Periodic Table and Periodicity of Properties
CLASSIFICATIONS OF ELEMENTS
Q1. Give the brief introduction about elements and periodic table?
Ans. Before the nineteenth century only a few elements were known. These elements could be easily studied individually with the passage of time, many more elements were discovered more and more of their compounds were prepared. Study of these elements and compounds individually became more difficult. When we look at the history of the chemical sciences, we see that the move significant achievements are in the development of the periodic table. It also gives us information of periodic properties of the elements. Periodic table gives us a basic frame work to understand the chemistry of the elements. The periodicity is well understood from the periodic table. The present shape of periodic table was completed through the efforts of various scientists.
Q2. Write the role of various scientists to classified periodic table.
Ans. As the number of discovered elements increased, the need to classify them also increased, because it became more difficult to remember the characteristics of the elements individually. The early classifications were full of errors, but with the passage of time, the more précised classification was presented by various scientists. Dobriener a German scientists in 1820 classified the elements into triads (where each group has three members) afterward an English scientists new land in 1865 presented the concept of classifying the elements based on law of octaves. A very important classification was presented by a Russian chemist Mendeleeve in 1869 which was based on the atomic masses of the elements. And after the discovery of protons, the classification based upon the number of protons built up which finally resulted in a comprehensive periodic table called the modern periodic table. This classification made very easy the understanding and remembering the properties of elements.
THE PERIODIC TABLE
Q3. Define periodic table.
Ans. The table which provides a basic frame work to study the periodic behavior of physical and chemical properties of elements as well as their compounds is called periodic table.
Periods and Groups
Horizontal rows of the elements in the periodic table are called periods and vertical columns of the elements are called groups. There were 12 periods and 18 groups in Mendel eve’s periodic table. Mendel eve’s left many empty spaces in his table for undiscovered elements. Mendel eve’s also predicted the properties of undiscovered elements e.g. after calcium having atomic mass 40 there should be titanium with mass 48 and it should be below aluminum having atomic mass 27. But properties of titanium were similar to silicon having atomic mass 28. So Mendeleeve placed titanium below silicon and left an empty space below aluminum. He predicted that atomic mass of the undiscovered elements would be the average of the atomic masses of calcium and titanium i.e. (40 + 48) / 2 = 44. Later on in 1875 this element was discovered and named as gallium.
Q4. What is Modern Periodic Table?
Ans. In 1913, Mosley gave the concept of atomic number for the elements. Elements are arranged in increasing order of their atomic numbers. The present form of the periodic table is called “long form of the periodic table” because it has eighteen groups and seven periods instead of eight groups and twelve periods.
Groups:- vertical columns of elements in periodic table are called groups. There are total eight groups (I to VIII), each group is divided into two sub groups A and B. normal elements are placed in sub group A and transition elements in sub group B. now international union of pure and applied chemistry (IUPAC) has numbered the group I to XVIII.
Periods: – there are seven periods in the table first period contains only two elements (H & He) second and third periods contains eight elements each. Sixth period is the largest period. It contains 32 elements. Seventh period is incomplete. It contains 24 elements with atomic number 87 to 109. Two horizontal rows are at the bottom of the table. Each of them contains 14 elements. They belong to 6th and 7th periods. Elements in 6th period are called lanthanide and elements in 7th period are called actinides.
Blocks: – periodic table is divided into four blocks i.e. S, P, d and f-blocks. The elements which have valance electron in s-orbital are called s-block elements. Elements of 1A and IIA are s-block elements. The elements which have valance electrons in p-orbital are called p-block element IIIA to VIIA and VIIIA group elements are p-block elements. (Except Helium) the elements having valance electrons in d-orbital are called d-block elements. The elements of sub group B are d-block elements. The elements having valance electrons in f-orbital are called f-block elements. Lanthanides and actinides are f-block elements. S and P-block elements are called normal. D-block elements are called outer transition and f-block elements are called inner transition elements.
PERIODICITY OF PROPERTIES
Q5. What is meant by periodicity of properties?
Ans. The repetition of similar properties after regular intervals in the periodic table is called periodicity of properties. Physical and chemical properties of elements are periodic function of their atomic numbers. There are certain properties which are of atoms only.
(i) Atomic radius (ii) Ionization energy
(iii) Electron affinity (iv) Valency
(v) Electronegativity (vi) Shielding effect.
These atomic properties depend upon the location of the element in periodic table.
Q6. How atomic sizes are estimated?
Ans. The atomic sizes are estimated assuming that atoms are spherical. The atomic size of an element can be expressed as the radius of its atom and the atomic radius is the distance from the centre of the nucleus to the outer most shell of electrons in the atom of any element. The two factors affect the atomic radius:
- Nuclear charge on the atom
- Shielding effect.
Q7. Define Ionization Energy?
Ans. The amount of energy required to remove the most loosely held electron from an isoland gaseous atom in its state is called ionization energy. Ionization energy is measured in the units of electron-volt (ev) and 1ev per atom = 96.5 KJ / mole. The ionization energy depends on the nuclear charge. Atomic size and number of inner electron shells. The energy required to remove the first electron from an isolated gaseous atom in its ground state is called first ionization energy. The energy required to remove an electron from an isolated monopositive gaseous ion (M) is called second ionization energy. The ionization energies decrease in going form top to bottom in a group due to the nuclear charge increase, the atomic size increase and the number of inner electrons increase. The ionization energy of elements increases as we go from left to right in a period. It is due to the following reasons.
- The nuclear charge increases by one unit at each element.
- The added electrons enter into the same shell
- The shielding effect remains the same.
Q8. Define Electron Affinity.
Ans. The minimum amount of energy which is released or absorbed when an electron is added to an isolated neutral gaseous atom is its outer most shell to produce an anion is called electron affinity. Electron affinity is an energy releasing process. Electron affinity is measured in kilo jule per mole. The value of the electron affinity of elements decreases in going from top to bottom in a group due to the atomic size increases and the nuclear charge increases the value of electron affinity increase across a period from left to right.
Q9. Define Electronegativity.
Ans. The relative tendency of an atom to attract a shared pair of electrons towards itself is called its electronegativity. The value of the electronegativity of an element describes the ability of its atom to compete for electrons with another atom to which it is bonded. The most commonly used scale for measuring electronegativity is due to Pauling (1932) and Milliken (1934). Pauling assigned a value of 4.0 to the electronegativity of fluorine and calculated the values for other elements. Since electronegativity of other elements is calculated by comparing it with fluorine, therefore electronegativity is a ratio and has no unit. Electronegativity of elements decrease from top to bottom in a group and increases from left to right in a period.
Q10. Define Shielding Effect in atom.
Ans. The decrease in the attraction force exerted by the nucleus on the valance shell electrons due to the presence of the electrons lying between the nucleus and valance shell is called shielding effect. The electrons present in the outer most shell are called valance shell electrons. The magnitude of the shielding effect is greatly affected by the number of inner shell electrons. When we travel from left to right in a period no new shell is formed therefore shielding effect does not change in period. When we move down a group, the number of inner-shells electrons increase and hence the shielding effect also increase.
Q11. Define Noble Gases.
Ans. The gaseous elements of group 18 or zero groups are called noble gases. These are He, Ne, Ar, Kr,Xe and Rn. The outermost electronic configuration of He is 1S2. The outer most shell of all noble gases is completely filled. It is because of their stable configurations that noble gases do not show any chemical reactivity however, Xenon (Xe) shows few chemical reactions under some conditions.