[Chemistry Class Notes] on Atomic Mass Formula Pdf for Exam

Matter is made up of very small parts called atoms. It is described as something that takes up space and has Mass. A physical property of matter is Mass. The Atomic Mass is referred to as the Mass of an atom or a molecule. In this article, we will study the Atomic Mass formula, the formula for molar Mass, and the average Atomic Mass formula that will help to calculate the subAtomic particles and also the Mass of an atom.

 

Formula of Atomic Number

Atomic number represents the no. of protons in the nucleus of an atom. The formula for Atomic number is-

 

Atomic number = no. of protons present in the nucleus of an atom.

 

Atomic Mass

The sum of the Masses of protons, neutrons, and electrons in an atom or group of an atom is the Atomic Mass. In contrast with protons or neutrons, the electrons have much less Mass, so the Mass of electrons is not included in the calculation.

 

Atomic Mass formula = Mass of protons + Mass of neutrons + Mass of electrons

 

The relative Atomic Mass of an element is the total Mass of the element’s naturally occurring isotopes relative to the Mass of a 12C atom that means a relative Atomic Mass of exactly 12 is given to one atom. 

 

Through, the Mass of the electrons are negligible and hence not always considered. Thus the Atomic Mass of an atom is generally said as the Mass of all the protons and neutrons present combined together. But due to the Mass of electrons, there is a little difference between the actual Mass of the atom as written on the periodic table than the Atomic Mass found by the ways of calculating Atomic Mass taught to the students.

 

To Calculate the Atomic Mass 

There are three different methods to calculate the Atomic Mass-

1. By having Reference to the Periodic Table 

In the periodic table, an Atomic number is typically indicated under the representation of an element.

For Example-

 

()

 

In general, however, the Atomic Mass of an atom will be very similar to its Mass number, although the decimal places will have some variation.

 

2. Addition of Protons and Neutrons

In order to calculate the Mass of a single atom of an element, one can consider using the number of the protons and neutrons present in their. By adding the Masses of each proton and neutron present in the nucleus of a given atom, One can extract the Atomic Mass of that atom. In the vague terminology, Atomic Mass can also be defined as the number of the protons and neutrons.

 

For example – consider if the student has to find the Atomic Mass of the isotope of the atom of oxygen with 10 neutrons present in their nucleus. The student will be able to find via the periodic table that the atoms of oxygen have 8 protons and 8 electrons in them. And thus the sum of these two will result into 18, which will be the Atomic Mass of that certain atom of oxygen Isotopes in amu terms (Atomic Mass units). The average Atomic Mass of this isotope is 17.999 or 18 (approx.).

 

3. The average Atomic Mass of various elements are determined by multiplying the Atomic Mass of each isotope by its fractional abundance and adding the value obtained. For example, chlorine contains two types of atoms having relative Masses of 35u and 37u. The relative abundance of these isotopes in nature is in the ratio 3:1. Thus the Atomic Mass of chlorine is the average of these different relative Masses.

 

Thus, the formula of the average Atomic Mass of chlorine = {(35 x 3) + (37 x 1)} / 4 = 35.5u

 

Atomic Mass of Elements

 

Relative Atomic Mass

The relationship between the Mass of an element and the number of atoms it contains is the element’s Relative Atomic Mass. To measure the Masses of distinct atoms, the relative Atomic Mass scale is used. 

 

A Relative Atomic Mass of 1 was initially allocated to the hydrogen atom, the lightest atom, and the relative Atomic Mass of other atoms as compared to this.

 

Relative Atomic Mass of Elements

 

Solved Examples

Example 1: Find the element Mass number whose atomic number is 15 and the number of neutrons present is 15.

Solution: This can be calculated using the formula for molar Mass

Atomic number= number of protons present

Mass number= No. of protons + No. of neutrons

                        =15+15=30

 

Conclusion

In chemistry, the definition of Atomic weight is important, as most chemical reactions take place in accordance with simple numerical relationships between atoms. Since it is not possible to explicitly count the atoms in a substance, chemists quantify reactants and products by measuring and using Atomic weight measurements to draw their conclusions.