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Atomic Mass

In the early 18th century, it was discovered that an atom is made up with its elementary particles and it is considered as the basic unit of molecules. The atomic weight scale is a relative scale which was first given by John Dalton in 1803. Initially, it was used for hydrogen atom but later, the atomic weight was given by taking standard of an isotope of oxygen for atomic mass unit (amu). But after that, the isotope of carbon C-12 is taken as standard for atomic mass unit and new symbol u is used rather than amu.

The unified atomic mass unit is the current unit of atomic mass which is shown with u. It is similar to Dalton. It was first given by Dalton, so it is named after him. As mass is the basic physical property of matter, so now it is referred as molecular weight or atomic weight. Let’s discuss atomic mass, the process to find it, its relation with mass number, and problem for calculating atomic mass.

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What is Atomic Mass?

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The Atomic Mass is the mass of an atom of an element in the periodic table. The mass of different isotopes of the elements is different from each other. The atom is made up of nucleus and the electrons revolving around it. The Nucleus is made of protons and neutrons. Thus,
The Atomic Mass is also defined as the sum of masses of protons, neutrons and electrons present in the atom.The Atomic Mass of an atom is measured in the unified Atomic Mass Units(amu or represented mostly as u or Da). It is the unit used for measuring the mass at atomic or molecular level.

The atomic mass is mistaken used as to refer the relative atomic mass or atomic weight or average atomic mass.

Average Atomic Mass
It is another term to standard atom weight which refers to the weighted average of mass of all the isotopes of an element with respect to their abundance in the earth atmosphere.

Relative atomic mass/Atomic Weight

It is the relative quantity and is referred to as ratio of average mass of an atom to the $\frac{1}{12}$ th of the mass of an atom of C-12.

How to Find Atomic Mass?

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We know that Mass number A = Z + N
where Z = Proton number and
N = Neutron number.
As described above the atomic mass is the sum of masses of protons, neutrons and electrons and to find the mass of an atom we need to follow the below mentioned steps:

Step – 1
Find the number of the protons:
The number of protons of an atom can be obtained by knowing its atomic number which signifies the number of protons in the atom.

Step – 2
Find the number of electrons:
Since we know that the atom is neutral or it does not have charge which indicates that the number of protons and electrons are equal in it. So the number of electrons is equal to the atomic number of an atom.

Step – 3
Find the number of neutrons in the nucleus of an atom:
To obtain the number of neutrons in an atom is a tricky one. We know that the mass number is given as the sum of number of neutrons and number of protons in the nucleus of an atom.
i.e., A = Z + N
$\therefore$ N = A - Z.
Subtracting mass number with the proton number, we get the neutron number.

Step – 4
In this step we will multiply the respective masses of each particle with their number in the atom.
Following values remains same:
The mass of neutron is given by 1.674927351×10−27 kg,
The mass of proton is 1.67262158 × 10-27 kg and
The mass of electron is 9.10938188 × 10-31 kg.
It is evident from the previous statement that the mass of electron is negligible as compared to the mass of proton or neutron and hence mostly in calculations the mass of electron is neglected.

So the Atomic mass is given by:
U = (Number of proton $\times$ 1.67262158 × 10-27) + (Number of neutron $\times$ 1.674927351×10−27) + (Number of electron $\times$ 9.10938188 $\times$ 10-31) kg.
U = (Z $\times$ 1.67262158 $\times$ 10-27) + ( N $\times$ 1.674927351×10−27) + ( e $\times$ 9.10938188 $\times$ 10-31) Kg.

Let us take the Krypton element as an example to determine its atomic mass:
Atomic Mass of Krypton

From the above diagram we can clearly obtained the number of the protons. The number of protons in an atom is equal to its atomic number so
No. of Protons in Krypton Molecule is = 36

As we know that the krypton molecule is neutral in charge and hence the number of electrons in its atom is equal to number of protons so
No of Electron in Krypton Molecule is = 36

For finding the number of neutron in the krypton we need to find the Mass Number of the Krypton.
We know that the Atomic Mass is the weighted average of the Mass Number of all the isotopes of the element and here we can approximate the Mass Number by rounding the Atomic Weight to the nearest whole number which is 84.

Now we can obtain the number of neutron by using the relation of mass number, as shown under:
Mass Number = Number of Neutron + Number of Protons.
A = Z + N.
84 = 36 + N.
N = 84 – 36
N = 48.
So, the Number of the neutron in the krypton atom is 48.

So now we can calculate the atomic mass by the following equation:
U of krypton = (36 x 1.67262158 × 10-27) + (48 x 1.674927351×10−27) + (36 x 9.10938188 × 10-31) kg.
U of krypton = (60.192 + 80.352 + 0.0327924) x 10-27 kg
U of krypton = 140.5767924 x 10-27 kg.

Use below widget to calculate atomic mass.


Atomic Mass Number

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The Atomic Mass Number or simply the Mass Number of an atom is defined as sum of the number of the protons and the number of neutrons in the nucleus of the atom.
The number of protons in the nucleus is called atomic number (Z). The total number of protons and neutrons is called mass number (A). Hence (A-Z) gives the number of neutrons an is called neutron number (N).

It is different for each isotopes of the element because every isotope has different number of neutrons in the nucleus of the isotopes. The mass number is denoted by A represented as:
A = Z + N

What is the Atomic Mass of an Element?

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The atomic mass of an element is referred to as the average mass of atoms present in the element. It is measured the ratio of the mass of the element and number of the atoms in the element.

If X is the chemical symbol of the element then it is represented by $^{A} _{Z} X $.
Where A = Mass Number and
Z = Proton Number.

Atomic Mass of Gold

The atomic number of protons Z = 79
The atomic number of neutrons N = 118.
The atomic mass of the gold is 196.96655 amu.

Carbon Atomic Mass

The Proton number Z = 6
The Neutron number N = 6
The atomic mass of the carbon atom is 12.0107 amu.

Oxygen Atomic Mass

The Proton number Z = 8
The Neutron number N = 8
The atomic mass of the oxygen atom is given by 15.9994 amu.

Atomic Mass of Sulfur

The Proton number Z = 16
The Neutron number N = 16
The atomic mass of the sulphur or sulphur is 32.066 amu.

Atomic Mass of Nitrogen

The Proton number Z = 7
The Neutron number N = 7
The atomic mass of atom of nitrogen is given by 14.00674 amu.

Atomic Mass of Copper

The Proton number Z = 29
The Neutron number N = 35
The atom of copper has atomic mass given by 63.546 amu.

Atomic Mass of Chlorine

The Proton number Z = 17
The Neutron number N = 18
The atomic mass of the chlorine atom is 35.4527 amu.

Atomic Mass of Sodium

The Proton number Z = 11
The Neutron number N = 12
The atom of sodium is given by 22.98977 amu.

Atomic Mass of Iron

The Proton number Z = 26
The Neutron number N = 30
The atomic mass of the atom of an Iron molecule is 55.845 amu.
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