Everything in this world is made up of elements. Air is a mixture of elements like oxygen and nitrogen. Our body has been made by the chemical combination of carbon, oxygen, hydrogen, nitrogen and many other elements.

All the material things are made up of elements. About 109 elements have been discovered so far, out of these nearly 90 elements occur naturally while the remaining elements have been produced artificially. Hydrogen, oxygen, nitrogen, carbon, iron, copper, calcium and sulphur are examples of the naturally occurring elements.

Concept of John Dalton:

According to an English chemist, John Dalton, matter is composed of extremely tiny, indivisible particles called atoms. Atoms of an element are all identical.

Further Developments:

Later on, John Dalton’s ideas were proved to be wrong. The experiments performed by the scientists during the late nineteenth century showed that atom is divisible and it contains a number of tiny particles. Every atom is composed of three fundamental particles known as electron, proton and neutron.Electron is a very light particle and it is negatively charged. A proton is 1836 times heavier than an electron and is positively charged. Neutron has no charge and its mass is almost equal to that of a proton.

Rutherford’s Model:

Lord Rutherford performed a number of experiments to find out the structure of an atom. He suggested that an atom has a very small, very dense, central part known as `Nucleus’. A nucleus consists of protons and neutrons and almost the whole mass of an atom is present in it. The electrons are revolving around the nucleus in different orbits.

An electrically neutral atom has as many number of electrons revolving outside the nucleus as there are protons present inside it.


All the atoms are composed of the same fundamental particles, electron, proton and neutron. So the total mass of an atom will depend upon the number of electrons, protons and neutrons present in it. Greater the number of these particles in an atom, greater will be the mass of that atom. For example, hydrogen atom has only one proton and one electron present in it, so it is the lightest element known. An oxygen atom is heavier than hydrogen since it has eight protons, eight neutrons and eight electrons, we can say that more the number of fundamental particles an atom will have, the heavier it will become.

Since an electron is a very light particle as compared to proton or neutron, so the actual mass of an atom depends mostly on the number of protons and neutrons present in its nucleus. The total number of protons and neutrons in the nucleus of an atom is called its atomic mass or mass number and is

represented by A.


The number of protons present in the nucleus of an atom is called its atomic number or proton number and is represented by Z. Each element has its own atomic number and it is different from the other element. So atomic number serves as a tool to identify elements. Since an atom is neutral so the total number of negatively charged electrons present must be equal to the total number of positively charged protons.

Hydrogen is the lightest element with one proton present in its nucleus. It is the only atom that does not have any neutron in its nucleus. Its atomic number is 1. Helium atom has two protons present in its nucleus hence, its atomic number will be 2. Similarly, atomic number of carbon is 6 because it contains six protons in its nucleus. ,

Complete Idea of Atomic Structure:

The atomic mass (A) and the atomic number (Z) of an element can give us a complete picture of its atomic structure.

Atomic mass (A) of oxygen is 16 and its atomic number (Z) is 8. In other words, there are 8 protons present in its nucleus and it will have 8 electrons revolving around the nucleus. The number of neutrons present in its nucleus can be found out as follows:

Atomic mass (A) = Number of protons (Z) + Number of neutrons (N)

Number of neutrons (N)           = Atomic mass (A) —Number of protons (Z)

= A — Z = 16 — 8 = 8

It shows that there are eight protons, eight neutrons and eight electrons present in an atom of oxygen.



As you know, electrons     are

continuously revolving around the nucleus of an atom. The paths of their movements are known as shells or energy levels.- These shells are designed as K,’ L, M, .. . and so. on.. They are present at ‘different distances from the nucleus and can have different number of electrons as well. The first shell is known as K. • It is closest to the nucleus and can accommodate only 2 electrons. The seed d shell is called L-shell and can hold upto 8 electrons. The third shell is known as .M-shell and . it can also accommodate upto 18 electrons.

The electrons fill the K-energy level first as it is closer to the nucleus and has the lowest energy. When this is completely filled up, then the remaining electrons will enter the higher level’s L, M arid so on.

The following table shows the arrangement of electrons of the first 10 elements.




Dalton’s atomic theory considered all the atoms of an element exactly alike have no difference in their masses and properties. Later on, it was found that all the atoms of an element may or may not be alike. The atoms of any element may have different atomic masses. This is because the nuclei of these atoms may have the same number of protons but :different number of neutrons. Thus, we , can say that “the atoms of an element can have same atomic number but different atomic masses.”

For example, element of hydrogen comprises three kinds of atoms. The ‘lightest one has one proton in its nucleus and is knoWn as protium. Deuterium is the heavier atom. of hydrogen with one proton and one, neutron present in its nucleus. Tritium being the heaviest one consists- of one proton and two neutrons in its nucleus. Ordinary hydrogen contains protium atom in very large excess whereas deuterium and tritium are present in very small amounts. The isotopes of an element have similar chemical properties but show . different physical. properties.


All the elements are made up of atoms. – When atoms of different elements combine together. chemically, new compounds are formed. We use ..these compounds in our daily life.

Valency:.                           .

Chemical combining capacity of anelement with other elements is called its valency.

When . :different atoms combine together, it is not necessary that one atom of an element combines with one atom of another element. Atoms ‘Combine in different ratio. In order to know the ratio in which

atoms combine together, an atom of hydrogen (being the simplest one) has been chosen as a standard. Thus, the number of hydrogen atoms that may combine chemically with one atom of an element is known as the valency of that element. For example, lithium has a valency of 1 because it can combine with one atom of hydrogen. Similarly, beryllium can combine chemically with two , atoms of hydrogen; therefore, its valency is 2.

 Valency of Elements



ElementsValencyNumber of
electrons in


The valency of an atom is also related to the number of electrons present in its outermost shell, e.g., there is one electron in the outer shell of lithium and two electrons in the outer shell of beryllium thus the valencies of lithium and beryllium are 1 and 2, respectively. Similarly, boron and carbon have three and four electrons in their outermost shells. So they will shOw 3 and 4valencies respectively. The outermost shell of nitrogen atom has five electrons. Its valency will be determined in a different way.

In order to find out its valency, we will see how many electrons it needs to complete its outermost shell. Since an atom can have at the most eight electrons in its outermost shell; so nitrogen atom will need three more electrons to complete its outermost shell. Hence, its valency will be 3. The fluorine atom possesses seven electrons in. its outermost shell. It needs only one electron to complete this shell. Its valency will, therefore, be one.