Calcium
Atomic number 20 means that every Calcium nucleus contains 20 protons. That proton count is the element's identity: change it and you no longer have Calcium.
Start with the nucleus
Atomic number 20 means that every Calcium nucleus contains 20 protons. That proton count is the element's identity: change it and you no longer have Calcium.
Calcium (Ca) is chemical element 20. It belongs to the alkaline-earth metals, and its reference state at room temperature is a solid. Those basic facts are a useful starting point for its chemistry and applications.
In practice, it turns up most often in bones and teeth, cement and lime and biology.
See the atom
Conceptual visual: electron shells are a learning aid, not a literal picture of quantum orbitals.
What this number means
Atomic number 20 means that every Calcium nucleus contains 20 protons. That proton count is the element's identity: change it and you no longer have Calcium.