The periodic table is a tool which helps us to study about 118 elements whether natural or synthetic and their compounds in a systematic way.
History of classification of elements : Till 18th century only a few elements and their compounds were known. so, it was easier to remember them. But, from the beginning of 19th century, the rate of discovery of elements and their compounds became fast and hence, the classification of elements became essential so that we can easily study about the properties of elements and their compounds.
Several chemists tried to classify elements. Some of them are as follows :
1. Doebereiner’s Triads : In 1817, a German chemist Johann Wolfgang Doebereiner presented the first classification of elements. In this classification, he prepared several groups of three elements called ‘Triads’ having similar properties. He told that the atomic weight of the middle element was the average of the rest two elements.
Ex : 1st Triads : Li(7), Na(23) & K(39) , 2nd Triad : Ca(40), Sr(87.5) & Ba(137) etc.
2. Newland’s Law of Octaves : In 1865, an English chemist John Alexander Newland gave another idea of classifying elements. He showed that when elements are arranged in the increasing order of their atomic weights each 8th element has similar properties as the 1st one. This is known as his law of octaves.
We can see that Li & Na have similar properties and so on.
3. Lothar Meyer’s Curve : In 1865, a German chemist Lothar Meyer plotted a graph between atomic volumes and atomic weights of elements. He found that elements with similar properties occupy the similar places on the curve. For example, the alkali metals lie on the top points on the curve.
4. Mendeleev’s Periodic Table : The most effective and systematic classification of elements was presented by a Russian chemist Dmitri Ivanovich Mendeleev. He proposed a law which is known as Mendeleev’s periodic law stated as follows :
The properties of elements are periodic functions of their atomic weights.
He arranged elements according to their increasing atomic weights and found that elements with similar properties are repeated after certain intervals. The key points related to his periodic table are as follows:
- There are nine vertical columns which are called groups. These groups are denoted as 0, I, II, III,…….VIII.
- Each group except 0 and VIII has been divided in two sub-groups A and B.
- There are 7 horizontal rows which are known as periods.
Importance of Mendeleev’s periodic table : The periodic table developed by Mendeleev made several contributions to the study of chemistry. Some of its important contributions are as follows :
- Systematic study of chemistry : This periodic table made the study of elements and their compounds easy and simple. One can easily know the elements with similar properties by the help of groups. Also the periodicity in the properties of elements can be understood through the arrangement of elements in groups and periods.
Updated on 31 July 2019…
Electromagnetic radiations(EMR) : Charged particles on moving with acceleration produce alternating electrical and magnetic fields which transmit in the form of waves and are called electromagnetic radiations or electromagnetic waves. James Maxwell was the first scientist who described EMR in 1870.
Properties of EMR : The properties of electromagnetic radiations are as follows :
1. The electric and magnetic fields generated by oscillating charged particles are perpendicular to each other and also perpendicular to the direction of propagation.
2. EMR do not require any medium to travel i.e. they can travel even in vacuum.
3. Electromagnetic radiations constitute spectrum which has been divided into different regions and Each EMR lies in a particular region.
4. Electromagnetic radiations are expressed by the help of their characteristic properties as follows :
● Wavelength : The distance between two consecutive crests and troughs is called the wavelength. It’s denoted by λ(Lambda) and its unit is ‘m’.
● Frequency : The number of waves passing through a point per second is called frequency. It generally denoted by ν. Its unit is Hz or s–
● Wave number : The number of wavelengths per unit of length is called the wave number. It is denoted by ⊽.The unit of wave number is m-1.
Key point : If the velocity of light is c, wavelength is λ and frequency is ν; then c = νλ
Electromagnetic spectrum : When different types of electromagnetic radiations are arranged according to their decreasing frequencies or increasing wavelengths, then the arrangement is called the electromagnetic spectrum.
After the discoveries of electron, proton and neutron, scientists made efforts to understand their arrangement inside an atom. The arrangement of subatomic particles inside an atom is called an atomic model.
Thomson model : The British physicist J J Thomson presented his atomic model in 1898 which is also called watermelon model or raisin pudding model. According to this model :
1. The positive charge is uniformly distributed inside an atom.
2. The negative charge i. e. electrons are distributed in such a way that the atom becomes neutral.
Limitations of Thomson model : Thomson model was able to describe the neutrality of an atom but it didn’t answer many other questions like –
Why do only electrons involve in a chemical reaction?
Why are there electric and magnetic fields of an atom?
Rutherford’s nuclear atomic model : Rutherford presented his model in 1911 on the basis of his alpha scattering experiment.
Experiment : In this experiment a very thin (about 100 nm) gold foil was taken and it was bombarded with α particles. Gold foil was surrounded by a fluorescent screen of ZnS. A tiny flash of light is observed when α particles strike the screen.
Observations : In the experiment Rutherford made following observations :
I. Maximum α-particles pass through the foil undeflected.
II. A few α-particles are deflected with small angles.
III. A very few α-particles (1 in 20000) bounced back.
Conclusions : On the basis of above observations, Rutherford made some conclusions given below :
I. Most of α-particles pass without any deflection. It means that maximum space inside an atom is vacant.
II. Few α-particles are deflected. It means that there is a small positive charge in the centre of an atom and α-particles face repulsion.
Bohr’s atomic model for Hydrogen: In 1913, Neils Bohr presented his model for Hydrogen atom on the basis of his research and experiments. The postulates of his atomic model are as follows:
1. The electron moves on a circular path called an orbit. Orbits can also be called stationary states or energy states because they have fixed energy. All orbits are concentric and have fixed radius with nucleus as the centre.
2. In an orbit, the energy of an electron is fixed. But, when it jumps from the lower energy state to the higher one, it absorbs energy. It loses energy when it falls from the higher energy state to the lower one.
Quantum mechnical model : To answer many questions which were not entertained by Bohr’s model, another model was presnted by Shrodinger which is called Quantum mechanical model. The postulates of this model were as follows :
1. Electrons show dual nature i.e. particle and wave nature.
2. To find the correct location and velocity of an electron simultaneously is not possible.