Periodic classification class 10 notes
Elements are classified on the basis of similarities in their properties. At present, 118 elements are known to us, out of vihich 98 are naturally occurring while remaining are synthesised artificially by man. Main objective of classification of elements is to make the study of 112 elements more convenient, systernatic.and organised.
The arrangement of elements in such a way so that elements having similar properties reappear at a regular interval, is called periodic classification or periodic arrangement of elements.
Döbereiner grouped the elements into triads and said that the atomic weight and properties of the middle element is the average of atomic weights or properties of other two elements.
Newlands gave the law of octaves and told that when elements are arranged in increasing order of their atomic weights, every eighth element resembles in properties with the first one just like musical notes. But all these were unable to arrange all the known elements. The first impressive attempt in this direction was given by Mendeléev.
Mendeléev’s Periodic Table
Mendeléev studied the formula and properties of hydrides and oxides of different elements and on the basis of their comparative study, he gave a law called the Mendeleev’s periodic law.
It states that the properties of elements are the periodic function of their atomic masses or atomic weights i.e., if elements are arranged in increasing order of atomic weights, after a regular interval the elements having similar properties reappear.
Mendeléev’s periodic table is the tabular representation of Mendeleev’s periodic law. It contains vertical columns called groups and horizontal rows called periods. In this table, there were 8 groups and 7 periods. Mendeléev placed elements with similar nature in the sarne group with respect to atomic weight.
Characteristics of Mendeléev’s Periodic Table
To place certain elements into correct group from the point of view of their chemical properties. Mendeléev reversed the order of some pairs of elements. eg., cobalt (atomic mass 58.9) appeared before nickel (atomic mass 58.7)
Mendeléev left some gaps in his periodic table for new elements that had not been discovered at that time. e.g., Eka-boron, Eka-aluminium and Eka-silicon, the properties of which had been found similar to the scandium, gallium and germanium discovered later.
Advantages of Mendeléev’s Periodic Table
This table was found helpful
- for the study of elements conveniently.
- for the prediction of properties of new elements.
- for predicting the valency of the elements.
- for calculating actual atomic weight.
Limitations of Mendeléev’s Classification
- He could not assign a correct position to hydrogen in his table.
- Isotopes of all the elements posed a challenge to Mendeléev’s periodic law.
- Atomic masses do not increase in a regular manner in going from one element to the next. So, it was not possible to predict how many elements could be discovered between two elements.
- Some elements having similar properties had been placed in different groups like Cu and Hg; Ag and Tl; Au and Pt were ; Ag and TI; Au and Pt were placed separately. Similarly, some elements having different properties had been placed together. e.g., in group 8, block of three elements had been placed together but they differ in properties. Similarly, copper, silver and gold are placed with Inert, chemically dissimilar alkali metals in group 1.
- Metals and non-metals were not placed separately in this i periodic table.
Modern Periodic Table
It was given by British chemist Moseley in 1913 on the basis of his discovery that atomic number is the most fundamental property. It is a tabular form of modern periodic law, according to which, “the physical and chemical properties of the elements are the periodic function of their atomic numbers”. This table removed almost all the drawbacks of Mendeléev’s periodic table.
Numerous forms of periodic table have been devised from time to time. A modern version, so called long form of the periodic table of elements, which is based on the electronic configuration of elements, is the most convenient and widely used.
Characteristics of Long Form of Periodic Table
- The horizontal rows are called periods and the vertical columns are called groups.
- The groups are numbered from 1 to 18 and there are altogether 7 periods.
- Elements having similar outer electronic configurations in their atoms are arranged in the vertical columns, i.e., groups or families. That’s why elements of a group possess similar chemical properties.
- The period number corresponds to the highest principal quantum number (n) of the elements in the period and each period marks a new electronic shell getting filled.
- The first period contains 2 elements and the subsequent periods consist of 8, 8, 18, 18 and 32 elements respectively and seventh period is incomplete.
- In this form of the periodic table, 14 elements of both six and seventh periods (lanthanoids and actinoids respectively) are placed in separate panels at the bottom.
Characteristics of Periods.
- The number of valence electrons in elements increases from 1 to 8 on moving from left to right in a period.
- The elements in a period have consecutive atomic numbers.
- The valency of element increases from 1 to 4 and then decrease to 0 (zero)on moving from left to right in a period, with respect to hydrogen.
- Atomic size, electropositive nature, metallic nature, reducing nature of elements and basic nature of oxides all decrease from left to right in a period.
- Electronegative nature, non-metallic nature, acidic nature of oxides, ionisation potential all increase from left to right in a period. In a period, electron affinity also increases from left to right.
Characteristics of Groups
- All the elements of a group of the periodic table have the same number of valence electrons and hence, have almost similar chemical properties.
- Atomic size, electropositive nature, metallic nature, reducing nature of elements and basic nature of oxides all increase from top to bottom in a group.
- Electronegative nature, ionisation potential, electron affinity, non-metallic nature and acidic nature of oxides all decrease down a group with increasing atomic number.
- Reactivity of metals increases down the group but that of non-metals decreases down the group.
Trends in Modern Periodic Table (Periodic Properties)
The properties which are repeated at regular intervals are known as periodic properties. These properties show a regular order along a group and period.
It generally increases from 1 to 7 in a period with respect to hydrogen but with respect to oxygen, it first increases from 1 to 4 and then decreases to o.
For alkali metals (i.e., sodium, potassium, etc) it is 1, for alkaline earth metals (i.e., magnesium, calcium, etc) it is 2, for aluminium it is 3 and for nitrogen it varies from -3 to +5.
2. Atomic Size
It refers to the radius of an atom. It generally increases on moving down the group because new shells are being added as we go down the group. It decreases along a period from left to right. This is due to an increase in nuclear charge which tends to pull the electrons closer to nucleus and reduces the size of the atom. Thus, size of alkali metals is largest and that of halogens is smallest in a period. Size of noble gases is larger as compared to corresponding halogens.
It is the energy required to remove an electron from an isolated gaseous atom in its ground state. It generally increases along a period from left to right due to increase in effective nuclear charge but ionisation energy of group-2 elements (Be, Mg, Ca, Sr) is larger than the ionisation energy of group-3 (B, Al, Ga, In) elements.
Similarly, ionisation energy of group-15 elements (N, P, As) is larger than ionisation energy of group-16 (0, S, Se) elements because of the stable configuration of group-2 elements (ns) and group-15 elements ( ns2,np3) than that of group-3 elements (ns2,np1) and group-16 elements (ns2,np4) respectively. generally decreases along a group on moving downwards due to increase in atomic size.
4.Electron Gain Enthalpy
It is defined as the enthalpy change accompanying the process when an extra electron is added to neutral gaseous atom to convert it into an anion. The energy released during the process is called electron affinity (EA).
EA increases across a period from left to right but EA of group-2, group-15 and group-O is zero or positive. It decreases on moving down the group. It is highest for chlorine.
It is the tendency of an atom in a molecule to attract the shared pair of electrons towards itself. It increases regularly along a period from left to right and decreases on moving down a group. It is highest for fluorine.
It is the tendency of an element to form cation by the loss of electrons. It decreases along a period from left to right and increases in a group on moving downwards. Thus, metallic elements occupy the left hand columns of the periodic table.
It is the tendency of an element to form anion by the gain of electrons. It increases along a period from left to right and decreases in a group on moving downwards.
It is defined as mass per unit volume and increases on moving down a group as well as along a period from left to right but density of gold is higher than that of mercury. The order of density of steel, mercury and gold is steel < mercury – gold.