Atomic Number: Introduction

Atoms are the vital building blocks of all matter and are composed of protons, neutrons, and electrons. The atoms are electrically neutral, the number of positively charged protons is equal to the number of negatively charged electrons.

Subsequently neutrons do not affect the charge, the number of neutrons is not reliant on the number of protons and will differ even amongst atoms of the same element.

History of Atomic Number

In 1910, Henry Gwyn-Jeffreys Moseley gave the concept of atomic number that was evolved from historic research. While experimenting on several chemical elements with X-rays.

He observed the pattern formed by reflected rays. After this experiment, he discovered that the wavelength of the reflected X-rays decreased in a regular predictable pattern with the increase in atomic mass.

According to his hypothesis, the regular variation in wavelength from element to element was triggered by an increase in the positive charge on atomic nuclei in going from one element to the next-heavier element.

After the discoveries made by Moseley, another scientist named Dmitri Mendeleev in 1850 proposed a new considerate understanding of the periodic law.

Mendeleev said that the properties of elements differ in a regular and expectable pattern when the elements are organized according to their atomic masses.

His theory was somewhat correct as the periodic table on this basis had a most important flaw. Certain pairs of elements seem to be misplaced when arranged according to their masses.

Various difficulties disappear when the atomic number rather than atomic mass is used to construct the periodic table. An element`s chemical properties rest on the number and arrangement of electrons in its atoms.

The number of electrons in an atom, is determined by the nuclear charge. Thus, the number of protons in a nucleus governs the chemical properties of an element.

What is Atomic Number?

The atomic number is represented by the letter Z . It is defined as the number of protons in the nucleus of each atom of that element.

The one characteristic that makes each element unique compared to all other elements is the number of protons. The elements are different because of their atomic number.

For example, any atom with an atomic number of 8 (its nucleus contains 8 protons) is an oxygen atom, and an atom with a different number of protons would be a different element.

Figure below showing the periodic table that displays all of the known elements and is arranged in order of increasing atomic number. In the below figure of the periodic table, the atomic number is displayed above the elemental symbol.

Figure: The periodic table is classified elements by atomic number

For example, Hydrogen at the upper left of the table has an atomic number of 1. Each hydrogen atom has one proton in its nucleus. Subsequent on the table is helium, whose atoms have two protons in the nucleus. Lithium atoms have three protons, beryllium atoms have four protons, and so on.

Meanwhile atoms are neutral, the number of electrons in an atom is equal to the number of protons. Hydrogen atoms normally have one electron that occupies the space outside of the nucleus.

Helium having two protons, will have two electrons. The count of proton will always be equal to an atom’s atomic number. This value will not vary unless the nucleus decays or is been bombed.

Mass Number

Several experiments have shown that the majority of the mass of an atom is been concentrated in its nucleus that is composed of protons and neutrons.

The mass number is represented by the letter A. It is also defined as the total number of protons and neutrons in an atom.

Table below shows the data from the first six elements from the periodic table. If we take the example of helium here. The atomic number of helium is 2 so that means it has 2 protons in its nucleus. And the nucleus also contains 2 neutrons.

So, the mass number of helium is 4. So this concludes helium atom contains 2 electrons as the number of electrons is equal to the number of protons.

Another example taken here can be of Lithium that has three protons and four neutrons and here the mass number will be equal to seven.

Table: The atoms of the first six elements

Name	Symbol	Atomic Number (Z)	Protons	Neutrons	Electrons	Mass Number (A)
Hydrogen	H	1	1	0	1	1.01
Helium	He	2	2	2	2	4.00
Lithium	Li	3	3	4	3	6.94
Berylium	Be	4	4	5	4	9.01
Boron	B	5	5	6	5	10.18
Carbon	C	6	6	6	6	12.01

Atomic Number Calculation

Therefore, the formula can be made from this, by knowing the mass number and an atomic number of an atom we can determine the number of neutrons present in that atom by subtraction.

Number of neutrons = Rounded Mass number – Atomic number

So, this shows how can we calculate the atomic number and mass number of a given element.