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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 their unique atomic number that helps to distinguish between two different elements. Let us study further to know about its significance.

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History of Atomic Number

We all have heard about periodic tables and how the elements are arranged in a periodic table in a very informative order.  The history of periodic table dates back to late 1860’s when Dmitri Mendeleev first discovered the periodic law. This soon became a major organizing concept of chemical sciences. However, 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 present in the atomic nucleus.

This hypothesis forms the basis of atomic number. Each and every element has a unique number that represents the number of protons present within an atomic nucleus.

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Atomic Number

We know that an atom consists of electrons, protons, and neutrons. Atomic number is one of the fundamental properties of an atom. Each atom can be characterized by a unique atomic number. It is represented by the letter “Z.”

The total number of protons present in the nucleus of an atom represents the atomic number of a particular atom. Every atom of a particular element is composed of the same number of protons and therefore have the same atomic number. However, atoms of different elements have unique atomic numbers that vary from one element to the other.

An atom does not have any net charge and is thus electrically neutral. This means that the number of electrons will be equal to the number of protons present in an atom thereby making an atom electrically neutral.

“Atomic Number = No. of Protons = No. of Electrons”

For example, each atom of oxygen has 8 protons in their nucleus so the atomic number is 8. Similarly, the atomic number of carbon is 6 because all atoms of carbon have 6 protons in their nucleus.

Atomic numbers are whole numbers because it is the total number of protons and protons are generally units of matter. It ranges from 1 to 118. It starts with hydrogen and ends with the heaviest known element Oganesson (Og).

Theoretically, the 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.

Importance of Atomic Numbers

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

Examples of Atomic Numbers

It does not matter how many electrons or neutrons present in an atom. Atomic number is always determined by the number of protons. For example

  • If an atom has one proton then the atomic number is 1 and the element is hydrogen.
  • Every carbon atom will have 6 protons and the atomic number of carbon is 6.
  • Every cesium atom has 55 protons and hence the atomic number is 55.

Atomic numbers of few elements:

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

Finding the Atomic Number

It can be calculated on the basis of information given. Firstly, an atomic number can be calculated by the number of protons in an atom of an element.

Secondly, if an element name or symbol is given then it can be identified by using a periodic table chart. It is a simple positive and whole number and is denoted by the letter z. For example, if the symbol Al (aluminum) is given. If you search the periodic table you can see the atomic number of Al is 13. It is easy to find out in a periodic table because the elements are present in increasing order.

identification of atomic number

(Source Credit: Wikipedia)

Thirdly it can be identified by the isotope symbol. Isotope symbol can be written in many ways but the symbol is always given while writing an isotope. For example, if an isotope of carbon is given as 14C or 146C.

  • 14C- If the isotope is written this way we know that the symbol represents carbon atom so the atomic number of carbon atom is 6.
  • 146C- If the isotope is written this way the atomic number will be the whole number and smaller of the two numbers which is 6. It is usually the subscript of the element symbol.

Solved Examples for You

Question 1: If the atomic number of sodium is 11. Find out how many electrons and protons are present in a calcium atom.

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

Thus, number of electrons=11 and number of protons =11

Question 2: What is the atomic number of chlorine?

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

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

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