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Chemistry > Structure of Atom > Atomic Number
Structure of Atom

Atomic Number

Elements are the building blocks for all the matter you see in the world. Now, the question is how can you distinguish these elements? The answer is “atomic number.” Every element has a unique atomic number that helps to distinguish between elements. Let us study further to know about its significance.

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History of Atomic Number

We all know that, in the periodic table, the elements are arranged in a very informative order. The history of periodic table dates back to the late 1860s. In a time when Dmitri Mendeleev first discovered the periodic law. The periodic law soon became a major organizing concept of chemical sciences. But, the law had some drawbacks. Some of the elements when arranged according to Mendeleev’s law were out of sequence.

It was not until Wilhelm Röntgen and his discovery of X-rays in 1895 that helped other scientists to further research on Mendeleev’s discovery. English physicist H.G.J. Moseley in 1913 studied wavelengths of X-rays emitted by different chemical elements. Moseley hypothesized the modern periodic table given by Mendeleev was on the basis of the number of protons in the nucleus of an atom.

And this hypothesis forms the basis of atomic number. Each element has a unique number. This number represents the number of protons in the nucleus of an atom.

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Atomic Number Definition

The total number of protons present in the nucleus of every atom of a chemical element represents the atomic number of that element. Each element is characterized by a unique atomic number.  It is represented by the letter “Z.”

As a matter of fact, every atom of an element consists of the same number of protons. And so, has the same atomic number. But different elements have different atomic numbers. For example, each atom of oxygen has 8 protons in its nucleus. So, the atomic number of oxygen is 8. Similarly, the atomic number of carbon is 6. As all of its atoms have 6 protons in its nucleus. 

We know that an atom consists of protons, neutrons and electrons. Also, it does not have any net charge. Thus, it is electrically neutral. This means that the number of electrons is equal to the number of protons present in an atom. Therefore,

“Atomic Number(Z) = No. of Protons = No. of Electrons”

Furthermore, atomic numbers are whole numbers. Because it is the total number of protons. And protons are generally units of matter. As a matter of fact, it ranges from 1 to 118. It starts with hydrogen and ends with the heaviest known element, Oganesson (Og).

Theoretically, atomic numbers can be increased if more elements are discovered. However, with the addition of more number of protons and neutrons, the elements become prone to radioactive decay.

You can download Atomic Number Cheat Sheet by clicking on the download button below

 Atomic Number

Importance of Atomic Number

  • Helps in the identification of a particular element of an atom.
  • Forms the basis of the arrangement of the elements in the periodic table. The elements are arranged in order of increasing atomic numbers.
  • Helps in the determination of the properties of any element. But, the valence electron determines the chemical bonding behaviour of an element.

Examples of Atomic Number

It does not matter how many neutrons and electrons are present in an atom. The Z is always determined by the number of protons. For example,

  • If an atom has one proton then the Z is 1 and the element is hydrogen.
  • Every iodine atom has 53 protons and the Z of iodine is 53.
  • Every caesium atom has 55 protons and hence the Z is 55.

Atomic number of few elements

  • Hydrogen= 1
  • Helium=2
  • Lithium=3
  • Beryllium=4
  • Boron=5
  • Carbon=6
  • Nitogen=7
  • Oxygen=8

Finding the Atomic Number

From the given information, we can find out the atomic number of an element in the following ways. 

First, if we know the number of protons in an atom of an element, we can find out the atomic number. Because the number of protons is equal to the atomic number of an element.

Second, if we know the element name or symbol, we can find the atomic number using the periodic table. For example, say you want to know the atomic number of symbol Al (aluminium). For that, spot Al in the periodic table. You can see the atomic number of Al there. It is a positive whole number denoted by letter z. For AL, it is 13. It is easy to find out the atomic number of an element using a periodic table because the elements are present in order of increasing atomic number.

identification of atomic number

(Source Credit: Wikipedia)

Last, if we know the isotope symbol of the element, we can find out the atomic number. There are many ways in which we can write the isotope symbol. For example, 14C or 146C are two ways to write the isotope of carbon. Either way, we can find out the atomic number as the isotope symbol will consist of the chemical symbol. To clarify,

  • If given 14C, then we know that the symbol represents the carbon atom. So, the atomic number is 6.
  • If given 146C, then the atomic number will be the whole number which is smaller of the two numbers. In this case, it is 6. The atomic number is usually the subscript of the element symbol.

Solved Examples for You

Question 1: If the Z of sodium is 11. Find the number of electrons and the number of protons present in a sodium atom.

Solution: We know that “Z = No. of Protons = No. of Electrons”

Thus, the No. of electrons=11 and No. of protons =11

Question 2: What is the Z of chlorine?

  1. 18
  2. 19
  3. 17
  4. 16

Solution: The correct answer is “C”. The Z of chlorine is 17 because the number of protons in a chlorine atom is 17.

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