To calculate the number of atoms in MgCO3, we need to consider the number of atoms of each element present in one molecule of MgCO3.

- Magnesium (Mg): There is one magnesium atom in each molecule of MgCO3.

- Carbon (C): There is one carbon atom in each molecule of MgCO3.

- Oxygen (O): There are three oxygen atoms in each molecule of MgCO3.

So, in total, there are \(1 + 1 + 3 = 5\) atoms in one molecule of MgCO3.