Atomic Number and Mass
Atoms of each element contain a characteristic number of protons and electrons, which are equal. The number of protons determines an element’s atomic number (Z) and distinguishes one element from another. The number of neutrons can vary and result in isotopes, which are different atoms of the same element that are identified by the number of neutrons in their nucleus.
An element's mass number (A) is the sum of the number of protons and the number of neutrons (Figure 1). The small contribution of mass from electrons is disregarded in calculating the mass number. This approximation of mass can be used to easily calculate how many neutrons an element has by simply subtracting the number of protons from the mass number. Isotopes of the same element will have the same atomic number but different mass numbers.
Since an element’s isotopes have slightly different mass numbers, scientists can determine the atomic mass by calculating the mean of the mass numbers for its naturally-occurring isotopes. Often, the resulting number contains a fraction. For example, the atomic mass of chlorine (Cl) is 35.45 because chlorine is composed of several isotopes, some (the majority) with an atomic mass of 35 (17 protons and 18 neutrons) and some with an atomic mass of 37 (17 protons and 20 neutrons).
Given an atomic number (Z) and mass number (A), you can find the number of protons, neutrons, and electrons in a neutral atom. For example, a lithium atom (Z=3, A=7) contains three protons (found from Z), three electrons (as the number of protons = the number of electrons), and four neutrons (7 – 3 = 4).