Final answer:
When a calcium atom loses two electrons, it becomes a positive ion, specifically a 2+ charged cation named Ca²⁺, isoelectronic with the noble gas argon.
Step-by-step explanation:
When a calcium atom loses two electrons, it becomes a positive ion. This is because the loss of electrons (which are negatively charged) leaves the atom with more protons (which are positively charged) than electrons, resulting in a net positive charge. Such positively charged ions are known as cations. For instance, calcium (atomic number 20) has 20 protons and, when neutral, it has 20 electrons. Upon losing two electrons, the calcium atom becomes Ca²⁺, which is a cation with 20 protons and only 18 electrons, giving it a 2+ charge. Ca²⁺ is isoelectronic with the noble gas argon, because it has the same number of electrons as argon does.