A calcium atom contains 20 electrons and is electrically neutral. A calcium ion contains only 18 electrons and is positively charged. A calcium atom becomes a calcium ion by losing two electrons.
Calcium Atoms via kwout
Atoms are the smallest unit of matter that can't be broken down chemically. Molecules are groups of two or more atoms that are chemically bonded. Ions are atoms or molecules that have gained or lost one or more of their valence electrons and therefore have a net positive or negative charge.
An atom can be an ion, but not all ions are atoms. There are distinct differences between an atom and an ion.
When you view an atom, the first question you might ask is ‘what kind of atom is it?’ What you are really asking is ‘which element am I looking at?’ You can answer this by counting the number of protons in the atom.
The number of protons, neutrons and electrons is the same in a neutral atom- that is, an atom with no charge. So, in a regular Calcium atom, there are 20 protons, 20 neutrons and 20 electrons. The electrons orbit around the nucleus and have a negative charge. An isotope is a version of the element with a different number of neutrons.
When the number of protons differs from the number of electrons, you have an ionic charge. A Calcium atom with 20 protons and 18 electrons has a charge of 2+ because it has 2 extra protons, which have a positive charge. A charged atom is called an ion. When the charge is positive, it is called an cation.
If the Calcium gained two more electrons it would have a charge of 2- because it has 2 extra electrons and electrons have a negative charge. A negatively charged ion is called an anion. Read more at Wiredcosmos.com.