Nucleus – Atomic number

Introduction

The matter is composed of elements distinguished by the physical and chemical properties of its fundamental component — the atom. An atom is the ‘basic building block of matter’. The atoms which constitute all matter around us cannot exist independently.

Atoms form ions and combine with other atoms to form molecules. Molecules of different properties react according to the laws of chemistry and form compounds. Many compounds combine to form the matter we see, touch, and feel around us.

History

• John Dalton, in 1808, postulated the atomic theory and considered the atom to be the ‘smallest’ and ‘indivisible’ particle of matter. 
• In 1897, J. J. Thomson gave the first atomic model. It’s called plum pudding model and it says that the atom is a sphere of positive charge, and negatively charged electrons are embedded in it to balance the total positive charge.
• Ernest Rutherford, in 1911 discovered a positively charged dense central core in the atom and named its nucleus. Around the atomic nucleus is present an electron ‘cloud’ which revolves around the nucleus in definite orbits. 

Rutherford discovered a positively charged particle in the atomic nucleus and named it proton from the Greek word ‘Proto’.

• In 1932, James Chadwick discovered another particle in the nucleus, similar to the proton but electrically neutral, and named its neutron.

The protons and the neutrons in the atomic nucleus together are called nucleons.

What is the Atomic Number?

Let us discuss the atomic number definition. It is the number of protons in the nucleus of an atom. Hence, the atomic number is also called the proton number. 

The number of protons in the atomic nucleus equals the number of electrons revolving around the nucleus, thus making the atom electrically neutral. Hence, the atomic number equals the number of electrons revolving around the atomic nucleus.

The atomic number can also be seen as the charge number of the nucleus, as neutrons carry no net electrical charge. 

Every element of the periodic table has its atomic number. It distinguishes one element from another. Atomic numbers are always whole numbers and cannot be infractions as protons are the unit of matter.

Representation of Atomic Number 

We can represent a nuclide as

 ZAX 

Where 

X represents the element of the periodic table.

Z denotes the atomic number, i.e., the number of protons or electrons in an electrically neutral atom.

A means the atomic mass or mass number, i.e., the number of nucleons (protons+neutrons) in the atomic nucleus.

Atomic number Z can be used to calculate the number of neutrons in an atomic nucleus.

Number of neutrons N = Mass number (A) – Atomic number (Z)

The representation of the atomic number as Z came from the German word ‘Zahl’, meaning ‘numbers’. It described the position of an element in the periodic table. 

After 1915, suggestions and evidence showed that Z was also the nuclear charge and determined the chemical properties of elements. Hence, the word AtomZahl having an English equivalent of atomic number and represented by Z became common.

Range of Atomic Number 

At present, the atomic number ranges from 1 (for Hydrogen) to 118 (for the heaviest element).

The most common atomic numbers are as below.

The range of atomic numbers would increase beyond 118 with the discovery of more elements. Theoretically, there cannot be a maximum limit for atomic numbers. But as the number of protons and neutrons increase in an atomic nucleus, it becomes more unstable. 

Unstable nuclei undergo radioactive decay to gain stability. The decay of unstable nuclei may result in products with a smaller atomic number. In contrast, nuclear fusion may produce atoms with a more significant number.

Significance of Atomic Number

Let us understand the significance of the atomic number.

• The atomic number is essential to identify the elements of an atom.
• The modern periodic table organised elements in the increasing order of their atomic numbers.
• It represents the charge number of an atomic nucleus.
• It is the critical factor in determining the chemical properties of elements.

Conclusion

Atomic number, defined as the number of protons in the nucleus of an atom, is also called a Proton number. In an electrically neutral atom, the number of protons equals the number of electrons. Hence, the atomic number is also equal to the number of electrons.

Z represents the atomic number; it comes from the German word ‘Zahl’, meaning ‘numbers’. Presently, the atomic number (Z) ranges from 1 to 118. 

The atomic number is exceptionally vital in determining the chemical properties of elements. Also, the increasing order of atomic numbers forms the basis of the modern periodic table.