In the previous section 3.11, we completed a discussion on the periodic trends in valence. In this section, we will see Anomalous properties of 2nd period elements. We will also see the periodic trends in chemical reactivity
1. Consider the table 3.8 below:
• It shows the elements of two periods: Period 2 and Period 3
• We see only the ‘first two elements’ of groups 1, 2, 13, 14, 15, 16, 17 and 18
• Note that, these groups constitute the s and p blocks of the periodic table
2. Take out the first element from each of those groups (except group 18):
♦ Li from group 1
♦ Be from group 2
♦ B from group 13
♦ C from group 14
♦ N from group 15
♦ O from group 16
♦ F from group 17
3. Let us analyze each element from the above set:
• Consider the first entry of the above set: Li of group 1
♦ We have learnt that all elements of a group will have similar properties
♦ But Li differs in many respects from the other members of the group 1
• Consider the next entry of the above set: Be of group 2
♦ We have learnt that all elements of a group will have similar properties
♦ But Be differs in many respects from the other members of the group 2
• Consider the next entry of the above set: B of group 13
♦ We have learnt that all elements of a group will have similar properties
♦ But B differs in many respects from the other members of the group 13
- - - -
- - - -
• Consider the last entry of the above set: F of group 17
♦ We have learnt that all elements of a group will have similar properties
♦ But B differs in many respects from the other members of the group 13
4. It is clear that, all first elements differ from their group members
■ Let us see some examples:
• Lithium mostly form covalent compounds
♦ But all other members of the group 1 forms ionic compounds
• Beryllium also mostly form covalent compounds
♦ But all other members of the group 2 forms ionic compounds
5. Here scientist noticed 3 interesting points:
(i) Behavior of Li is more similar to the second element of the following group
• Let us elaborate on this:
• Li belongs to group 1
♦ So the ‘following group’ is group 2
♦ The ‘second element’ of the group 2 is Mg
• So Li is more similar to Mg
• This is indicated by the yellow arrow in the table below:
(ii) Behavior of Be is more similar to the second element of the following group
• Let us elaborate on this:
• Be belongs to group 2
♦ So the ‘following group’ is group 3
♦ The ‘second element’ of the group 3 is Al
• So Be is more similar to Al
• This is indicated by the pink arrow in the table above
(iii) In the same way, B is more similar to Si
• Let us elaborate on this:
• B belongs to group 13
♦ So the ‘following group’ is group 14
♦ The ‘second element’ of the group 14 is Si
• So B is more similar to Si
• This is indicated by the blue arrow in the table above
6. The diagonal relationship:
• We see that, Li and Mg are diagonal elements
• Also, Be and Al are diagonal elements
• Again, B and Si are diagonal elements
■ So this type of similarity is commonly referred to as diagonal relationship in the periodic properties
1. Imagine that, we move along the period 2
• Let us move from Li to Be
2. We know that, the size of the atom decreases
• So Be will be smaller than Li
3. Now imagine that, from Be, we move downwards along the group 2
• Let us move from Be to Mg
4. We know that, the size of the atom increases
• Mg will be larger than Be
5. So we have two effects:
(i) ‘Decreasing size’ when moving horizontally towards the right from Li to Be
(ii) ‘Increasing size’ when moving downwards from Be to Mg
6. These two effects are opposite to each other and so cancel out
• The result of this ‘cancelling effect’ can be written in 3 steps:
(i) Imagine that we move in a ‘L-shaped’ path from Li to Mg (through Be)
(ii) We will find that the size decreases and then ‘increases back’ to nearly the original size
(iii) That means, the atomic size of Li and Mg are nearly the same
• We find that, it is indeed true:
♦ Metallic radii of Li, Be and Mg are 152, 111 and 160 pm respectively
♦ Note that, 160 is close to 152
7. Consider the next set: Be and Al
• Let us move in a ‘L-shaped’ path
♦ We start from Be and reach Al (through B)
♦ When B is reached, the size becomes small
♦ But when Al is reached, the size ‘increases back’ to nearly the size of Be
8. A similar analysis can be written in the case of the next set 'B and Si' also:
• Let us move in a ‘L-shaped’ path
♦ We start from B and reach Si (through C)
♦ When C is reached, the size becomes small
♦ But when Si is reached, the size ‘increases back’ to nearly the size of B
9. We will see more about 'diagonal relationship' when we discuss in detail about s and p block elements in later chapters
A. Physical properties:
(i) atomic radii
(ii) Ionic radii
(iii) Ionization enthalpy
(iv) Electronegativity
B. Chemical properties:
(i) Valence
(ii) Oxidation state
■ By now, we know that, the periodic trends is a result of this:
Systematic arrangement of elements on the basis of electronic configuration
• We have one more item to learn:
The chemical reactivity
• The four physical properties and the two chemical properties that we have seen, can be used to explain chemical reactivity
• We will write it in steps:
1. Recall the 4 horizontal arrows that we saw in fig.3.14 in an earlier section 3.9
• For convenience, that fig.3.14 is shown again below:
2. Out of the four arrows, two are important for our present discussion:
(i) The arrow for ionization enthalpy
(ii) The arrow for electron gain enthalpy
3. Consider the arrow for ionization enthalpy:
■ It indicates that:
In a period, the ionization enthalpy is least for the element at the extreme left end
• That means:
In a period, the element at the extreme left end can lose it’s electron in the most easiest way
• It follows that:
In a period, the element at the extreme left end can become a cation in the most easiest way
• It follows that:
In a period, the element at the extreme left end will have the highest chemical reactivity
♦ Chemical Reactivity is the ‘readiness to take part in chemical reactions’
4. Consider the arrow for electron gain enthalpy
■ It indicates that:
In a period, the electron gain enthalpy is 'highest negative' for the element at the extreme right end
• That means:
In a period, the element at the extreme right end can gain an electron in the most easiest way
• It follows that:
In a period, the element at the extreme right end can become an anion in the most easiest way
• It follows that:
In a period, the element at the extreme right end will have the highest chemical reactivity
5. So we have two extremes:
• One at the left end and the other at the right end
♦ Both have the highest reactivities
6. The 'variation of reactivity' can be written in 3 steps:
(i) The reactivity decreases (initially) as we go from left to right across a period
(ii) It reaches the 'lowest reactivity' at the center of the periodic table
(iii) After passing the center, it again increases and reaches the 'highest reactivity' at the extreme right end
7. We can write a note on oxidation and reduction also:
(i) Oxidation:
• 'Losing electrons' is oxidation
♦ That is., 'elements which lose electrons' are said to be oxidised
• The elements at the left easily loses electrons
♦ So we can say: Elements at the left are easily oxidised
(ii) Reduction:
• 'Gaining electrons' is reduction
♦ That is., 'elements which gain electrons' are said to be reduced
• The elements at the right easily gains electrons
♦ So we can say: Elements at the right are easily reduced
8. We can write a note on metallic and non-metallic character also:
(i) Metallic character:
• Metals are electron donors
♦ That is., metals lose electrons easily
• Elements at the left lose electrons easily
♦ So we can say: Elements at the left are more metallic
(ii) Non-metallic character:
• Non-Metals are electron acceptors
♦ That is., non-metals accept electrons easily
• Elements at the right accepts electrons easily
♦ So we can say: Elements at the right are more non-metallic
1. Consider the table 3.8 below:
 |
| Table 3.8 |
• We see only the ‘first two elements’ of groups 1, 2, 13, 14, 15, 16, 17 and 18
• Note that, these groups constitute the s and p blocks of the periodic table
2. Take out the first element from each of those groups (except group 18):
♦ Li from group 1
♦ Be from group 2
♦ B from group 13
♦ C from group 14
♦ N from group 15
♦ O from group 16
♦ F from group 17
3. Let us analyze each element from the above set:
• Consider the first entry of the above set: Li of group 1
♦ We have learnt that all elements of a group will have similar properties
♦ But Li differs in many respects from the other members of the group 1
• Consider the next entry of the above set: Be of group 2
♦ We have learnt that all elements of a group will have similar properties
♦ But Be differs in many respects from the other members of the group 2
• Consider the next entry of the above set: B of group 13
♦ We have learnt that all elements of a group will have similar properties
♦ But B differs in many respects from the other members of the group 13
- - - -
- - - -
• Consider the last entry of the above set: F of group 17
♦ We have learnt that all elements of a group will have similar properties
♦ But B differs in many respects from the other members of the group 13
4. It is clear that, all first elements differ from their group members
■ Let us see some examples:
• Lithium mostly form covalent compounds
♦ But all other members of the group 1 forms ionic compounds
• Beryllium also mostly form covalent compounds
♦ But all other members of the group 2 forms ionic compounds
5. Here scientist noticed 3 interesting points:
(i) Behavior of Li is more similar to the second element of the following group
• Let us elaborate on this:
• Li belongs to group 1
♦ So the ‘following group’ is group 2
♦ The ‘second element’ of the group 2 is Mg
• So Li is more similar to Mg
• This is indicated by the yellow arrow in the table below:
 |
| Table 3.9 |
• Let us elaborate on this:
• Be belongs to group 2
♦ So the ‘following group’ is group 3
♦ The ‘second element’ of the group 3 is Al
• So Be is more similar to Al
• This is indicated by the pink arrow in the table above
(iii) In the same way, B is more similar to Si
• Let us elaborate on this:
• B belongs to group 13
♦ So the ‘following group’ is group 14
♦ The ‘second element’ of the group 14 is Si
• So B is more similar to Si
• This is indicated by the blue arrow in the table above
6. The diagonal relationship:
• We see that, Li and Mg are diagonal elements
• Also, Be and Al are diagonal elements
• Again, B and Si are diagonal elements
■ So this type of similarity is commonly referred to as diagonal relationship in the periodic properties
• Let us see the reason for this type of a similarity. We will write it in steps:
• Let us move from Li to Be
2. We know that, the size of the atom decreases
• So Be will be smaller than Li
3. Now imagine that, from Be, we move downwards along the group 2
• Let us move from Be to Mg
4. We know that, the size of the atom increases
• Mg will be larger than Be
5. So we have two effects:
(i) ‘Decreasing size’ when moving horizontally towards the right from Li to Be
(ii) ‘Increasing size’ when moving downwards from Be to Mg
6. These two effects are opposite to each other and so cancel out
• The result of this ‘cancelling effect’ can be written in 3 steps:
(i) Imagine that we move in a ‘L-shaped’ path from Li to Mg (through Be)
(ii) We will find that the size decreases and then ‘increases back’ to nearly the original size
(iii) That means, the atomic size of Li and Mg are nearly the same
• We find that, it is indeed true:
♦ Metallic radii of Li, Be and Mg are 152, 111 and 160 pm respectively
♦ Note that, 160 is close to 152
7. Consider the next set: Be and Al
• Let us move in a ‘L-shaped’ path
♦ We start from Be and reach Al (through B)
♦ When B is reached, the size becomes small
♦ But when Al is reached, the size ‘increases back’ to nearly the size of Be
8. A similar analysis can be written in the case of the next set 'B and Si' also:
• Let us move in a ‘L-shaped’ path
♦ We start from B and reach Si (through C)
♦ When C is reached, the size becomes small
♦ But when Si is reached, the size ‘increases back’ to nearly the size of B
9. We will see more about 'diagonal relationship' when we discuss in detail about s and p block elements in later chapters
Chemical reactivity
So far in this chapter, we have seen the periodic trends in the following items:(i) atomic radii
(ii) Ionic radii
(iii) Ionization enthalpy
(iv) Electronegativity
B. Chemical properties:
(i) Valence
(ii) Oxidation state
■ By now, we know that, the periodic trends is a result of this:
Systematic arrangement of elements on the basis of electronic configuration
• We have one more item to learn:
The chemical reactivity
• The four physical properties and the two chemical properties that we have seen, can be used to explain chemical reactivity
• We will write it in steps:
1. Recall the 4 horizontal arrows that we saw in fig.3.14 in an earlier section 3.9
• For convenience, that fig.3.14 is shown again below:
2. Out of the four arrows, two are important for our present discussion:
(i) The arrow for ionization enthalpy
(ii) The arrow for electron gain enthalpy
3. Consider the arrow for ionization enthalpy:
■ It indicates that:
In a period, the ionization enthalpy is least for the element at the extreme left end
• That means:
In a period, the element at the extreme left end can lose it’s electron in the most easiest way
• It follows that:
In a period, the element at the extreme left end can become a cation in the most easiest way
• It follows that:
In a period, the element at the extreme left end will have the highest chemical reactivity
♦ Chemical Reactivity is the ‘readiness to take part in chemical reactions’
4. Consider the arrow for electron gain enthalpy
■ It indicates that:
In a period, the electron gain enthalpy is 'highest negative' for the element at the extreme right end
• That means:
In a period, the element at the extreme right end can gain an electron in the most easiest way
• It follows that:
In a period, the element at the extreme right end can become an anion in the most easiest way
• It follows that:
In a period, the element at the extreme right end will have the highest chemical reactivity
5. So we have two extremes:
• One at the left end and the other at the right end
♦ Both have the highest reactivities
6. The 'variation of reactivity' can be written in 3 steps:
(i) The reactivity decreases (initially) as we go from left to right across a period
(ii) It reaches the 'lowest reactivity' at the center of the periodic table
(iii) After passing the center, it again increases and reaches the 'highest reactivity' at the extreme right end
7. We can write a note on oxidation and reduction also:
(i) Oxidation:
• 'Losing electrons' is oxidation
♦ That is., 'elements which lose electrons' are said to be oxidised
• The elements at the left easily loses electrons
♦ So we can say: Elements at the left are easily oxidised
(ii) Reduction:
• 'Gaining electrons' is reduction
♦ That is., 'elements which gain electrons' are said to be reduced
• The elements at the right easily gains electrons
♦ So we can say: Elements at the right are easily reduced
8. We can write a note on metallic and non-metallic character also:
(i) Metallic character:
• Metals are electron donors
♦ That is., metals lose electrons easily
• Elements at the left lose electrons easily
♦ So we can say: Elements at the left are more metallic
(ii) Non-metallic character:
• Non-Metals are electron acceptors
♦ That is., non-metals accept electrons easily
• Elements at the right accepts electrons easily
♦ So we can say: Elements at the right are more non-metallic
So we have completed a discussion on ‘periodic trends (along periods) in reactivity’. In the next section, we will see some examples to demonstrate those trends. We will also see ‘periodic trends (along groups) in reactivity’
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