Physics Lesson 20.1.7 - Atomic Mass (Weight) Number

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Welcome to our Physics lesson on Atomic Mass (Weight) Number, this is the seventh lesson of our suite of physics lessons covering the topic of Atomic Nucleus and Its Structural Properties, you can find links to the other lessons within this tutorial and access additional physics learning resources below this lesson.

Atomic Mass (Weight) Number

Another number that appear near each element (below its symbol) in the periodic table is the atomic mass or atomic weight number A. In general, it is not a whole number and shows a mean value of all isotopes of a given element based on the percentage of their existence. However, the value of A is easily found by rounding the value shown in the periodic table (under the name of element) to the nearest whole number. Atomic weight is not used to identify the name of element (Z is enough for this) but the corresponding isotope instead. Hence, Z = 1 and A = 3 means we are dealing with a hydrogen isotope (known as deuterium) which has 1 proton and 2 neutrons in the nucleus.

The unit used to indicate the atomic mass is not kilogram but amu (atomic mass unit) or simply u instead. Thus, in scientific terms, 1 u represents 1/12 of the mass of a C-12 nucleus and it is a rough value to indicate the mass of one proton or one neutron. The conversion factor between amu and kilogram is

For example, in the periodic table shown above, you see the number 195.085 below the Platinium (Pt) element (Z = 78). This means a Platinium nucleus is about 195 times heavier than 1/12 of a C-12 nucleus. Another conclusion we draw from the above value is that A (Pt) = 195, so the number of neutrons in this nucleus is N = A - Z = 195 - 78 = 118.

Example 1

A chemical element X has 26 protons and 57 particles in the nucleus.

1. What element is it?
2. What is the number of neutrons in the nuclei of X?
3. What is the atomic weight of this element?

Solution 1

1. The atomic number Z of the unknown element must be 26. Therefore, we identify this element in the periodic table. It is shown below. Obviously, the name of element is iron (its symbol is Fe).
2. Since the element has 57 particles in the nucleus (A = 57), the number of neutrons N is
   A = Z + N
   N = A - Z
   = 57 - 28
   = 29
   Thus, the given element is one of many isotopes of iron.
3. The figure extracted from the periodic table shows the value 55.845 below the name of element. This shows that the atomic weight (mass) of this element is 55.845 amu.

You have reached the end of Physics lesson 20.1.7 Atomic Mass (Weight) Number. There are 9 lessons in this physics tutorial covering Atomic Nucleus and Its Structural Properties, you can access all the lessons from this tutorial below.

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