Atomic Number and Mass Number: Definition, Formula and Examples

Atomic Number and Mass Number of Elements: Atoms are the building blocks of matter. They are responsible for the format of all living and non-living things. Understanding their molecular structure and terminologies is the initial step to learning chemistry. Two important concepts in atomic structure are the atomic number and the mass number. Let’s explore what these terms mean and how they are used.

What is Atomic Number?

• The atomic number is a fundamental property of an atom. It is defined as the number of protons found in the nucleus of an atom. 
• It is represented by the symbol ‘Z’.
• Atoms of the same atomic number and chemical properties combine to form an element. 
• Different elements are formed by different types of atoms.

Formula for Atomic Number

Calculating atomic number is simple. The number of protons in an atom is the direct math to know the atomic number. Thus, the formula for atomic number would be:

Atomic Number (Z)=Number of Protons

Example:

• Hydrogen: The simplest atom, hydrogen, has 1 proton in its nucleus. Therefore, the atomic number of hydrogen is 1.
• Carbon: Carbon atoms have 6 protons. Thus, the atomic number of carbon is 6.

Every element on the periodic table has a unique atomic number. This number is usually written above the element symbol on the periodic table.

What is Mass Number?

• The mass number is called the atomic mass number of nuclear number.
• The atomic mass number is the total number of protons and neutrons in an atom’s nucleus. 
• It is represented by the symbol ‘u’ (earlier abbreviated as ‘amu’, but according to th latest IUPAC recommendations, it is now written as ‘u’ – unified mass).
• Neutrons are neutral particles, meaning they have no charge, but they add to the mass of an atom along with protons.

Formula for Atomic Mass Number

The calculation of atomic mass is easy. Just add the number of protons and neutrons of an atom to get its atomic mass. Thus, the formula for atomic mass would be:

Mass Number (u)=(Number of Protons)+(Number of Neutrons)

Or,

Mass Number (u)=(Atomic Number)+(Number of Neutrons)

Example:

• Hydrogen: Most hydrogen atoms have 1 proton and 0 neutrons, so the mass number of hydrogen is 1.
• Carbon: A common form of carbon has 6 protons and 6 neutrons. Thus, the mass number of this carbon atom is 12.

The mass number is not listed on the periodic table because it can vary for different atoms of the same element, known as isotopes.

Isotopes

Isotopes are atoms of the same element that have same atomic number but varies in atomic masses due to the difference in their numbers of neutrons.

Example:

Carbon Isotopes:

• Carbon-12 has 6 protons and 6 neutrons (mass number = 12).
• Carbon-14 has 6 protons and 8 neutrons (mass number = 14).

Calculating Atomic Number

Example 1: Oxygen

Oxygen is a common element found in the air we breathe. To find its atomic number:

• Look at the periodic table and find oxygen (O).
• The atomic number of oxygen is listed as 8.
• This means oxygen has 8 protons in its nucleus.

Example 2: Sodium

Sodium is a metal commonly found in table salt. To find its atomic number:

• Look at the periodic table and find sodium (Na).
• The atomic number of sodium is listed as 11.
• This means sodium has 11 protons in its nucleus.

Calculating Mass Number

Example 1: Chlorine

Chlorine is often used in cleaning products. To calculate its mass number:

• Chlorine atoms typically have 17 protons.
• A common isotope of chlorine has 18 neutrons.
• Using the formula:

Mass Number=Number of Protons+Number of Neutrons

Mass Number=17+18=35

So, the mass number of this chlorine isotope is 35.

Example 2: Potassium

Potassium is an essential nutrient for plants and animals. To calculate its mass number:

• Potassium atoms typically have 19 protons.
• A common isotope of potassium has 20 neutrons.
• Using the formula:

Mass Number=Number of Protons+Number of Neutrons

Mass Number=19+20=39

So, the mass number of this potassium isotope is 39.

Atomic Number and Mass Number of First 30 Elements

Students are generally advised to memorise the first 30 elements from the periodic table. And thus, expected to be aware of the atomic number and the atomic mass of those 30 elements. Here, the list of those 30 elements are provided along with atomic number, atomic mass, name and symbol. 

Hopefully this article helped to understand the basics of molecular chemistry. To get for such information check the links below:

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