Importance of chemical periodicity in Academic Curriculum

• Periodicity Definition in Chemistry
• What Is Periodicity on the Periodic Table?
• What is the reason for the periodicity of the elements?
• What is the law of chemical periodicity?
• Guide for students to solve thechemical periodicityproblem

Periodicity: Repetition of similar properties after a fixed interval. Exact meaning of Periodicity is a particular property is repeated in a system after at fixed regular interval. As for example, as in the case of pendulum, it will pass through its mean position after a fixed interval of time.

There is the repetition of similar property element after a fixed interval in periodic table. When we classified the elements and start to arrange all the elements according to their increasing atomic number, same properties of the elements repeat at a definite interval or we can say that we observe the similar property element after a definite interval so we have to arrange all the similar property elements at a same place.  In other words, we can define it as; similar properties of elements are observed when we arrange the elements in the interval of 2 or 8 or 8 or 18 or 18 and 32 (i.e. atomic number difference). Similar properties are observed when we arrange elements in Group and period by increasing atomic number, after a fixed definite interval, there is the repetition of similar properties. Similar properties like melting point, boiling point, density, atomic volume, valence etc., repeat after certain regular interval of time. As we place Potassium below sodium because after argon we found similar property element potassium, so we have to place sodium and potassium in same group. The periodicity or periodic trends like screening effect, effective nuclear charge, ionisation enthalpy, electro negativity etc.

We can discuss some of the periodic properties. The outer most shell electrons are attracted towards the nucleus of the atom but these electrons are repelled by inner shell of electrons. Due to this repulsion of electrons valence shell electrons feel less force of attraction from the nucleus, this decrease in nuclear force of attraction is known as screening effect or shielding effect. The value of screening effect depends upon the number of inner shell electrons i.e., higher value of inner electrons, higher the value of screening effect. Due to the screening effect, nuclear charge decreases. This decrease in nuclear charge is called effective nuclear charge (Z). The value of effective nuclear charge increase in a period (left to right).

Another periodic property is ionization enthalpy and ionisation potential (I.E. or I.P.).We know that electrons are attracted towards the nucleus, as electrostatic attraction between the nucleus and electrons is strong so energy is required to remove one electron from the valence shell. This amount of energy in gaseous isolated atom is called I.P. or I.E. In a group as atomic size decreases there is increase in ionization energy. Larger the atomic size, smaller is the value of ionisation enthalpy. Atoms having half filled or full filled orbitals are highly stable so large amount of energy is required to remove one electron from valence shell.

So we can say that all the properties of elements directly or indirectly change according to the electronic configuration i.e. dependent on the electronic configuration of the element.

In Modern periodic table we arrange the element according to the increasing order of atomic number and arrange the elements in group and period. In a group similar property element are present. Like chemical properties of the elements remain same in a group due to the same number of valence shell electron but there is gradual change in few physical properties of elements in a group due to the difference in their atomic size. In a period, due to the gradual change of electronic configuration from element to element. After a fixed interval of time, there is the repetition of similar electronic configuration i.e. same number of valence shell electrons. As K (19) have electronic configuration of 1s², 2s², 2p⁶, 3s²,3p⁶, 4s¹ having same configuration as Na (11) 1s², 2s², 2p⁶, 3s¹. So we have to place Na and K in a same group. The physical and chemical properties of the elements are periodic function of their atomic number. In modern periodic table there is arrangement of elements according to the increasing atomic number.

In modern long form of Periodic table, there are 18 groups and 7 periods. Most of them elements are metals and few of them are non-metal. Some of them are metalloids. Modern periodic table is divided into four blocks: s-block, p-block, d-block and f-block. After definite interval fixed element is placed below first element according to the similar properties.

eg. Q. With the help of atomic number write the element present in second period and thirteen groups.

Q. An element with atomic number 82, belong to which group of the modern periodic table.

Hint: first element of sixth period is Cs (caesium, atomic number 55), and this period have 32 elements so atomic number 82 element belongs to sixth period.

Q.  Which property is same in group of periodic table?

Hint: As elements in the periodic table arrange according to the increasing order of atomic number. Same group elements have similar electronic configuration i.e. same number of valence electrons.

Q. Find out the position of element having atomic number 114.

Q. The atomic number of the last member of seventh period of the extended form of periodic table.

Q. Name the elements which are present on the right side of the long form of periodic table.

Hint : on the right hand side s-block elements are present, which are Metals known as Alkali metals and Alkaline earth metals like Na, K, Ca etc.

Q. The electronic configuration of element A is 1s², 2s², 2p⁶,3s² and element B is 1s², 2s², 2p⁵ , the formula of the compound formed by A and B .

Hint : According to the electronic configuration the valency of A and B are 2 and 1 so the formula will be AB₂.

Q. The value of effective nuclear charge in the alkali metals from Na to Cs :___________.

Hint: The value of effective nuclear charge remains same, as there is increase in the number of shells in a group so increase in the value of screening effect.

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