What is Atomic Number | How to Find it

The number of protons present in the nucleus of each atom of an element is called atomic number of an element. Generally, it is represented by the symbol “Z”.

The concept of the atomic number was first introduced by the English physicist Henry Moseley in 1913. He conducted an experiment using X-ray spectroscopy to find out a positive charge on the nucleus of an atom.

Moseley found that the frequency (or wavelength) of the X-rays emitted by different elements is directly related to the charge on the nucleus. He stated that an atomic number of an element is the number of unit positive charge carried by the nucleus of an atom.

Since the presence of a positive charge on the nucleus is due to the protons in the nucleus, and each proton carries one unit positive charge, therefore, atomic number of an element is equal to the number of protons present in the nucleus of its atom.

Since the atom is electrically neutral, the number of electrons present in an atom is equal to the number of protons found in the nucleus. Thus,

Atomic number (Z) of an element is equal to the

  • number of positive charge present in the nucleus of its atom.
  • number of protons present in the nucleus of its atom.
  • number of electrons in a neutral atom.

Importance of Atomic Number

The atomic number plays several important roles in chemistry:

(1) Identifies an Element: Atomic number helps to identify an element in the periodic table because every element has a unique atomic number. For example, hydrogen has an atomic number of 1, while helium has an atomic number of 2. Atomic number is a unique identifier to identify elements in the periodic table.

(2) Determines Chemical Properties: The atomic number plays a vital role in determining chemical behavior. The number of protons and the number of electrons in a neutral atom affects how an atom interacts with others. This is because chemical reactions mainly involve the outermost electrons (i.e. valence electrons).

(3) Organizes the Periodic Table: Elements in the periodic table are arranged in order of increasing atomic number, which makes a recurring patterns (periodicity) of chemical properties across elements. Elements with similar chemical properties are grouped together in vertical columns called groups. These patterns repeat at regular intervals as the atomic number increases. This periodic arrangement helps to predict the behavior and characteristics of elements based on their position in the table.

How to Find Atomic Number of an Element?

There are several ways by which you can find out the atomic number of an element. They are as:

(1) Locate the Element on the Periodic Table:

  • Use the periodic table to find the atomic number of an element by looking up the element symbol that you are interested in.
  • The atomic number is usually located at the top of the element’s symbol. This is always a whole number. For example, hydrogen (H) has an atomic number of 1, helium (He) has an atomic number of 2, lithium (Li) has 3, as you can see periodic table shown below. This number represents the number of protons in the nucleus of an atom of that element. If you are not able to see in the below picture, then zoom it.
Periodic table to find the atomic number of various elements.

You can confirm the atomic number of an element based on nearby the atomic number of elements. This is because the elements in the periodic table are arranged in increasing order of the atomic number. This sequential order helps in determining the atomic number of an element based on its neighboring elements.

For example, if you want to find out the atomic number of carbon element which is 6, then the element to the left should be boron whose atomic number is 5 and the element to the right should have nitrogen whose atomic number is 7. If this pattern holds true, you’ve correctly identified the atomic number of the element.

(2) Using Formula:

  • The atomic number (Z) is simply the number of protons in an atom of the element.
  • Z = number of protons.

(3) For Neutral Atoms:

  • The atomic number also tells you the number of electrons in a neutral atom.
  • In a neutral atom, number of electrons = number of proton.

Examples of Finding Atomic Numbers

Example 1: Hydrogen (H)

  • Step 1: Look up hydrogen in the periodic table. You will find that its atomic number is 1.
  • Step 2: This means that every atom of hydrogen has exactly 1 proton in its nucleus.
  • Step 3: Since hydrogen is neutral, it also has 1 electron.

Example 2: Carbon (C)

  • Step 1: Find carbon element on the periodic table. You will see that its atomic number is 6.
  • Step 2: This means that every carbon atom has 6 protons in its nucleus.
  • Step 3: In a neutral carbon atom, there are 6 electrons which orbits around the nucleus in different energy levels or orbits.

Example 3: Oxygen (O)

  • Step 1: Locate oxygen element in the periodic table. You will see that its atomic number is 8 in the above periodic table.
  • Step 2: This means that each oxygen atom contains 8 protons in its nucleus.
  • Step 3: In a neutral oxygen atom, there are also 8 electrons which revolves around the nucleus in different energy levels or orbits.

Atomic Number vs. Mass Number

The atomic number (Z) tells us the number of protons present in the nucleus of an atom, while the mass number (A) tells us the total number of protons and neutrons present in the nucleus of an atom. The mass number is used to calculate the atomic mass of an element.

Mass Number (A) = Number of Protons (Z) + Number of Neutrons (n)

Unlike the atomic number, the mass number can vary between atoms of the same element because of isotopes, which have different numbers of neutrons.

Key Points about Z

  • Atomic number is the number of protons present in an atom’s nucleus.
  • It is represented by the letter Z.
  • Every element has a unique atomic number.
  • All the atoms of a specific element have the same number of protons. Therefore, Z is also the same. For example, all boron atoms have the atomic number of 5, whereas all atoms of carbon atoms have 6 protons in their nucleus.
  • Atoms of distinct elements have different atomic numbers.
  • In the periodic table, distinct elements have been arranged in order of increasing atomic number.
  • In a neutral atom, Z is equal to the number of electrons. This is because the number of electrons (which are negatively charged) balances the number of protons (which are positively charged), making the atom electrically neutral.
Share your love