photo.jpg

# Atomic Mass and Atomic Number A typical oxygen atom, which has an atomic number of 8, contains eight protons and eight neutrons. Mass numbers are useful because they tell us the number of subatomic particles in the nuclei of different atoms. However, they do not tell us the actual mass of the a...

# Atomic Mass and Atomic Number A typical oxygen atom, which has an atomic number of 8, contains eight protons and eight neutrons. Mass numbers are useful because they tell us the number of subatomic particles in the nuclei of different atoms. However, they do not tell us the actual mass of the atoms. For example, they do not take into account the masses of the electrons or the slight difference between the mass of a proton and that of a neutron. The actual mass of an atom is known as its atomic weight. When you are asked for the mass of a particular atom, we will ignore the mass of the electron and the slight difference between the neutron and the proton. We assign the proton a mass of one AMU (atomic mass unit) and the neutron one atomic mass unit. Let us consider two examples: Oxygen and Chlorine, for example. ### Oxygen Oxygen has an atomic number of 8. We know that it must have 8 protons. Since atoms tend to be electrically neutral, oxygen will also have 8 electrons which balance the positive charge of the 8 protons. Oxygen also has a mass of 16. Thus there must be a total of 8 neutrons. 8 protons + 8 neutrons = 16 amu ### Chlorine Chlorine, on the other hand, has an atomic number of 17 and an atomic mass of 35. Remember the atomic number represents the number of protons. 17 + 18 = 35amu Thus, Chlorine has 17 protons and 18 neutrons. # Notes - Blank space for notes. # Atomic Structure Atoms normally contain equal numbers of protons and electrons.