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# Atomic Weight and Atomic Mass

Atomic mass and atomic weight do not mean the same but still some people use the terms interchangeably. Let’s see how they are different.

## What Is Atomic Weight?

A single element can have several isotopes. To calculate atomic weight it is necessary to consider masses of isotopes. The percentage of isotopes may differ. While calculating atomic weight, isotope relative abundance and isotope mass have to be considered. Atomic weight, also referred to as relative atomic mass, is the ratio of the mean mass of the atoms of a chemical element to a certain standard. The normal unit of atomic mass has been one-twelfth of the atomic mass of the carbon-12 isotope since the year 1961. An isotope is one of two or more species of atoms with different atomic mass numbers (total number of protons + total number of neutrons) of the same chemical element. Helium’s atomic weight is known to be 4.002602. This value is known to represent the average of the isotopes’ usual ratio of natural abundances. Atomic weight is measured in units of atomic mass (usually abbreviated to AMU), also known as Daltons.

## History of Atomic Weight Scale

• John Dalton considers the value 1 for hydrogen as the base of his scale of atomic weights.
• The Swedish chemistJ.J. Berzelius used 100 for oxygen and the Belgian chemist J.S.Stas who carried out much quantitative analysis of compounds proposed 16 for oxygen and this base was used for many years.
• For several decades nuclidic masses were expressed on a scale based on 1/18th the mass of the neutral atom oxygen. The chemical atomic weight unit was then 1.000272 times the physical atomic mass unit.
• This period of confusion was brought to an end in 1961 by the acceptance of 1/12th the mass of carbon as the unit for both atomic weights and nuclidic masses.

## What Is Atomic Mass?

The total number of neutrons and protons of an atom is called atomic mass. Electrons are not counted and can be neglected as their size is very small. The atomic mass unit is Dalton. It is represented as Da and is the standard unit used to indicate the mass of an atom. The atomic mass of elements differs as it depends on the number of protons and neutrons in a particular element.

To make you understand how atomic mass and atomic weight are different from each other, here are some major differences between atomic mass and atomic weight:

## Difference between Atomic Mass & Atomic Weight

Difference between Atomic mass and Atomic weight
ATOMIC MASS ATOMIC WEIGHT
It is the sum of neutrons and protons present in an atom Average of ratio of atoms, present in an element
Isotopes are not included in the calculation Isotopes are included in the calculation
Calculated value is a whole number Calculated value may or may not be a whole number
Analysis is done by mass spectroscopy Analysis is done by mass spectroscopy

These were some important differences between atomic weight and atomic mass. To know differences between other topics in chemistry you can register to BYJU’S or download our app for simple and interesting content. India’s largest k-12 learning app with top-notch teachers from across the nation with excellent teaching skills.

## Recommended Videos ## Frequently Asked Questions – FAQs

### What is atomic number and atomic weight?

Therefore each atom can be given both an atomic number (the number of protons is equal to the number of electrons) and an atomic weight (about equal to the number of protons plus the number of neutrons).

### Where is the atomic mass?

Experimental evidence revealed that the vast majority of an atom’s mass is contained in its nucleus, which consists of protons and neutrons. The total number of protons and neutrons in an atom is known as the mass number (represented by the letter A).

### What is another name for atomic weight?

Atomic weight, also called relative atomic mass, is the ratio of the total weight of the atoms of a chemical substance to a norm. The normal unit of atomic mass has been one-twelfth of the atomic mass of the carbon-12 isotope since 1961.

### Why is the atomic mass important?

In chemistry, atomic mass is incredibly significant because it is the relation between mass, which we can calculate in the laboratory, and moles, which are atom numbers. In chemistry, much of what we research is determined by atomic ratios.

### What is difference between weight and mass?

The mass of an object is a measure of the inertial property of the object, or the sum of matter found therein. An object ‘s weight is a measure of the force imposed by gravity on the object, or the force required to sustain it.

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