In the previous section 2.20, we completed a discussion on the structure and electronic configuration of atoms. In this chapter, we will see Classification of elements and periodicity of properties
• By the beginning of the 19th century, more and more new elements were becoming known to scientists
• The number of known elements and the information about their compounds became very large
• So a need was felt to ‘organize the information’
• But a ‘method for organizing’ was not immediately known
• One method would be create groups based on properties
♦ That is, elements having similar properties could be put together in a group
• Another method would be to create groups based on atomic weights
♦ That is., elements having nearly equal atomic weights could be put together in a group
• Scientists tried various methods. Failure of a method did not stop them from pursuing the research. Because, failure of a method would point the direction towards a new approach
3. Now we can do the calculations:
(i) Take any group
(ii) Calculate $\mathbf\small{\frac{m_A+m_C}{2}}$
(iii) But $\mathbf\small{\frac{m_A+m_C}{2}}$ is the arithmetic mean of the first and last atomic weights
(iv) This arithmetic mean will be very close to the atomic weight (mB) of the middle element B
Let us see some examples:
Triad 1:
A. Lithium (Li); mA = 7
B. Sodium (Na); mB = 23
C. Potassium (K); mC = 39
$\mathbf\small{\frac{m_A+m_C}{2}=\frac{7+39}{2}=\frac{46}{2}=23}$
This arithmetic mean 23 is same as mB
Triad 2:
A. Calcium (Ca); mA = 40
B. Strontium (Sr); mB = 88
C. Barium (Ba); mC = 137
$\mathbf\small{\frac{m_A+m_C}{2}=\frac{40+137}{2}=\frac{177}{2}=88.5}$
This arithmetic mean 88.5 is very close to mB
Triad 3:
A. Chlorine (Cl); mA = 35.5
B. Bromine (Br); mB = 80
C. Iodine (I); mC = 127
$\mathbf\small{\frac{m_A+m_C}{2}=\frac{35.5+127}{2}=\frac{162.5}{2}=81.25}$
This arithmetic mean 81.25 is very close to mB
4. Also properties of the middle element B will be in between the properties of A and B
Let us write an example:
• Let A have a very high readiness to react with acids
• Readiness of C (to react with acids) will be very low
• Readiness of B will be half way between A and C
♦ That is., B will show less readiness than A and more readiness than B
5. The triads created by Dobereiner made a good impression among scientists
• The relationship between the members of the triads is known as Dobereiner's law of triads
• It states that:
The atomic mass of the middle element of a triad is the arithmetic mean of the atomic masses of the other two elements
6. But only a few elements obey this law. So it was not taken up further
1. In 1865, John Alexander Newlands arranged the elements in the increasing order of their atomic weights
• Let us number them as 1, 2, 3, . . .
• This is shown in fig.3.1 below:
2. Consider the element no.1
• Starting from this element no.1, the 8th element is element no.8
■ Properties of this element no.8
is similar to
the properties of element no.1
3. Consider the element no.2
• Starting from this element no.2, the 8th element is element no.9
■ Properties of this element no.9
is similar to
the properties of element no.2
4. Consider the element no.3
Starting from this element no.3, the 8th element is element no.10
■ Properties of this element no.10
is similar to
the properties of element no.3
5. So in general, we can write:
Consider any element. Starting from that element, the 8th element can be put together in a group with the starting element
• This is shown in the fig.b
6. This pattern can be continued for some more elements
• This is similar to the fact that, every 8th note resembles the 1st in octaves of music
• This relation discovered by Newlands is called the Law of octaves
6. But this law does not work for elements after calcium. So this law was not accepted
• Works of Russian scientist Dmitri Mendeleev and German scientist Lothar Meyer paved the way for the modern periodic law
• Mendeleev and Meyer worked independently
• Works of Mendeleev was published first. Also it had some additional information. Let us see the basic details about his works:
1. Mendeleev arranged elements in the increasing order of their atomic weights. Also he wanted 'elements with similar properties' to come together in groups
• This arrangement consists of horizontal rows and vertical columns
• The horizontal rows were called series and vertical columns were called groups
2. Mendeleev faced a problem
• The problem and the 'solution that he devised' can be written in 5 steps:
(i) Consider the table in fig.3.2 below:
• It consists of:
♦ Horizontal series: 1, 2, 3, . . .
♦ Vertical groups: 0, I, II, III, . . .
(ii) Consider the element iodine
• Based on it’s properties, iodine must be placed in the red rectangle shown in fig.3.2
• This is because, the elements (fluorine and manganese) already placed in that group has similar properties as iodine
(iii) But based on atomic mass, iodine does not fit there
(iv) Mendeleev gave more importance to properties
• He ignored the atomic mass. He assumed that, the atomic mass of iodine must have been wrongly calculated
(v) So he placed iodine at the red rectangle itself
3. Mendeleev faced another problem also
• The problem and the 'solution that he devised' can be written in 6 steps:
(i) Consider the two green rectangles. One is below aluminium and the other is below silicon
(ii) He could not find any elements that could be placed at those rectangles
(iii) So he boldly left those rectangles blank
• He said that, the 'elements that could be placed in those rectangles' were not yet discovered
(iv) He named the one below aluminium as Eka-Aluminium
• And the one below silicon as Eka-Silicon
(v) He even predicted the properties of those undiscovered elements
(vi) Later Gallium and Germanium were discovered
• Gallium has the properties very close to those predicted for Eka-Aluminium
• Germanium has the properties very close to those predicted for Eka-silicon
4. The table prepared by Mendeleev became a strong foundation for future developments
• It was first published in 1905
• He is often referred to as the Father of the Periodic Table
5. Mendeleev's Periodic Law states that:
The properties of the elements are the periodic function of their atomic weights
• In 1913, English scientist Henry Moseley conducted an important experiment
• Let us see the details of that experiment:
1. Various elements are forced to emit X-rays
2. The frequency ($\mathbf\small{\nu}$) of X-rays emitted by each element is recorded
3. Using the data, two graphs are plotted:
(i) The first graph is obtained by plotting $\mathbf\small{\sqrt{\nu}}$ against atomic number
(ii) The second graph is obtained by plotting $\mathbf\small{\sqrt{\nu}}$ against atomic mass
4. The first plot which uses atomic numbers, gives a straight line
• The second plot which uses atomic masses, does not give a straight line
■ Thus Moseley discovered that, atomic number is a more fundamental property of an element than it's atomic mass
• When atomic number was used instead of atomic mass, many problems that Mendeleev faced were solved with out the need for making any assumptions
• Based on Moseley's findings, Mendeleev's Periodic Law was modified
• The new law is known as the Modern Periodic law. It states that:
The physical and chemical properties of the elements are the periodic function of their atomic numbers
• We know that, atomic number is same as the 'number of protons' in the nucleus
• In a neutral atom, the 'number of protons' will be equal to the 'number of electrons'
• In the previous chapter, we saw that, the 'arrangement of electrons' follows a basic pattern:
♦ The orbitals with lower energies are filled first
• In the previous chapter, we saw these also:
♦ If the last electron is present in the s orbital, that atom will fall in the s-block of the periodic table
♦ If the last electron is present in the p orbital, that atom will fall in the p-block of the periodic table
♦ If the last electron is present in the d orbital, that atom will fall in the d-block of the periodic table
♦ so on . . .
• Thus the relation between the atomic number and the periodic table is clear
• We can write:
Arrangement of elements in the periodic table
depends on
Arrangement of electrons
which depends on
Number of electrons
which is same as
Number of protons
which is same as
Atomic number
• The most widely accepted one is called the Long form of the Periodic table
2. In the Mendeleev's periodic table, the horizontal rows were called Series
♦ In the Long form, the horizontal rows are called Periods
• In the Mendeleev's periodic table, the vertical columns were called Groups
♦ In the Long form, the vertical columns are called Groups or Families
3. Initially, the groups in the long form were numbered as: IA, IIA, IIIB, IVB, . . .
• But based on the recommendations of International Union of Pure and Applied Chemistry (IUPAC), the groups are now numbered from 1 to 18
Feature 1:
(i) Take any group
(ii) Take any one element in that group
(iii) Write down the last sub-shell of that element (s, p, d or f)
(iv) All elements in that group will be having that same last sub-shell
Feature 2:
(i) Take any group
(ii) Take any one element in that group
(iii) Write down the 'number of electrons' in it's last sub-shell
(iv) All elements in that group will be having the same 'number of electrons' in the last sub-shell
Feature 3:
(i) Take any period
(ii) Write down the 'number of that period'
(iii) Take any element from that period
(iv) The number of main-shells in that element will be equal to the 'number of that period'
• When a new element is discovered, it must be given a temporary name
• The IUPAC has given a set of rules so that, the temporary names can be easily assigned
• Let us see the details about those rules. We will write them in steps:
1. We will be required to assign temporary names for elements with atomic numbers:
101, 102, 103, . . .
2. To assign the temporary name, first write down the atomic number
3. For each digit in the atomic number, use the following code:
♦ In the place of '0', put 'nil'. The abbreviation of 'un' is 'n'
♦ In the place of '1', put 'un'. The abbreviation of 'un' is 'u'
♦ In the place of '2', put 'bi'. The abbreviation of 'un' is 'u'
♦ so on . . .
■ The full table is given below:
0 → nil (n)
1 → un (u)
2 → bi (b)
3 → tri (t)
4 → quad (q)
5 → pent (p)
6 → hex (h)
7 → sept (s)
8 → oct (o)
9 → enn (e)
4. After replacing the digits in order, write 'ium' at the end
5. Based on the above code, we can write the temporary name of any element (starting from 101)
• An example:
Temporary name of the element with atomic number 105:
(i) We have:
1 → un (u)
0 → nil (n)
5 → pent (p)
(ii) So temporary name = (un + nil + pent + ium) = Unnilpentium
(iii) Symbol for this temporary name = (u + n + p) = Unp
• Another example:
Temporary name of the element with atomic number 117:
(i) We have:
1 → un (u)
1 → un (u)
7 → sept (s)
(ii) So temporary name = (un + un + sept + ium) = Ununseptium
(iii) Symbol for this temporary name = (u + u + s) = Uus
• Another example:
Temporary name of the element with atomic number 120:
(i) We have:
1 → un (u)
2 → bi (b)
0 → nil (n)
(ii) So temporary name = (un + bi + nil + ium) = Unbinilium
(iii) Symbol for this temporary name = (u + b + n) = Ubn
6. Thus, when a new element is discovered, it first gets a temporary name and corresponding symbol
• Later, permanent names are suggested, based on the country/state where the element was discovered or to pay tribute to a notable scientist
• These suggested names are put to vote by the members of the IUPAC
• Thus a suitable name is finalized
• By the beginning of the 19th century, more and more new elements were becoming known to scientists
• The number of known elements and the information about their compounds became very large
• So a need was felt to ‘organize the information’
• But a ‘method for organizing’ was not immediately known
• One method would be create groups based on properties
♦ That is, elements having similar properties could be put together in a group
• Another method would be to create groups based on atomic weights
♦ That is., elements having nearly equal atomic weights could be put together in a group
• Scientists tried various methods. Failure of a method did not stop them from pursuing the research. Because, failure of a method would point the direction towards a new approach
Dobereiner's Triads
1. In 1829, Dobereiner classified some elements into groups called Triads
• Each group had three elements
• Those elements were written in increasing order of atomic weights
2. Let the three elements be A, B and C
• Let their masses be mA, mB and mC
• Since they were arranged in the increasing order of atomic weights, we can write:
mA < mB < mC
1. In 1829, Dobereiner classified some elements into groups called Triads
• Each group had three elements
• Those elements were written in increasing order of atomic weights
2. Let the three elements be A, B and C
• Let their masses be mA, mB and mC
• Since they were arranged in the increasing order of atomic weights, we can write:
mA < mB < mC
(i) Take any group
(ii) Calculate $\mathbf\small{\frac{m_A+m_C}{2}}$
(iii) But $\mathbf\small{\frac{m_A+m_C}{2}}$ is the arithmetic mean of the first and last atomic weights
(iv) This arithmetic mean will be very close to the atomic weight (mB) of the middle element B
Let us see some examples:
Triad 1:
A. Lithium (Li); mA = 7
B. Sodium (Na); mB = 23
C. Potassium (K); mC = 39
$\mathbf\small{\frac{m_A+m_C}{2}=\frac{7+39}{2}=\frac{46}{2}=23}$
This arithmetic mean 23 is same as mB
Triad 2:
A. Calcium (Ca); mA = 40
B. Strontium (Sr); mB = 88
C. Barium (Ba); mC = 137
$\mathbf\small{\frac{m_A+m_C}{2}=\frac{40+137}{2}=\frac{177}{2}=88.5}$
This arithmetic mean 88.5 is very close to mB
Triad 3:
A. Chlorine (Cl); mA = 35.5
B. Bromine (Br); mB = 80
C. Iodine (I); mC = 127
$\mathbf\small{\frac{m_A+m_C}{2}=\frac{35.5+127}{2}=\frac{162.5}{2}=81.25}$
This arithmetic mean 81.25 is very close to mB
4. Also properties of the middle element B will be in between the properties of A and B
Let us write an example:
• Let A have a very high readiness to react with acids
• Readiness of C (to react with acids) will be very low
• Readiness of B will be half way between A and C
♦ That is., B will show less readiness than A and more readiness than B
5. The triads created by Dobereiner made a good impression among scientists
• The relationship between the members of the triads is known as Dobereiner's law of triads
• It states that:
The atomic mass of the middle element of a triad is the arithmetic mean of the atomic masses of the other two elements
6. But only a few elements obey this law. So it was not taken up further
Newland's law of Octaves
• Let us number them as 1, 2, 3, . . .
• This is shown in fig.3.1 below:
 |
| Fig.3.1 |
• Starting from this element no.1, the 8th element is element no.8
■ Properties of this element no.8
is similar to
the properties of element no.1
3. Consider the element no.2
• Starting from this element no.2, the 8th element is element no.9
■ Properties of this element no.9
is similar to
the properties of element no.2
4. Consider the element no.3
Starting from this element no.3, the 8th element is element no.10
■ Properties of this element no.10
is similar to
the properties of element no.3
5. So in general, we can write:
Consider any element. Starting from that element, the 8th element can be put together in a group with the starting element
• This is shown in the fig.b
6. This pattern can be continued for some more elements
• This is similar to the fact that, every 8th note resembles the 1st in octaves of music
• This relation discovered by Newlands is called the Law of octaves
6. But this law does not work for elements after calcium. So this law was not accepted
Towards the modern periodic table
• Mendeleev and Meyer worked independently
• Works of Mendeleev was published first. Also it had some additional information. Let us see the basic details about his works:
1. Mendeleev arranged elements in the increasing order of their atomic weights. Also he wanted 'elements with similar properties' to come together in groups
• This arrangement consists of horizontal rows and vertical columns
• The horizontal rows were called series and vertical columns were called groups
2. Mendeleev faced a problem
• The problem and the 'solution that he devised' can be written in 5 steps:
(i) Consider the table in fig.3.2 below:
 |
| Fig.3.2 |
♦ Horizontal series: 1, 2, 3, . . .
♦ Vertical groups: 0, I, II, III, . . .
(ii) Consider the element iodine
• Based on it’s properties, iodine must be placed in the red rectangle shown in fig.3.2
• This is because, the elements (fluorine and manganese) already placed in that group has similar properties as iodine
(iii) But based on atomic mass, iodine does not fit there
(iv) Mendeleev gave more importance to properties
• He ignored the atomic mass. He assumed that, the atomic mass of iodine must have been wrongly calculated
(v) So he placed iodine at the red rectangle itself
3. Mendeleev faced another problem also
• The problem and the 'solution that he devised' can be written in 6 steps:
(i) Consider the two green rectangles. One is below aluminium and the other is below silicon
(ii) He could not find any elements that could be placed at those rectangles
(iii) So he boldly left those rectangles blank
• He said that, the 'elements that could be placed in those rectangles' were not yet discovered
(iv) He named the one below aluminium as Eka-Aluminium
• And the one below silicon as Eka-Silicon
(v) He even predicted the properties of those undiscovered elements
(vi) Later Gallium and Germanium were discovered
• Gallium has the properties very close to those predicted for Eka-Aluminium
• Germanium has the properties very close to those predicted for Eka-silicon
4. The table prepared by Mendeleev became a strong foundation for future developments
• It was first published in 1905
• He is often referred to as the Father of the Periodic Table
5. Mendeleev's Periodic Law states that:
The properties of the elements are the periodic function of their atomic weights
Moseley's Experiment
• In 1913, English scientist Henry Moseley conducted an important experiment
• Let us see the details of that experiment:
1. Various elements are forced to emit X-rays
2. The frequency ($\mathbf\small{\nu}$) of X-rays emitted by each element is recorded
3. Using the data, two graphs are plotted:
(i) The first graph is obtained by plotting $\mathbf\small{\sqrt{\nu}}$ against atomic number
(ii) The second graph is obtained by plotting $\mathbf\small{\sqrt{\nu}}$ against atomic mass
4. The first plot which uses atomic numbers, gives a straight line
• The second plot which uses atomic masses, does not give a straight line
■ Thus Moseley discovered that, atomic number is a more fundamental property of an element than it's atomic mass
• Recall that, Mendeleev used atomic mass as the basis for his table
• Based on Moseley's findings, Mendeleev's Periodic Law was modified
• The new law is known as the Modern Periodic law. It states that:
The physical and chemical properties of the elements are the periodic function of their atomic numbers
• So atomic number is an important property
• In a neutral atom, the 'number of protons' will be equal to the 'number of electrons'
• In the previous chapter, we saw that, the 'arrangement of electrons' follows a basic pattern:
♦ The orbitals with lower energies are filled first
• In the previous chapter, we saw these also:
♦ If the last electron is present in the s orbital, that atom will fall in the s-block of the periodic table
♦ If the last electron is present in the p orbital, that atom will fall in the p-block of the periodic table
♦ If the last electron is present in the d orbital, that atom will fall in the d-block of the periodic table
♦ so on . . .
• Thus the relation between the atomic number and the periodic table is clear
• We can write:
Arrangement of elements in the periodic table
depends on
Arrangement of electrons
which depends on
Number of electrons
which is same as
Number of protons
which is same as
Atomic number
1. Based on the atomic number, various forms of the periodic table have been proposed from time to time
2. In the Mendeleev's periodic table, the horizontal rows were called Series
♦ In the Long form, the horizontal rows are called Periods
• In the Mendeleev's periodic table, the vertical columns were called Groups
♦ In the Long form, the vertical columns are called Groups or Families
3. Initially, the groups in the long form were numbered as: IA, IIA, IIIB, IVB, . . .
• But based on the recommendations of International Union of Pure and Applied Chemistry (IUPAC), the groups are now numbered from 1 to 18
• If we take a closer look at the long form, we will see some interesting features. We will see the basic details about each of those features:
(i) Take any group
(ii) Take any one element in that group
(iii) Write down the last sub-shell of that element (s, p, d or f)
(iv) All elements in that group will be having that same last sub-shell
Feature 2:
(i) Take any group
(ii) Take any one element in that group
(iii) Write down the 'number of electrons' in it's last sub-shell
(iv) All elements in that group will be having the same 'number of electrons' in the last sub-shell
Feature 3:
(i) Take any period
(ii) Write down the 'number of that period'
(iii) Take any element from that period
(iv) The number of main-shells in that element will be equal to the 'number of that period'
Naming of elements with atomic numbers greater than 100
• The IUPAC has given a set of rules so that, the temporary names can be easily assigned
• Let us see the details about those rules. We will write them in steps:
1. We will be required to assign temporary names for elements with atomic numbers:
101, 102, 103, . . .
2. To assign the temporary name, first write down the atomic number
3. For each digit in the atomic number, use the following code:
♦ In the place of '0', put 'nil'. The abbreviation of 'un' is 'n'
♦ In the place of '1', put 'un'. The abbreviation of 'un' is 'u'
♦ In the place of '2', put 'bi'. The abbreviation of 'un' is 'u'
♦ so on . . .
■ The full table is given below:
0 → nil (n)
1 → un (u)
2 → bi (b)
3 → tri (t)
4 → quad (q)
5 → pent (p)
6 → hex (h)
7 → sept (s)
8 → oct (o)
9 → enn (e)
4. After replacing the digits in order, write 'ium' at the end
5. Based on the above code, we can write the temporary name of any element (starting from 101)
• An example:
Temporary name of the element with atomic number 105:
(i) We have:
1 → un (u)
0 → nil (n)
5 → pent (p)
(ii) So temporary name = (un + nil + pent + ium) = Unnilpentium
(iii) Symbol for this temporary name = (u + n + p) = Unp
• Another example:
Temporary name of the element with atomic number 117:
(i) We have:
1 → un (u)
1 → un (u)
7 → sept (s)
(ii) So temporary name = (un + un + sept + ium) = Ununseptium
(iii) Symbol for this temporary name = (u + u + s) = Uus
• Another example:
Temporary name of the element with atomic number 120:
(i) We have:
1 → un (u)
2 → bi (b)
0 → nil (n)
(ii) So temporary name = (un + bi + nil + ium) = Unbinilium
(iii) Symbol for this temporary name = (u + b + n) = Ubn
6. Thus, when a new element is discovered, it first gets a temporary name and corresponding symbol
• Later, permanent names are suggested, based on the country/state where the element was discovered or to pay tribute to a notable scientist
• These suggested names are put to vote by the members of the IUPAC
• Thus a suitable name is finalized
So we have completed a basic discussion on periodic table. In the next section, we will see how the periodic table is related to electronic configuration of the elements
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