Here's the breakdown of the elements and their number of atoms in each compound:

1. NaCl (Sodium Chloride)

* Sodium (Na): 1 atom

* Chlorine (Cl): 1 atom

2. H2O2 (Hydrogen Peroxide)

* Hydrogen (H): 2 atoms

* Oxygen (O): 2 atoms

3. Fe3O4 (Iron Oxide - Magnetite)

* Iron (Fe): 3 atoms

* Oxygen (O): 4 atoms

4. SiO2 (Silicon Dioxide - Quartz)

* Silicon (Si): 1 atom

* Oxygen (O): 2 atoms