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Atomic Number Mass Number - Definition, Example, Formula & Calculation, FAQs

Atomic Number Mass Number - Definition, Example, Formula & Calculation, FAQs

Edited By Team Careers360 | Updated on Jun 17, 2022 06:10 PM IST

Mass Number Definition: Basically, the atomic number can be defined as the protons present in the nucleus which can be denoted by the symbol Z, and mathematically it can be defined as;

Z= protons present in the nucleus.

Simply as we can better understand the atomic number by taking an example i.e. if we see the hydrogen atom which contains one proton so the atomic number of hydrogen atom will be one, similarly for the sodium atom it will be 1.

So, from the above fact we can say that the dependency of the atomic number is based only on the protons in the element not on the electrons or neutrons, so by using this statement w can create a relation mathematically as;

Z = no. of protons = no. of electrons

 atomic mass and atomic number(Element Symbol)

NCERT Chemistry Notes :

Here in the above image of the element we can see the representation of atomic mass and atomic number/weight number of the element. This is the way we can represent atomic mass and atomic number.

How to find atomic number: For atomic mass of an element, we can say that it is the sum of protons and neutron in the nucleus and is denoted b the symbol A as shown in the above image, so mathematically we can write as:

A = Protons + Neutrons

This combination of protons and neutrons is also called nucleons.

We can also write.

A = Z + Neutrons

Neutrons = A – Z

In the periodic table, the hydrogen element is only the element which does not have the neutron and has only one proton.

Brief Introduction :

The atomic mass of an element is actually very small because atoms are extremely small. Today we have sophisticated techniques i.e., mass spectroscopy for determining the atomic masses fairly and accurately. But in the 19th century scientists could determine the mass of an atom relative to another by experimental means as has been mentioned earlier.

Name of Elements

Symbol

Atomic number

Number of Electrons

Number of Protons

Number of neutrons

Atomic Mass

Hydrogen

H

1

1

1

-

1

Helium

He

2

2

2

2

4

Lithium

Li

3

3

3

4

7

Beryllium

Be

4

4

4

5

9

Boron

B

5

5

5

6

11

Carbon

C

6

6

6

6

12

Nitrogen

N

7

7

7

7

14

Oxygen

O

8

8

8

8

16

Fluorine

F

9

9

9

10

19

Neon

Ne

10

10

10

10

20

Sodium

Na

11

11

11

12

23

Magnesium

Mg

12

12

12

12

24

Aluminium

Al

13

13

13

14

27

Silicon

Si

14

14

14

14

28

Phosphorus

P

15

15

15

16

31

Sulphur

S

16

16

16

16

32

Chlorine

Cl

17

17

17

18

35.5

Argon

Ar

18

18

18

22

40

Potassium

K

19

19

19

20

39

Calcium

Ca

20

20

20

20

40

Also check-

The above table contains the element name, atomic number, atomic mass, number of protons, number of electrons, and number of neutrons, so we can easily differentiate between them, in inorganic chemistry we can also find such type of data and we can clearly learn the increasing or decreasing number of atom or mass by going from up to down the periodic table. An element’s number is adequate to the number of protons in the nuclei of any of its atoms. The table of elements in periodic table gives the atomic number of every element. The number may be a integer usually written above the chemical symbol of every element within the table. The number for hydrogen is 1ecause every hydrogen atom has proton.

The number for helium is because every helium atom has 2 protons, the number of elements within the table, that are arranged as of accelerating number of protons within the nucleus. Accordingly, the protons, which is usually adequate to the number of electrons within the neutral atom, is additionally the number, for a while let’s say iron has 26 no of electron it means it has its atomic number 26. As we have discussed above the number of an element is the sum of proton and neutron now, we will discuss on the calculation the atomic number and mass no.

Calculation of Atomic Number and Mass

Also read :

Calculation of Atomic Number and Mass Number:

Our great scientists calculated the mass of atom by the mean of mass number from the isotopes of the elements that occurs naturally. Often the decimal number is obtained in the results. For instance, the atomic mass of chlorine is 35.45amu due to its various isotopes.

Let us understand with an example say an atomic number (Z) and mass number (A), in a neutral atom we can find the number of protons, neutrons and electrons, for instance a lithium atom (Z=3, A=7 amu) contains three protons (from Z), three electrons (as proton = electron), and four neutrons (7 – 3 = 4).

Atomic number and Mass Number

More Summary About Atomic number and Mass Number:

It is well known that protons are present within the nucleus of an atom. atomic number of any element is determined by the amount of proton present in atom, it's denoted by ‘Z’. All atoms of a component have an equivalent number, Z. Infect, elements are defined by the amount of protons they possess. For hydrogen, Z = 1, because in atom, just one proton is present within the nucleus. Similarly, for carbon, Z = 6. Therefore, the number is defined because the total number of protons present within the nucleus of an atom.

By studying the properties of the subatomic particles of an atom, we can easily say that the total mass of an atom is practically thanks to protons and neutrons alone. that's by protons and neutrons also are called as nucleons. So therefore, the total mass of an atom resides in its own nucleus. for instance, mass number of carbons is 12amu due to its 6 protons and 6 neutrons. In nature, variety of atoms of some elements are identified, which have the same number of atom but different mass numbers which are called the isotopes for example hydrogen element has its isotopes named deuterium and tritium denoted by simply D and T.

Let us consider two elements calcium (Ca), atomic number 20, and argon (Ar), atomic number 18. the number of electrons in these atoms is different, but the nucleon number of both these elements is 40.

Also read -

Frequently Asked Questions (FAQs)

1. What is atomic number?

The atomic number can be defined as the protons present in the nucleus which can be denoted by the symbol Z, and mathematically it can be defined as; Z= protons present in the nucleus or in the other words an element’s number is adequate to the number of protons in the nuclei of any of its atoms.

2. What is mass number?

Atomic mass of an element is the sum of protons and neutron in the nucleus and is denoted b the symbol A as shown in the above image, so mathematically we can write as;

A = Protons + Neutrons  

This combination of protons and neutrons is also called nucleons.

We can also write.

A = Z + Neutrons

Neutrons = A – Z

In the periodic table, hydrogen element is only the element which has not the neutron and have only one proton. 

3. Explain by giving at least one example of each: atomic number and mass number.

Atomic number: The number of a component is adequate to the number of protons within the nucleus of its atom. e.g., example; Carbon contains 6 protons, so it has atomic no. = 6.Mass number: The nucleon number of an atom is adequate to the number of protons and neutrons in its nucleus. Nucleons = protons + neutrons, for Example carbon: Protons + Neutrons = Nuclei = nucleon number 6 + 6 = 12

4. Explain Isotopes and Isobars. Give any two uses of isotopes.

Isotopes: Isotopes are one of two or more species of atoms of a chemical element with the same atomic number and position in the periodic table and nearly identical chemical behaviour but with different atomic masses and physical properties.

Isobars: Isobars are atoms having an equivalent nucleon number instead of atomic numbers. Both calcium and argon have same nucleon number but different atomic number. 

Two uses of isotopes are: isotope can be used in the treatment of goitre and as fuel in the nuclear reactor.

5. If there are two different elements then, can they have an equivalent atomic number?

Atoms from two different elements may have an equivalent neutron count, but never an equivalent proton count. Atomic number = number of protons = number of electrons

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