Relative Atomic Mass - Some Basic Concept of Chemistry for NEET

Welcome to the interesting world of chemistry! In this article, we will be discussing some basic concepts related to atomic masses. Before that let us know what an atom is? Atoms are the building blocks of matter. It is the littlest unit of matter that's composed of three subatomic particles: the proton, the neutron, and therefore the electron. 

In this particular article we will be explaining how to find atomic mass and how to use the atomic mass formula to calculate the relative atomic masses of different elements. Finally, we will discuss what relative atomic mass is and what it means for chemical reactions. 

What is Atomic Mass?

Earlier atoms were considered as the smallest particle of matter that can take part in a chemical reaction. Atoms are not capable of independent existence. Two or more atoms combine together to form different types of compounds such as ionic compounds, metal crystals, covalent compounds and coordination compounds.

Every particle of matter has some amount of mass associated with it whether small or large. Everything is made up of atoms. Mass of an atomic particle is called the atomic mass. The atomic mass of an element is the number of times a molecule of that element is heavier than an atom of carbon taken as 12. 

The atomic mass of an element can be defined as the number which indicates how many times the mass of one atom of the element is heavier in comparison to the mass of one atom of hydrogen.

Atomic mass of an element =\[\dfrac{\text{Mass of One Atom of the Element}}{\text{mass of }\dfrac{1}{12}\text{th Part of the Mass of C-12 Atom}}\]

Atomic Mass Unit

The quantity $\dfrac{1}{12}$th mass of an atom of carbon-12 is known as the atomic mass unit and is abbreviated as amu. The actual mass of one atom of carbon-12 is 

$1.9924\times {{10}^{-23}}g\text{ or }19924\times {{10}^{-26}}kg$

Thus 1 amu =$\dfrac{1.9924\times {{10}^{-23}}}{12}=1.66\times {{10}^{-24}}g$

Atomic Mass of an Element = $\dfrac{\text{Mass of one Atom of the Element}}{\text{1 amu}}$

Actual Mass of an Element = $\text{Atomic mass(amu) }\times \text{ 1}\text{.66}\times \text{1}{{\text{0}}^{-24}}g$

How to Find Atomic Mass

To find the atomic mass of an element, you will need to know the element's atomic number and mass number. The atomic number is the number of protons in an atom's nucleus, while the mass number is the sum of the protons and neutrons in the nucleus. Armed with this information, you can use a periodic table to look up the element's atomic mass (listed under its symbol).

Once you have located the element on the periodic table, simply find its Atomic Number (Z) and Mass Number (A). These are usually listed above or below the elemental symbol. The Atomic Mass is then just A. So, for example, if we wanted to find out how much an atom of Carbon-12 weighed, we would locate Carbon on the periodic table and see that it has an Atomic Number of 6 and a Mass Number of 12 (6 protons and 6 neutrons).  The mass number and atomic number are almost the same. However, You can see in some cases such as in the case of carbon the atomic mass of carbon is 12.011 and mass number of carbon is 12. This is because carbon occurs in nature in more than one isotopic form. The atomic mass of these types of elements is the average mass per atom of a naturally occurring sample of atoms of the element. It is also referred to as average atomic mass.

The average atomic mass of an element can be calculated by taking the weighted average of the masses of all its isotopes, where the weighting factor is equal to the natural abundance of each isotope. For example, if an element has two isotopes with masses m₁ and m₂ and abundances A₁ and A₂ respectively, then its RAM will be given by: 

$RAM=\dfrac{{{A}_{1}}\times {{m}_{1}}+{{A}_{2}}\times {{m}_{2}}}{{{A}_{1}}+{{A}_{2}}}$

The average atomic mass of carbon will come out to be: ((0.98892) (12) + (0.01108) (13.00335) +(2 × 10^-12) (14.00317))/((0.98892+(0.01108)+(2 × 10^-12) ) = 12.011.

What is Relative Atomic Mass

The relative atomic mass of an element is the average mass of all atoms of that element compared to a standard unit. The standard unit for relative atomic mass is the carbon-12 atom, which has a relative atomic mass of 12. 

Relative atomic masses are used to determine the masses of molecules and other compounds made from different elements. For example, water (${{H}_{2}}O$) has a molecular weight of 18.015 amu (atomic mass units), which is made up of two hydrogen atoms with relative atomic masses of 1.008 amu each, and one oxygen atom with a relative atomic mass of 15.999 amu.

Relative atomic mass is the ratio of the average mass of an atom to $\dfrac{1}{12}th$ of the mass of an atom of carbon-12. 

Summary

Atoms are the building blocks of matter. It is the littlest unit of matter that's composed of three subatomic particles: the proton, the neutron, and therefore the electron. Every particle of matter has some amount of mass associated with it whether small or large. Everything is made up of atoms. Mass of an atomic particle is called the atomic mass. The relative atomic mass of an element is the average mass of all atoms of that element compared to a standard unit. The standard unit for relative atomic mass is the carbon-12 atom, which has a relative atomic mass of 12.