All substances are present in the form of elements, mixtures, and compounds. Elements: Composed of the same type of atoms. So far 118 elements are known. Of these, 94 elements are found naturally. we will discussed Periodic classification of element
Need for classification:
Elements were categorized to make them easier to read and study in a chronological way.
Primitive Classification of Elements
(Dobereiner’s Law of Triads)
• In 1817 Wolfgang Döbereiner selected some groups of three elements and described those groups as triads.
• As a rule, if the three elements of the triad are arranged in increasing order of their atomic masses, then the atomic mass of the middle element is approximately the average of the atomic masses of the other two elements.
• This method was not successful because till then only three triples could be detected among the known elements.
Newland’s law of octaves
• In 1866, Newlands arranged the elements in order of increasing atomic mass and found that each eighth element had the same properties as the first element.
• This work started with the element hydrogen with the lowest atomic mass and ended with the 56th element thorium.
• He compared this repetition to the octave of music, so this theory is called Newlands’s octave theory.
• Example- The properties of lithium and sodium were similar in Newlands’ octave. Because sodium is the eighth element after lithium.
• This rule applies only to light metals such as calcium.
• The properties of the new elements did not match this table.
• Newlands put two elements together in the array. And also put some dissimilar elements together.
• Example – Cobalt and nickel were placed in the column where fluorine and chlorine were, but their properties are different from those two elements.
|Co and Ni
|Ce and La
Mendeleef’s Periodic Table
The main credit for the classification of elements goes to Dmitry Ivanovich Mendeleev. He contributed significantly to the early development of the periodic array of elements.
Mendeleev arranged the elements in his table on the basis of similarity in their basic properties, atomic mass and chemical properties.
The physical and chemical properties of elements are periodic functions of their atomic masses.
It has eight vertical columns called groups. And there are 6 horizontal rows called periods.
Achievements of Mendeleef’s periodic table- (significance of Meendeleef’s periodic table)
1) Sequential study of properties of elements Elements with similar properties were put together.
2) search for new elements omitted blanks for unknown elements; such as scandium (Sc), gallium (Ga) and germanium (Ge).
3) Without leaving the previous arrangement, inert gases could be placed in a separate group if they were detected.
- Status of isotopes not clarified.
- The location of hydrogen is not fixed.
Improper ordering of some elements according to atomic masses.
Modern periodic table
Moseley modified Mendeleev’s periodic law in 1913 to give the modern periodic law. According to this law, “the properties of elements are a periodic function of their atomic number.”
Atomic number was used instead of atomic mass for the classification of elements.
That is, the elements were placed in increasing order of their atomic number instead of their mass number.
In this way the defects of Mendeleev’s periodic table were removed and a new array was formed which is called the modern periodic table.
position of elements in the modern table
The modern periodic array has 18 vertical columns called ‘groups’. And 7 horizontal rows called ‘periods’.
Atoms of all elements of the same period have the same number of shells. The number of shells present in an element is equal to the number of its period.
Periodic Number = Number of Cells
All the elements present in a group have the same valence electron and valency.
The number and size of cells increases as you move from top to bottom in a group, and decreases in size as you move from left to right.
Metallic property increases from top to bottom and decreases from left to right.
Trends of the Modern Periodic Table
• The number of electrons present in the outermost shell of an atom is called valency.
Change in Valency
1) The valency remains the same as the number of valence electrons remains the same from top to bottom in the group. From left to right in a period, the valency first increases and then decreases because it depends on the number of electrons that are given or taken away.
The distance from the center of the nucleus of an atom to its last shell is called atomic size.
Change in Atomic Radius
1) Atomic size increases from top to bottom in the group as the number of shells increases.
2) Atomic size decreases from left to right in a period as the positive charge in the nucleus increases and the attractive force increases.
Metallic and Non-metallic character
Metallic property means the ability of an atom of an element to give up electrons.
1) The metallic property increases from top to bottom in the group because the electron-giving capacity increases.
2) Moving from left to right in a period, the metallic property decreases because of the decrease in the ability to lose an electron.
Non-metallic property means the ability of an atom to accept electrons.
1) Going from top to bottom in group-I, the metallic property decreases because the effective nuclear charge decreases, which reduces the ability to accept electrons.
2) The non-metallic property increases from left to right in the period because the tendency to accept electrons increases due to increase in the effective nuclear charge.