Gist of the lesson:
Classification of elements:the arrangement of element in such manner that element with similar properties are grouped together while elements with dissimilar properties are separated .
Early attempt to classify elements:
He arranged the elements with similar properties in a group of three known as triad in such a manner that the atomic mass of the middle element was approximately the average of the other two elements
Only three triads were identified from the element known at that time .hence this classification was not useful.
NEWLAND’S LAW OF OCTAVES :
He arranged the element in the order of increasing atomic masses starting with hydrogen(least atomic mass ) and ended with thorium having atomic mass 56 . According to him ,the properties of every eighth element are similar to the first element . Iit was compared to music notation sa,re ,ga ,ma, pa ,da ,ni ,sa,and thus the name Newlands law of octaves(notes of music ).
1. It was applicable only for lighter element having atomic mass upto 40 amu ,i.e.upto calcium .
2. He believed that only 56 elements existed in nature but later on more element were discovered whose properties did not fit into Newland law of octaves.
3. Some elements having different properties were grouped together like cobalt and nickel have been placed with halogens .
Due to above limitations, Newland law of octave was rejected
MANDELEEV’S PERIODIC TABLE :
He arranged the elements in order of increasing atomic masses , similarity in physical and chemical properties of element . properties of hydrides and oxides of different element were studied and elements with similar properties were grouped together .
He classified the elements in table consisted of vertical columns called groups and horizontal rows called periods . there were 7 groups in table and group is subdivided into subgroups A and B except group 7 which has three sets of elements in 4th , 5th , 6th period.
LIMITATIONS OF MENDELEEV, PERIODIC TABLE :
1. Position of hydrogen was not assigned correctly .
2. No separate position has been given to isotopes of an element .
3. Some element having higher atomic mass are placed before the elements with lower atomic mass .
MODERN PERIODIC TABLE :
Mosely modified the Mandelleve’s periodic table by taking atomic number as the fundamental property instead of atomic mass.
Modern periodic table consists of 18 vertical columns known as group , and 7 horizontal rows known as periods .
Elements in group one are called alkali metal s.
Elements in group 2 are called alkaline earth metals .
Elements in group 17 are called halogens .
Group 18 element are called inert gasses or noble gases.
Significance of group in the periodic table is that an element in a group has same number of valance electron ,valency and thus identical chemical properties .
1ST PERIOD – 2 elments and is called very short period .
2nd PERIOD- 8 elements and are called short period .
3rd PERIOD – 8 elements and are called short period .
4th PERIOD – 18 elements and are called long period .
5th PERIOD – 18 elements and are called long period .
6th PERIOD – 32 elements and are called very long period .
7th PERIOD- incomplete period .
The number of shell present in the element indicates the period to which it belongs .
It is defined as the combing capacity of an atom of an element to acquire noble gas configuration. it is equal to the number of electron lost , gained or shared during the formation of chemical compound .
ATOMIC SIZE / ATOMIC RADII:
It is defined as the distance from the centre of the nucleus to the outer most cell of atom . it is generally expressed in pico meter ( pm) .
On moving down the group the atomic radii increases.
Because on moving down the group a new energy shell is added which increases the distance between the outermost electron and the nucleus . All though the nucleus charge also increases , but it is compensated by the additional shell being added thus , increasing the size of the atom .
Across the period the atomic radii decrease. Due to the increase nuclear charge , the pull on the electron increases and hence, they are pulled closer to the nucleus thus, decreasing the atomic size .
Oxides and its nature. Metal reacts with oxygen to form oxides by loss of electrons. These oxides on dissolution in water from bases.
REACTIVITY OF ELEMENTS.
Down the group reactivity of metal increases as the tendency to lose electron increases due to the increased atomic size .
Reactivity of non metals decreases down the group because of the increased atomic size and the tendency to gain electron decreases.
On moving across the period the reactivity first increases due to the decreased in the metallic character and increase in nonmetallic character.
FORMATIVE ASSESSMENT III
TIME: 1 HRS
Q.1what is the position of hydrogen in the modern periodic table? (1)
Q.2 where are the isotopes of the same elements having different atomic masses placed in the periodic table ? (1)
Q.3 An element M is the third group of the periodic table. Write the formula of its oxide? (1)
Q.4 What is the valency of magnesium with atomic no. 12 and chlorine with atomic no. 17? (1)`
Q.5what is the difference in number of shell in magnesium and sulphur? (1)
Q.6 on the basis of electronic configuration , how will you select (1)
i)the terminating member in a period .
ii) the chemically similar elements .
Q.7Give reason as to why the atomic radii of elements increase in a group while moving from top to bottom ? (2)
Q.8 element in a group of periodic table have similar chemical properties why ? (2)
Q.9 explain why atomic number is more important than atomic weight in determining chemical properties ? (2)
Q. 10 where in periodic table do we find :
i)elements classified as non metal .
ii) elements forming negative ions .
iii)elements with high melting points .
iv)elements forming positive ions . (2)
Q.11 in a group reactivity of metals increases while those of non metals decreases . Explain. (2)
Q.12 elements in a group of periodic table have similar chemical properties why (2)
Q.13 elements of group 18 are called zero group. Why? (2)
Q.14 write the electronic configuration of atoms of
A)potassium (K) B)argon (Ar) C)lithium (li) D)fluorine (F) E)chlorine (Cl) (5)
Q.15i)Why is potassium more reactive than lithium ?
ii)why is fluorine is more reactive than chlorine ?
iii)which is smaller in size Cl or Ar ?
iv)which is smaller in size Li or F ?
v)which is more electronegative F or Cl?
Q.16The atomic no. of an element is 17.
i)what is its valency?
ii) Whether it is a metal or non-metal?
iii) Whether it is bigger or smaller in size then an element of atomic no.18?
iv) What type of bonds it will form with elements of group 18?
v) How would its oxide behave with litmus solution? (5)
Q.1 an element has two electron in its M shell:
i) Identify the element. ?
ii) What type of ion will it form ?
iii) What will be the formula of its chloride ?
iv) Predict the solubility of its chloride ?
Q.2 which among the following elements whose atomic number are given below belong to the
same period ? give the reason 17,10,20,12,19,15
Q.3 element X with atomic 12 and element Y with atomic number 17 reacts with hydrogen to form
hydrides . Which of them is expected to have high melting points?
Q.4 why is position of hydrogen not justified in modern periodic table?
FORMATIVE ASSESSMENT IV
Q.1 Name the element with atomic number 12.
Q.2 Name a metal in making cans and a member of group14.
Q.3Name the most electronegative element in the periodic table.
Q.4 Name the horizontal rows in the periodic table .
Q.5 on moving across the period , atomic size of the element increase or decrease.
Q.6 who gave the classification on the basis of musical note .
Q.7Name two elements belonging to group one which can be cut with the help of knife .
Q.8 what name is given to the elements belonging to group 2 of the periodic table and why?
Q.9 Name the lustrous non metal having 7 valence electron .
Q.10 Name the highly reactive metal that give violet colour to flame.
Q.11 Name the gas used in coloured advertising lights having 2 valence electron .
DEBATE AND DISCUSSION
A) Drawbacks of Mendeleev’s and modern periodic table.
B) Achievements of Mendeleev’s and modern periodic table.
C) Advantages of modern periodic table in understanding chemistry.
1 Power point Presentation on the following topics:
1. Modern Periodic Table based on the similarity of properties of elements
2. Contribution by various Scientists towards the development of Periodic Table.
3. PERIODIC CLASSIFICATION
1.Making flash cards to study atomic numbers electronic configuration and other properties of elements.
2.Make an outline sketch of the Modern Periodic Table.
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