To understand the definition of atomic number, structure of an atom is important to understand.
What is an element?
Element is a key component of matter. In the early ages, Carbon, silver, gold, tin, lead, mercury, iron Copper, and sulphur were identified to be the only elements, which were considered unbreakable into simpler units by any chemical reactions. However, over time, more elements were discovered and the concept of the element being unbreakable changed. To date, 118 elements have been discovered and arranged in the periodic table.
According to modern definition, an element is a substance made up of the same types of atoms, has the same atomic number, and cannot be interconverted or divided into small substances by chemical reactions.
Structure of atom
Atom is made up of three main components i.e., proton, electron, and neutron.
- Protons are positively charged particles present in the center of an atom called the nucleus.
- Neutrons are also present in the nucleus but they do not carry any charge or are neutral.
- While electron is negatively charged particles that roam around the nucleus of an atom in a specific path called an orbit.
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| An atom consits of electons, protons and neutrons. |
Definition of atomic number
The atomic number of an element is the sum of the number of protons residing in the nucleus of an atom of that element. The atomic number is also known as the proton number and represented by the Z symbol. The standard symbol Z may come from the German word “Atomzahl”, where “Zahl” means “number”.
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| Atomic number representation of an atom. |
The atomic number is unique to each element because each element has the same atoms and each atom has the same number of protons present in the nucleus. So atomic numbers can also be used to identify the element.
Where is the atomic number in the periodic table?
In the periodic table, all the elements are arranged in ascending order of their atomic number. So, the atomic number is written above the symbol of an element in the periodic table. The atomic number of hydrogen and helium displayed in the periodic table are shown in the figure below.
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| Atomic number of hydrogen in periodic table. |
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| Atomic number of helium in periodic table. |
Video lesson
Examples of atomic number
Here are examples of the atomic number of some elements.
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Elements |
Symbols |
Number of protons |
Atomic number (Z) |
|
Hydrogen |
H |
1 |
1 |
|
Helium |
He |
2 |
2 |
|
Carbon |
C |
6 |
6 |
|
Oxygen |
O |
8 |
8 |
|
Fluorine |
F |
9 |
9 |
|
Sodium |
Na |
11 |
11 |
|
Sulphur |
S |
16 |
16 |
Read about atomic mass here.
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