Certainly! The atomic number of an element represents the number of protons in the nucleus, while the atomic mass (atomic weight) is the average mass of all the isotopes of that element, taking into account their abundance.
There are a few elements where the atomic number is greater than the atomic mass. This occurs when the element has a dominant isotope with a relatively low atomic mass and a few less abundant isotopes with higher atomic masses. Here are a few examples:
Chlorine (Cl):
- Atomic number: 17
- Atomic mass: 35.45 (approximately) Chlorine has two stable isotopes: chlorine-35 (75.78% abundance) and chlorine-37 (24.22% abundance). The average atomic mass is calculated by considering the abundance of each isotope, resulting in an atomic mass slightly greater than the atomic number.
Potassium (K):
- Atomic number: 19
- Atomic mass: 39.10 (approximately) Potassium has three isotopes: potassium-39 (93.26% abundance), potassium-40 (0.012% abundance), and potassium-41 (6.73% abundance). The presence of the relatively heavy potassium-40 isotope contributes to the higher atomic mass.
Cobalt (Co):
- Atomic number: 27
- Atomic mass: 58.93 (approximately) Cobalt has one stable isotope, cobalt-59 (approximately 100% abundance). Although the atomic mass is close to the atomic number, it is slightly higher due to the presence of other less abundant isotopes.
It's worth noting that the atomic mass is usually reported as a decimal number due to the isotopic mixture, while the atomic number is always an integer. The examples given above illustrate the cases where the atomic mass is slightly higher than the atomic number.