In Chemistry, the Atomic number is defined as the number of protons found in the nucleus of an atom of a chemical substance. It is the charge number of the nucleus as neutrons carry no net electric charge and also the number of electrons it can support. In an uncharged atom the total number of protons is equal to the number of electrons.

As we know, electrons are responsible for almost all chemical reactions. The atomic number indirectly sets all the physical and chemical properties of an element. The atomic number uniquely identifies a chemical element. It is denoted by the capital letter Z.

Did you know: The atomic number is also known as the proton number.

The atomic mass number A is the total number of protons and neutrons in an atom. This is easy to find from the periodic table. Just take the atomic weight and round off to the nearest whole number. This is because protons and neutron are very close to 1 atomic mass unit and electrons are very close to 0 atomic mass unit. The atomic mass number is a decimal number.

Atomic number and atomic mass

Atomic number and atomic mass | Image source:

How to find atomic number
If you take a look at the periodic table you will notice that every single element has a different atomic number. Periodic tables have the full name of each of the elements and their chemical symbol along with the atomic number. The atomic number is located at the top left or top right of the element. The periodic table is arranged in order of the atomic number.

How to find atomic mass
There are three ways to find the atomic mass.

First is to look into the periodic table. The atomic mass is usually below the element's symbol. To find the atomic mass of an element look for the atomic symbol. The decimal number which is the atomic mass of the element.  The value is in atomic mass units or AMU.

Example: The value for Carbon is 12.01 atomic mass unit in the periodic table.

To calculate the mass of a single atom add the number of protons and neutrons. To calculate the atomic mass find out the atomic number from the periodic table and the number of neutrons. Now you can add the two to get the atomic mass.

Example: To find the atomic mass of an isotope of carbon that has 7 neutrons: Find the atomic number from the periodic table. It is 6. Add the two to get the atomic mass. 6 +7 = 13.

The atomic mass of an element is the weighted average of its isotopes based on their natural abundance. Atomic mass can be calculated by multiplying each isotope mass by its abundance. Add all these values to get the atomic mass or the atomic weight of an element.

Example: For a sample containing 98% canbon-12, 2% carbon-13 the atomic number can be calculated as:
Multiply the atomic mass of each isotope with the amount of the element present in the sample.
0.98 X 12 = 11.76
0.3 X 13 = 0.26

Now add the two to get the total atomic mass
11.76 +0.26 = 12.02

The atomic mass of the sample is 12.02 g/mol.


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