Periodic Classification Of Elements Class 10 Notes

[UPDATED] Periodic Classification Of Elements | Class 10 Science Chapter 5 Notes

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Periodic Classification of Elements Class 10 Notes

Classification:means identifying similar species and grouping them together. 
Lavoisier:divided elements into two main types known as metals and nonmetals. 

Doberiner’s Law of Triads: 

According to this law, “in certain triads (grout) of three elements) the atomic mass of the central element was the arithmetic mean of the atomic masses of the other two elements.” But in some triads all the threc elements possessed nearly the same atomic masses, therefore the law was rejected.

e.g., Atomic masses of Li, Na and K are respectively 7, 23 and 39, thus the mean of atomic masses of I St and 3rd element is

Limitations of Doberiner’s Triads:

He could identify only a few such triads and so the law could not gain importance. In the triad of Fe, Co, Ni, all three elements have a nearly equal atomic mass and thus does not follow the above law 6. 4. 

Newland’s Law of Octaves: 

According to this law “the elements are arranged in such a way that the eighth element starting from a given one has properties which are a repetition of those of the first if arranged in order of increasing atomic weight like the. eight note of musical scale.” 

Drawback of Newland’s law of Octaves: 

  • According to Newland only 56 elements exists in nature and no more elements would be discovered in the future But later on several new element were discovered whose properties did not fit into law of octaves.
  • In order to fit new elements into his table Newland adjust two elements in the same column, but put some unlike elements under the same column.
  • Applicable only upto Calcium
  • Properties of new elements couldn’t fit in it.
  • In some cases properties of the elements were not same as defined by octave. eg:
  • Fe was placed far away from Co and Ni.
  • Worked well only with lighter elements. Thus, Newland’s classification was not accepted.
Mendeleev arranged 63 elements known at that time in the periodic table. According to Mendeleev “the properties of the elements are a periodic function of their atomic masses.” The table consists of eight vertical column called ‘groups’ and horizontal rows called ‘periods’.

Merits of Mendeleev’s Periodic Table: 

(i) In some places the order of atomic weight was changed in order to justify the chemical and physical nature. 
(ii) Mendeleev left some gaps for new elements which were not discovered at that time. 
(iii) One of the strengths of Mendeleev’s periodic table was that, when inert gases were discovered they could be placed in a new group without disturbing the existing order.

Limitations:

  • No fixed position for hydrogen
  • No place for isotopes
  • No regular trend in atomic mass.
  • Co was placed before Ni.

Characteristics of the Mendeleev’speriodic table :

Its main characteristics are :

  • In the periodic table, the elements are arranged in vertical rows called groups and horizontal rows called periods. 
  • There are eight groups indicated by Roman Numerals I, II, III, IV, V, VI, VII, VIII. The elements belonging to first seven groups have been divided into subgroups designated as A and B on the basis of similarities. The elements that are present on the left hand side in each group constitute sub-group A while those on the right hand side form sub-group B. Group VIII consists of nine elements arranged in three triads. 
  • There are six periods (numbered 1, 2, 3, 4, 5 and 6). In order to accomodate more elements, the periods 4, 5, 6 are divided into two halves. The first half ofthe elements are placed in the upper left corners and the second half occupy lower right corners in each box.

Achievements of Mendeleev’s periodic table:

  • The arrangement of elements in groups and periods made the study of elements quite systematic in the sense that if the properties of one element in a particular group are known, those of the others can be easily predicted. 

Prediction of new elements and their properties : 

  • Many gaps were left in this table for undiscovered elements. However, the properties of these elements could be predicted in advance from their expected position. This helped in the discovery of these elements. The elements silicon, gallium, and germanium were discovered in this manner. 

Correction of doubtful atomic masses : 

  • Mendeleev corrected the atomic masses of certain elements with the help of their expected positions and properties.

Limitations of Mendeleev’s classification : 

    • He could not assign a correct position of hydrogen in his periodic table, as the properties of hydrogen resemble both with alkali metals as well as with halogens.
    • The isotopes of the same element will be given different positions if the atomic number is taken as basis, which will disturb the symmetry of the periodic table 
    • The 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.

Modern Periodic Law : 

This law was given by Henry Moseley in 1913. it states,“Properties of the elements are the periodic function of their atomic numbers”. 

Cause of periodicity : 

Periodicity may be defined as the repetition of the similar properties of the elements placed in a group and separated by certain definite gap of atomic numbers. The cause of periodicity is the resemblance in properties of the elements is the repetition of the same valence shell electronic configuration.

Modern Periodic Table:

Moseley proposed this modern periodic table and according to which “the physical and chemical properties of elements are periodic function of their atomic number and not the horizontal rows called “periods”. The groups have been numbered 1, 2, 3 …. 18 from left to right. 

  • The elements belonging to a particular group make a family and are usually named after the first member. In a group, all the elements contain the same number of valence electrons. 
  • In a period all the elements contain the same number of shells, but as we move from left to right the number of valence shell electrons increases by one unit. 
  •  Atomic Number – denoted by Z and equal to the no. of protons in the nucleus of an atom.
  • Modern periodic table contains 18 vertical columns known as groups and horizontal rows known as periods.
  • Elements in a group have the same number of valence electrons
  • No. of the shells increases as we go down the group.
  • Elements in a period have same number of shells.
  • Each period marks a new electronic shell getting filled.
  • No. of elements placed in a particular period depends upon the fact that how electrons are filled into various shells.
  • The maximum no. of electrons that can be accommodated in a shell depends on the formula where n is the no. of the given shell. e.g. K shell – 2 × (1) = 2 elements in the first period L shell – 2 ×(2) = 8 elements in the second period.
  • The position of the element in the periodic table tells about its reactivity.

Trends in Modern Periodic Table: Given by Henry Moseley in 1913.

 The trends observed in some important properties of the elements in moving down the group (from top to bottom of the table) and across a period (from left to right in a period) are discussed below : 
 
(i) Periodicity in Properties: The properties of elements depend upon the electronic configuration which changes along a period and down a group in the periodic table. The periodicity properties i.e., repetition of properties after a regular interval is due to similarity in electronic configuration.
 
(ii) Tendency to lose or gain electrons: Chemical reactivity of an element depends upon the ability of its atoms to donate or accept electrons.
 
(iii) Variations of a tendency to lose electrons down the group: Tendency to lose electron goes on increasing down the group. 
 Reason: It is due to the increase in the distance between the valence electrons and the nucleus as the atomic size increases down the group, the force of attraction between the nucleus and the valence electrons decreases, therefore, tendency to lose electron also increases down the group. 
 
(iv) Variation of a tendency to lose electrons along a period: It goes on decreasing generally along a period from left to right with a decrease in atomic size. 
 Reason: Due to a decrease in the atomic size, the force of attraction between the valence electrons and the nucleus increases and therefore, electrons cannot be removed easily. 
 
(v) Variation of a tendency to gain electrons down the group: It goes on decreasing down the group in general. 
 Reason: Due to increase in atomic size, the force of attraction between the nucleus and the electron to be added becomes less. 
 
(vi) Variation of a tendency to gain electrons along a period: It increases left to right in a period.
 Reason: It is due to a decrease in the atomic size which leads to an increase in the force of attraction between the nucleus and the electron to be added.
 
Metallic and non-metallic character: Group 1 to 12 are metals. Group 13 to 18 comprises non-metals, metalloids, and metals. 
 
Properties of Metals: 
(i) They are malleable. 
(ii) They are ductile. 
(iii) They are good conductors of heat and electricity. 
(iv) They have generally 1 to 3 valence electrons. 
(v) They have the same or less number of electrons in their outermost shell than the number of shells. 
(vi) They are mostly solids. 
 
Properties of Non-metals: 
(i) They exist in a solid, liquid, or gaseous state. 
(ii) Non-metals are generally brittle. 
(iii) They are non-conductors. 
(iv) They have 4 to 8 valence electrons. 
 
In short you can remember the given below trends which is important for the exam point of view
    • Valency: Valency may be defined as the combining capacity of the atom of an element with atoms of other elements in order to acquire the stable configuration (i.e. 8 electrons in the valence shell. In some special cases it is 2 electrons).
    • Atomic size: It refers to the distance between the center of nucleus of an isolated atom to its outermost shell containing electrons. The atomic radius decreases on moving from left to right along a period. This is due to an increase in nuclear charge which tends to pull the electrons closer to the nucleus and reduces the size of the atom.
  • In a group atomic size decreases from top to bottom due to an increase in a number of shells.
  • Metallic and non-metallic properties: In a period from left to right metallic nature decreases while the non-metallic character increases. In a group metallic character increases from top to bottom while non-metallic character decrease. 
  • Electronegativity: The relative tendency of an atom to attract the shared electron pair of electrons towards itself is called electronegativity. In a period from left to right, the value of electronegativity increases while in a group from top to bottom the value of electronegativity decreases.

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