The atomic number of an element refers to the number of protons present in the nucleus of an atom of that element. Each element has a unique atomic number, and it determines the element's identity. For example, hydrogen has an atomic number of 1, carbon has an atomic number of 6, and oxygen has an atomic number of 8.
On the other hand, the number of atoms refers to the count of individual atoms that make up a substance or a sample. It represents the quantity of atoms present and can vary depending on the size or amount of the substance.
The atomic number is a characteristic property of an element and remains constant for all atoms of that element. In contrast, the number of atoms can change depending on the sample size or the amount of the substance being considered.
For instance, let's consider a sample of oxygen. Oxygen has an atomic number of 8, meaning that each oxygen atom has 8 protons in its nucleus. If we have a sample containing 10 oxygen atoms, we can say that the sample contains 10 atoms of oxygen.
In summary, the atomic number is a fixed property of an element and represents the number of protons, while the number of atoms can vary and depends on the quantity of the substance being considered.