. It should fizz up as the oxygen in the acetic acid combines with the calcium carbonate. The bubbles are carbon dioxide that is formed when the oxygen from the acetic acid joins with the carbon from the calcium carbonate.
When it is done fizzing, you should see a layer of little particles of calcium acetate at the bottom of the cup. This is the calcium left over from the calcium carbonate, together with the hydrogen left over from the acetic acid.
Now try the same experiment with an egg. Eggshells are also made of calcium carbonate, so they should do the same thing. If you leave an egg in vinegar for two hours and then take it out, it will feel as if it were made of rubber, and you'll be able to see through it. Be careful so it doesn't break - the shell has dissolved. You may need to change the vinegar once or twice to get it to dissolve all of the shell.
To see that bones are also made of calcium carbonate, try dropping a scrubbed chicken bone in vinegar. How about somebody's baby tooth? Or a piece of marble?
By adding baking soda back to the calcium, you can make it back into calcium carbonate again. Baking soda is made of sodium bicarbonate - molecules of sodium combined with carbon. If you add baking soda and water to the dissolved calcium, the carbon from the baking soda will combine with the calcium to make calcium carbonate again. You should see white particles appear in the liquid.
To find out more about atoms, check out these books from Amazon.com or from your library: