Modern Periodic Table

Mendeleev made a successful effort in grouping elements in the form of his periodic table. He had many achievements, but there were many limitations in his periodic table as well.

Some limitations of Mendeleev’s periodic table are listed below: The position of hydrogen was not justified in Mendeleev’s periodic table. The discovery of isotopes revealed another limitation of Mendeleev’s periodic table. Although Mendeleev arranged the elements in the increasing order of their atomic masses, there were instances where he had placed an element with a slightly higher atomic mass before an element with a slightly lower atomic mass.

The limitations of Mendeleev’s periodic table forced scientists to believe that atomic mass could not be the basis for the classification of elements.

In 1913, Henry Moseley demonstrated that atomic number (instead of atomic mass) is a more fundamental property for classifying elements. The atomic number of an element is equal to the number of protons present in that element. Since the number of protons and electrons in an element is equal, the atomic number of an element is equal to the number of electrons present in a neutral atom.

Atomic number = Number of protons = Number of electrons

The number of protons or electrons in an element is fixed. No two elements can have the same atomic number. Hence, elements can be easily classified in the increasing order of their atomic numbers. In the light of this fact, Mendeleev’s periodic law was done away with. As a result, the modern periodic law came into picture.

Atomic Mass: The atomic mass of an element is the sum of neutrons and the protons present inside the nucleus of an atom.

Atomic mass = Number of protons + Number of neutrons

The modern periodic law states that the properties of elements are a periodic function of their atomic numbers, not their atomic masses.

The table that is obtained when elements are arranged in the increasing order of their atomic numbers is called the Modern Periodic Table orLong Form of the Periodic Table as shown in the figure.

The Modern periodic table

In the modern periodic table, which was prepared by Niels Bohr, elements are arranged in rows and columns. These rows and columns are known as periods and groups respectively. The table consists of 7 periods and 18 groups.

Do You Know? In the modern periodic table, hydrogen is placed above alkali metals because of a resemblance in their electronic configurations. However, it is never regarded as an alkali metal. This makes hydrogen a unique element.

If you look at the modern periodic table, then you will find that all elements in the same group contain the same number of valence electrons. Let us perform the following activity to understand better.

Activity 1: Look at group two of the modern periodic table. Write the name of the first three elements followed by their electronic configurations.

What similarity do you observe in their electronic configurations? How many valence electrons are present in these elements?

The first three elements of group two are beryllium, magnesium, and calcium. All these elements contain the same number of valence electrons in their electronic configuration. The number of valence electrons present in these elements is 2. On the other hand, the number of shells increases as we go down the group.

Again, if you look at periods in the modern periodic table, then you will find that all elements in the same period contain the same valence shell. Let us perform the following activity to understand better.

Activity 2: Look at the elements of the third period of the modern periodic table. Write the electronic configuration of each element and calculate the number of valence electrons present in these elements.

What do you observe from the given activity? Do these elements contain the same number of shells? How many valence electrons are present in these elements?

You will find that elements such as sodium, magnesium, aluminium, silicon, phosphorus, sulphur, chlorine, and argon are present in that period. All these elements contain the same number of valence shells, but they do not have the same number of valence electrons.

Name of the element	Electronic configuration (K, L, M)
Sodium	2, 8, 1
Magnesium	2, 8, 2
Aluminium	2, 8, 3
Silicon	2, 8, 4
Phosphorus	2, 8, 5
Sulphur	2, 8, 6
Chlorine	2, 8, 7
Argon	2, 8, 8

Thus, the number of electrons in the valence shell increases by one unit as the atomic number increases by one unit on moving from left to right in a period.

Let us calculate the number of elements that are present in the first, second, third, and fourth periods.

The number of electrons that a shell can hold can be calculated using the formula 2n². Here, n represents the number of shells from the nucleus. For example, n is equal to 1, 2, and 3 for K, L, and M shells respectively. Hence, the number of electrons that each of these shells can hold can be calculated by substituting the value of n in the given formula.

Number of electrons that K shell can accommodate = 2n²

                                                                                               = 

                                                                                               = 2

Hence, K shell can accommodate only 2 electrons and only two elements are present in the first period.

Similarly, second and third periods can accommodate 8 and 18 electrons respectively. Since their outermost shells can contain only 8 electrons, there are only 8 elements in both the periods.

Important Note:

The position of an element in the Modern Periodic Table tells us about its chemical reactivity. The valence electrons determine the kind and the number of bonds formed by an element.

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