Periodic Classification of Elements

 

Periodic Classification of the Elements

 

Classification:

Grouping of elements into different classes is called periodic classification of elements. Elements are arranged into groups (columns) and periods (Rows )based on their properties.  This method is requires arranging the elements that are alike and separating the elements that are unlike.

Need of Classification :

 It helps us understand how different elements form different compounds. The number of elements discovered has increased, making it difficult to remember the behavior and properties of these elements. Hence it is important to classify elements according to their properties.

 

Dobereiner’s  law Triads:
Johann Wolfgang Dobereiner, a German chemist, classified the known elements in groups of three elements on the basis of similarities in their properties. These groups were called triads.

According to this law, when elements are arranged in order of increasing atomic masses, groups of three elements, having similar properties are obtained. The atomic mass of middle element of the triad is nearly  equal to the average of the atomic masses of the other two elements.
For Example Li (6.9),         Na (23),    K (39).

 

Characteristics of Triads:

1.   Properties of elements in each triad were similar.

2.   Atomic mass of the middle element was roughly the average of the atomic masses of the other two elements.

Limitations:

 Dobereiner could identify only three triads. He was not able to prepare triads of all the known elements.

 

 

Newlands’ Law of Octaves:

John Newlands’, an English scientist, arranged the known elements in the order of increasing atomic masses. He found that the physical and chemical properties of every first element is very similar to that every eighth element.  He compared this to the octaves found in music. Therefore, he called it is the ‘ Law of Octaves’. It is known as ‘ Newlands' law of octaves ’ .

Characteristics of Newlands’ Law of Octaves:

·       It contained the elements from hydrogen to thorium.

·       Properties of every eighth element were similar to that of the first element.

Limitations of Newlands’ law of Octaves:

1.   The law was applicable to elements up to calcium (Ca).

2.   In order to fit elements into the table, Newlands’ adjusted two elements like cobalt and nickel in the same slot.

3.   At the time of Newlands' only 56 elements were known in nature and Newlands' assume that no more elements will be discovered the future But, later on , several new elements were discovered, whose properties did not fit into the law of octaves.

4.   Newlands' law of Octaves worked well with lighter elements only.

 

 Mendeleev’s Periodic Table:

 

 D. I. Mendeleev, a Russian chemist, was the most important contributor to the early development of a periodic table of elements wherein the elements were arranged on the basis of their atomic mass and chemical properties.

Mendeleev use the following criteria to formulate his periodic law.

1.   All the elements were arranged on the basis of their atomic masses.

2.   He reacted all then known elements with hydrogen (to form hydrides) and oxygen (to form oxides)

 

On the bases of the above two criteria he formulated the law called as the Mendeleev’s law of the periodic table:

“The properties of the elements are periodic functions of their atomic masses.”

 

Characteristics of Mendeleev’s Periodic Table:

1.   Mendeleev arranged all the 63 known elements in increasing order of their atomic masses.

2.   The table contained vertical columns called ‘groups’ and horizontal rows called ‘periods’.

3.   The elements with similar physical and chemical properties came under the same groups.

4.   Mendeleev’s stated that the properties of elements are the periodic function of their atomic masses.

 

 

 

Achievements of Mendeleev’s Periodic Table:

 

1.   Through this table, it was very easy to study the physical and chemical properties of various elements.

2.   Mendeleev left some gaps in his periodic table.
He predicted the existence of some elements that had not been discovered at that time. Later on elements like scandium, gallium and germanium were discovered to fill these gaps.

3.   The Nobel gases like helium, neon and argon, which were discovered later, were placed in a new group without disturbing the existing order.

Limitations of Mendeleev’s Periodic Table : 

1.   No fixed positions were given to hydrogen in the Mendeleev’s periodic table.

2.   Positions of Isotopes of all elements was not certain according to Mendeleev’s periodic table.

3.   Atomic masses did not increase in a regular manner in going from one element to the next.

4.   Anomalous pairs of elements: Cobalt (Co) has higher atomic weights but was placed before Nickel (Ni) in the periodic table.

5.   He could not explain the cause of periodicity among the elements

 

Modem Periodic Table:

 

Henry Moseley, gave a new  property of elements, ‘atomic number’ and this was I adopted as the basis of Modem Periodic Table.

Modern Periodic table.

1.   Elements are arranged in increasing order in their atomic numbers.

2.   These are 7 period and 18 groups.

3.   Insert gases are placed in a separate group i.e. 18 group.

4.   All the transitional elements are placed in 3 to 12 groups.

5.   Metal and non metals are widely separated from one another.

6.   Hydrogen is given a special position in modern Periodic table

 

Modem Periodic Law: 

“Properties of elements are a periodic function of  their atomic numbers”.

Characteristics of the modern periodic table

or

Long form of periodic table

1.   The modem periodic table consists of 18 groups and 7 periods.

2.   Elements present in any one group have the same number of valence electrons.

3.   The number of shells increases as we go down the group.

4.   Elements present in any one period, contain the same number of shells.

5.   Each period marks a new electronic shell getting filled.

 

Trends in the Modern Periodic Table:

 

Valency:  

Valency of an element is determined by the number of valence electrons present in the outermost shell of its atom. In  simple terms the combining capacity of the atom is called as valancy.

Group: Valency of elements in a particular group is same.

Period: Valency of elements in a period first increases from one to four and then decreases to zero.

Atomic Size: 

 

Atomic size refers to the radius of an atom. It is the distance from the center of the nucleus to the outermost electron present in the atom.

Group: In a group, atomic size and radii increases from top to bottom.

Period: In a period, atomic size and radii decreases from left to right.

 

Metallic and Non-metallic Properties:

 

The tendency to lose electrons from the outermost shell of an atom, is called metallic character of an element.

The tendency to gain electrons from the outermost shell of an atom, is called non-metallic character of an element.

 

Note:

Groups in Modern Periodic Table

The modern periodic table contains 18 vertical columns known as groups.

Group 1 elements are known as alkali metals.
Group 2 elements are known as alkaline earth metals.
Group 15 elements are known as pnicogens.
Group 16 elements are known as chalcogens.
Group 17 elements are known as halogens.
Group 18  elements are known as noble gases.

 

 

 

 

Non- metallic character:- The tendency to gain electrons from the outermost shell of an atom is called non-metallic character of an element.

Ionization energy:-

It is defined as the energy required to remove an electron completely from an isolated gaseous atom of an element. The energy required to remove the first electron is called first ionization energy.

Variation in a group:- Ionisation energy goes on decreasing down a group.

Variation in a period:- It goes on increasing generally along a period from left to right.

 

 

Page no. 112

Q.1. Yes, Dobereiner’s  triads also exist in the column of Newland’s octaves.

E.G.; Li(Lithium), Na(Sodium), K(Potassium) is a triad of Dobereiner also exist in the coloumn of Newland’s octaves.

Q.2. See back.

Page no. 116

Q.1.   Elements                                    Oxides

        1. K (Potassium)                            K20

        2. C (Carbon)                                  C02

             3. Al (Aluminium)                          Al203

        4. Si (Silicon)                                   Si02

        5. Ba (Barium)                                Ba0

Q.2. (i) Scandium (EKKA- boron): Sc

         (ii) Germanium (EKKA- silicon): Ge

Q.3. Mendeleev use the following criteria for his periodic table:

 (i) Mendeleev arranged the elements on the basis of their fundamental property i.e., the atomic mass.

(ii)  Mendeleev concentrated on the compound formed by elements with oxygen(oxides) and hydrogen (hydrides). He selected hydrogen and oxygen as they are very reactive and formed compounds with most elements.

Q.4. Noble gases are very less reactive element. These gases were discovered very late, because they are very inert and placing them in a separate group, does not disturb the existing order put forward by Mendeleev.

Page no. 120

Q.1. Modern periodic table removes various anomalies of Mendeleev’s periodic table.

         Achievements of modern periodic table:- See back

Q.2. The two elements  that show the chemical reactions similar to magnesium are:-

(i) Beryllium

(ii) Calcium

Q.3. (i)

(i) Sodium

(ii) Hydrogen

(iii)Lithium

(ii)

Beryllium

Magnesium

Calcium

Helium

Neon

Argon

 

Q.4. (i) Lithium(Li), Sodium(Na), Potassium(K) atoms belong to same group and have same numbers of electrons in their outermost shells.

(ii) Helium(He) and Neon (Ne) both belong to last group and have their outermost shell filled.

Q.5. Among the first ten elements only Lithium and Beryllium are metals.

Q.6. Be and  Ga are excepted to be most metallic out of be and Ga . Ga is bigger in size and hence has greater tendency to lose electrons than Be. Therefore, Ga is more metallic than Be.

Excerise 121

Q.2. ‘X’ is a metal element and is in the same group as (B) Mg(Magnesium) metal.

Q.3.( a) Neon (2,8)

(b) Magnesium(2+8+2=12)

(c) Silicon (2,8,4)

(d) Boron(2,3)

(e) Carbon(2,4)

Q.4. (a) All elements of this column have 3 electrons in their valence shell like Boron.

(b) All elements of this column have 7 electrons in their valence shell like fluorine.

Q.5. (A) its atomic number is 17(equal) to total number of electrons i.e. (2+8+7=17)

(B) Electron configuration of these elements are:-

N=2,5        F= 2,7          P= 2,8,5             AR= 2,8,8

These properties of this element are similar to F (Fluorine) element as both have same number of valence electrons (electrons is the outermost shell)

Q.6. ‘A’ is an non metal because group 17 contain non metal.

(b) ‘c’ is less reactive than ‘A’ as non metallic properties decreases on moving down in a group.

(c) ‘c’ will smaller in size than ‘B’ as atomic size decreases as we move from left to right in a period.

(d) Anion will be formed by A because non metal form anions only.

Q.7. The electronic configuration of nitrogen and phosphorus

Elements

Nitrogen

Phosphorus

Atomic No.

7

15

Electronic configuration

2,5

2,8,5

  Nitrogen will be more electro-negative because due to increases in size the electro negativity of non metals goes on decreasing on moving down the group

Q.8. We can determine the position of any element in the modern periodic table be its electronic configuration on number of electrons in the outermost shell shows the group number and total number of shell of an atom shows the period to which it belongs.

E.g. Atomic number of solution=11

Electronic Configuration=2,8,1

It contains 1 electron in outermost shell. So it belongs to group 1 and total number of shell are 3 So, it belong to 3rd period.

Q.9 The electronic configuration of all the elements are given below.

Atomicno.

       1  20

            21

                38

 

Electronic confuration

 2,8,2

 

 

2,8,8,1

 

 

2,8,8,1

 

 

2,8,8,3

 

 

2,8,8,8,2

 

So the elements having atomic number 12 and 38 resemble with the properties of calcium as both the elements have some number of valence electrons as calcium.

 

Q10.

Mendeleev periodic table

1.   Elements are arranged in increasing order of their atomic masses.

2.   These are 6 period and 8 groups.

3.   Inert gases were not known at the time of Mendeleev.

4.   All the transitional elements are placed in 8 groups.

5.   Many metals and non metals are group together.

6.   No fixed position was given to hydrogen in Mendeleev periodic table.