CaCO3 is an ionic compound, which consists of carbonate ion (CO3)2- and calcium ion (Ca)2+, in which carbonate ion is an atomic group and a whole bound by valence bonds. After CO2 obtains two electrons from calcium atom, the structure changes, each atom satisfies the stable structure of 8 electrons, and the atomic group is negatively charged. However, the calcium atom loses two electrons, which also satisfies the stable structure of 8 electrons and becomes a positively charged calcium ion. Calcium ions and carbonate ions are connected by ions, that is, by the mutual attraction of positive and negative charges.

If it is renamed calcium carbonate, it means that carbon and oxygen are ions respectively, which is inconsistent with reality, so it cannot be called this name.

An acid is a compound in which the cation released after being dissolved in water only contains hydrogen ions. The free cation is called so-and-so acid radical.

Beginners will ask similar questions ... students, don't worry, you will understand after learning. If there is any conceptual ambiguity, please ask the teacher for advice.