Define Avogadro's constant, relative atomic mass, relative molecular mass, and empirical formula.

Here are the definitions for the terms in chemistry:

3. Avogadro's Constant: Avogadro's constant ($N_A$) is the number of constituent particles (atoms, molecules, ions, or other particles) per mole of a substance. Its value is approximately $6.022 \times 10^{23} \text{ mol}^{-1}$.

4. Relative atomic mass: The relative atomic mass ($A_r$) of an element is the ratio of the average mass of atoms of that element to $\frac{1}{12}$ of the mass of an atom of carbon-12. It is a dimensionless quantity.

5. Relative molecular mass: The relative molecular mass ($M_r$) of a compound is the ratio of the average mass of molecules of that compound to $\frac{1}{12}$ of the mass of an atom of carbon-12. It is also a dimensionless quantity.

6. The empirical formula: The empirical formula of a chemical compound is the simplest whole-number ratio of atoms of each element present in the compound. It indicates the proportional relationship of elements in a compound.