The elements on the periodic table can occur in nature in one of two ways: as free elements, or in compounds. A compound is a chemical combination of the atoms of one element with those of another element, or elements. Elemental calcium is pure calcium, while calcium carbonate is an example of a compound of calcium and other elements.
Elements and Compounds
There are many examples in nature of pure elements. The oxygen you breathe is elemental, as is the nitrogen that comprises the bulk of Earth's atmosphere. Many metals occur as elements; copper, silver, and gold are all pure elements. Most of what you see when you look around, however, is made up of compounds. The carbohydrates, proteins, and fats in the food you eat are all examples of compounds, as are table salt, baking soda and most other forms of matter around us.
Calcium is an element but one that's rarely encountered in pure elemental form. The reason for this is that calcium is actually quite reactive, and readily forms compounds. In fact, elemental calcium is a soft metal, though you've probably never seen a chunk of shiny, gray calcium. The reason calcium tends to react with other elements is that it has two "extra" electrons -- two more than would be stable -- and when it reacts with other elements, it can lose those extra electrons.
One of the most common forms of calcium found in nature is the compound calcium carbonate, or CaCO3. Calcium carbonate is an ionic compound, or salt, meaning that it consists of charged particles. Calcium occurs in this compound as a cation, a positively charged particle, while carbonate is an anion, or negatively charged particle. Because of their opposite charges, they're attracted to one another. Calcium carbonate is the primary constituent of limestone, marble and many aquatic shells.
Calcium In Diet
Aside from occurring in rocks and shells, calcium is also an important mineral you need to maintain health and wellness. Even though you've probably assumed that needing calcium means you need elemental calcium, your body doesn't use elemental, metallic calcium -- it uses calcium cation. Calcium carbonate is one common compound of calcium that you can take to provide for your calcium needs. Other compounds of calcium that provide the mineral include calcium citrate and calcium phosphate.