What is Atomic Number?

Elements are the building blocks of all the matter that exist in the universe. Every element is made up of atoms that have protons, electrons, and a nucleus. In order to know the difference between the elements, an atomic number is used. Every element has its own individual atomic number. The atomic number is a single most important characteristic of an atom.

Introduction to Atomic Number

The atomic number is defined as the number of units of protons in the nucleus. It is mentioned by the alphabet Z.

Atomic number=Number of protons

With the help of the periodic table, we can find the atomic number of the chemical element. For example, if an atom has an atomic number 6, the element is carbon.

Suppose if it is a neutral atom, that means it has an equal number of protons and electrons, then the negative charge and the positive charge balance each other. For that case, the electrons determine how one atom interacts with the other atom. The protons determine the chemical properties of the element.

Atomic number=Number of protons=Number of electrons

The atomic number is also equal to the number of electrons

The mass of a proton is higher than the mass of an electron, but the amount of charge is the same and opposite to each other. In an atomic nucleus, one proton can hold only one electron.

As every element has its own individual atomic number, this number indeed looks like a fingerprint of the element. An atom with the same atomic number but different mass numbers are known as an isotope.

The isotopic notation of an element is written as, _ZX^A, where Z is the atomic number and it is always written at the subscript and A is the mass number and it is written at the superscript.

 For example,

 ₂He⁴                     

Here, A=four and Z=two for the helium atom.

Some examples are, if the number of protons present in the nucleus of iodine is 53, then its atomic number is 53. If the atom has 1 proton and its atomic number is 1 and the element is hydrogen.

Atomic Mass

Atomic mass or atomic weight is the total mass of an atom. The mass is approximately equal to the number of protons and neutrons of the nucleus, while electrons give a little contribution to the mass. It is expressed in atomic mass units ( a.m.u) or Dalton. It is defined as the $\frac{1}{{12}^{th}}$ of the mass of a carbon atom.

1 a.m.u=1.67377 x 10^-27 kg

Mass Number

The number of protons and the number of neutrons in the nucleus determine the mass number of an element. Electrons give a small amount of contribution to the mass, but it is not considered while calculating.

Mass number=Protons +Neutrons

If we want to calculate the atomic number, we can easily subtract the number of neutrons from the mass number. The atomic number is always smaller than the mass number.

If we know the atomic number and mass number, we can find the number of protons, neutrons, and electrons in the neutral atom.

Atomic Number and Periodic table

The periodic table is a table of elements arranged in the increasing orders of the atomic numbers of the elements. The table starts with the lightest element hydrogen and ends with the heaviest element organesson. When we go through the periodic table horizontally( from left to right), we are adding one more proton to the nucleus, which is increasing the order of atomic numbers.

In the 1860s, Dmitri Mendeleev discovered the periodic law. This law helps in organizing the elements in the table. While arranging, some of the elements came out of sequence. At that period, scientists have no clear view of the interior structure of an atom and they did not discover the nucleus. They only use the atomic numbers as the guide for arranging the chemical element.

After the discovery of x-rays, this query was solved. In 1895, Wilhelm Rontgen discovered x-rays. It helps other scientists to do further research on Mendeleev’s periodic law. In 1913, English physicist H.G.J.Moseley studied the wavelength of x-rays emitted by the different elements.

Moseley hypothesized the modern periodic table and arranged all the elements on the basis of a number of protons in the nucleus of their atom. We can increase the atomic number by adding more elements.

The periodic table helps to find the atomic mass and number of electrons that the element has. The elements are arranged in families and periods. In each family, the element has similar properties. By using this table, we can find the similar chemical and physical properties of the element.

How to find Atomic number

• Case 1: If we know the number of protons in an atom of an element, we can easily find their atomic number.
• Case 2: If we know the name or symbol of the element, then by using the periodic table, we can identify the atomic number. For example, if we want to know atomic number of helium (He), first we spot the helium in the periodic table, and there we can identify the atomic number written on the table. The atomic number of helium is two.
• Case 3: If we know the isotopes symbols of the element, it is easy to find the atomic number. There are many ways we can write the isotope symbol.

For example, the isotope of carbon is ¹⁴C or ¹⁴C_6, the two ways which we can find the atomic number from the isotope symbol is,

• If we know the symbol of the element, then by the symbol itself we can identify the atomic number from the periodic table. For ¹⁴C, the symbol C means carbon and its atomic number is six.
• The atomic number is always written at the subscript of the symbol. For ¹⁴C₆ in this case, the atomic number is always a whole number which smaller while comparing both the numbers. Here, the atomic number is six for the carbon atom.

Chemical properties

Each and every element has unique chemical properties as a consequence of the number of electrons present in the neutral atom, which means its atomic number. The configuration of these electrons is based on the principle of quantum mechanics. The total number of electrons in the electron shell (outermost valence shell), is the key factor to finding the chemical bonding. Therefore, the chemical behavior of the element is defined by its atomic number.

Importance of Atomic number

• The atomic number helps to identify the name of the chemical element of an atom.
• The atomic number helps to determine the chemical and physical properties of the element.
• This forms the basis for the arrangement of elements in the periodic table. In the table, the elements are arranged in the increasing order of atomic number.

Common Mistakes

One should not confuse between atomic mass and mass number, both are different. Atomic mass is the total weight of the atom while the mass number is the sum of the number of protons and neutrons.

Context and Applications

This topic is the basic necessary topic for all the graduates particularly for bachelors of science (physics and chemistry).

Related Concepts

• Mass number.
• Atomic mass.
• Atomic model.

Practice Problems

Question 1: What is the atomic number of hafnium?

(a) 86

(b) 72

(c) 106

(d) 98

Answer: The correct option is (b).

Explanation: The atomic number of hafnium is 72. It comes under group A (IVb) elements in the periodic table.

Question 2: The proton number of gadolinium is 64. What is its atomic number?

(a) 64

(b) 56

(c) 93

(d) 123

Answer: The correct option is (a).

Explanation: In an atomic nucleus, the atomic number is always equal to the proton number. Hence, the atomic number of gadolinium is 64. It belongs to the 4f lanthanide series.

Question 3: The atomic mass of rhenium is 186.2. What will be the atomic number?

(a) 98

(b) 74

(c) 75

(d) 88

Answer: The correct option is (c).

Explanation: Rhenium is a very rare earth metal and it belongs to group 7 (VIIb) of the periodic table. Its atomic number is 75.

Question 4: The atomic number of promethium is 61. How many protons are there?

(a) 61

(b) 58

(c) 66

(d) 85

Answer: The correct option is (a).

Explanation: Promethium is a rare earth element and is a radioactive element. It belongs to the lanthanide series in the f block on the periodic table. The atomic number is 61, therefore the number of protons is 61.

Question 5: If the atomic number of an element is 61. What is the name of the element?

(a) 45

(b) 56

(c) 74

(d) 66

Answer: The correct option is (d).

Explanation: The element dysprosium is a rare earth metal of the lanthanide series in the periodic table. The atomic number is 66.