In the previous section, we saw some solved examples related to the position of elements in the periodic table. In this section we will see a few more solved examples.
We have completed the basic discussion on the Modern periodic table. We will learn more details in higher classes. In the next section, we will discuss about Non-metals.
Solved example 4.3
Electron configuration of
elements P, Q, R and S are given below:
P – 2,2 Q – 2,8,2 R –
2,8,5 S – 2,8
(a) Which among these elements
are included in the same period?
(b) Which among these elements
are included in the same group?
(c) Which among these elements
is a noble gas?
(d) To which group and period
does the element R belong?
Solution:
(a) Elements in a same period
will have the same number of shells
• In the given problem, P and S
have both 2 digits in their electron configuration. So they have both
2 shells. Thus, they both belong to the Period 2
• Q and R have both 3 digits in
their electron configuration. So they have both 3 shells. Thus, they
both belong to the Period 3
(b) Elements in the same group
will have the same number of electrons in their outermost shells
In the given problem, P and Q
have both 2 electrons in their outermost shells. So they both belong
to Group II
(c) S has an octet
configuration in the outermost shell. So it is a noble gas
(d) • R has 5 electrons in it's
outermost shell. So it belongs to Group V
• It has 3 digits in it's
electron configuration. So it has 3 shells. Thus it belong to the
Period 3
Solved example 4.4
An incomplete form of the
periodic table is given below.
Write answers to the questions related to the position of elements in it.
Write answers to the questions related to the position of elements in it.
(a) Which is the element with the
biggest atom in group I?
(b) Which is the element with the
lowest ionisation energy in group I?
(c) Which is the element with the
smallest atom in period II?
(d) Which are the transition
elements?
(e) Which among the two elements L
and M, has the smallest electronegativity?
(f) Which among the two elements B
and I, has higher metallic nature?
(g) Which are the elements that
belong to the halogen family?
(h) Which type of ion will be formed from an atom of 'M'. Cation or anion?
(h) Which type of ion will be formed from an atom of 'M'. Cation or anion?
(i) Which is the element that
resembles E the most in it's chemical properties?
Solution:
(a) We have discussed the
periodic trend related to the 'size of atoms', above in this section. We can
write this:
• In a group, from top to
bottom, size of atom increases
• In a period, from left to
right, size of atom decreases
• In this problem, we have to
consider the variation within a group only. The bottom most element
will be the biggest. So, in group I, D is the biggest.
(b) We have discussed the
periodic trend related to the 'ionisation energy', above in this section. We
can write this:
• In a group, from top to
bottom, the ionisation energy decreases
• In a period, from left to
right, the ionisation energy increases
• In this problem, we have to
consider the variation within a group only. The bottom most element
will have the lowest ionisation energy. So, in group I, D will be
having the lowest ionisation energy.
(c) The variation of size was
written in problem (a) above. In this problem, we have to consider
the variation within a period only. The element at the extreme right
will be the smallest. So, in period 2, M is the smallest.
(d) The transition elements
fall between groups II and III. So G and H are transition elements.
(e) We have discussed the
periodic trend related to the 'electronegativity', above in this section. We
can write this:
• In a group, from top to
bottom, the electronegativity decreases
• In a period, from left to
right, the electronegativity increases
• In this problem, we have to
consider the variation within a period only. This is because, L and M
fall within a period. The left most element will have the lower
electronegativity. So, among L and M, the element L will be having
the lower electronegativity.
(f) We have discussed the
periodic trend related to the 'metallic nature'. We
can write this:
• In a group, from top to
bottom, the metallic nature increases
• In a period, from left to
right, the metallic nature decreases
• In this problem, we have to
consider the variation within a period only. This is because, B and I
fall within a period. The left most element will have the higher
metallic nature. So, among B and I, the element B will be having the
higher metallic nature.
(g) We have learned that,
halogen family is the group VII. So, among all the given elements, M
and N are the halogens.
(h) • We have lerned about cations and anions here. Positive ions are Cations and negative ions are anions.
• As we move from left to right in a period, the metallic character decreases.
• That means, the tendency to lose electrons decreases. That means, tendency to gain electron increases
• As M is further to the right in it's period, it will have a tendency to gain electrons. So it will become a negative ion. That is., it will become an anion.
(h) • We have lerned about cations and anions here. Positive ions are Cations and negative ions are anions.
• As we move from left to right in a period, the metallic character decreases.
• That means, the tendency to lose electrons decreases. That means, tendency to gain electron increases
• As M is further to the right in it's period, it will have a tendency to gain electrons. So it will become a negative ion. That is., it will become an anion.
(i) Both B and F are close to E. But F will resemble E more because, F, like E has two electrons in the outermost shell
Solved example 4.5
Solved example 4.5
Give examples for the
following:
(a) Two elements that have 1
electron in their outermost shells
(b) Three elements that have 2
electrons in their outermost shells
(c) Three elements that have
filled outermost shells
Solution:
(a) All elements in the group
I have 1 electron in their outermost shells. We can pick any two from
this group. Say lithium (Li) and sodium (Na)
(b) All elements in the group
II have 2 electrons in their outermost shells. We can pick any three
from this group. Say beryllium (Be), magnesium (Mg) and calcium (Ca)
(c) All elements in the group
VIII have filled electrons in their outermost shells. We can pick any
three from this group. Say neon (Ne), argon (Ar) and krypton (Kr)
Solved example 4.6
Lithium, sodium and potassium
are all metals which react with water to liberate hydrogen gas. Is
there any similarity in the atoms of these elements?
Solution: All the three
elements have one electron in the outermost shell. During chemical
reactions, this single electron is readily donated by these metals
Solved example 4.7
In the modern periodic table,
which are the metals among the first 10 elements?
Solution:
Lithium (Li) and Beryllium (Be).
Solved example 4.8
By considering their position
in the periodic table, which one of the following elements would you
expect to have the maximum metallic character?
Ga, Ge, As, Se, Br
Solution:
All the given elements belong
to a same period. Among them, Ga is the one which is at the left most
position. So It is the one with the maximum metallic character
Solved example 4.9
Which of the following
statements is not a correct statement about the trends when going
from left to right in any period
(i) The elements become less
metallic in nature
(ii) The number of valence
electrons increase
(iii) The atoms lose their
electrons more easily
Solution:
Statement (iii) is false. As we move from left to right in any period, the metallic character decreases. That is., tendency to lose electrons decreases.
Solved example 4.10
Which element has
(i) Two shells, both of which
are completely filled with electrons
(ii) Electron configuration
2,8,2
(iii) A total of 3 shells,
with 4 electrons in the valence shell
(iv) A total of 2 shells, with
3 electrons in the valence shell
(v) twice as many electrons in
it's second shell as in it's first shell
Solution:
(i) • Given that the element has
two shells. So they are K and L
• Given that they are completely
filled up. K-shell can hold a maximum of 2×12 = 2×1 = 2 electrons
• L-shell can hold a maximum of
2×22 = 2×4 = 8 electrons
• So the total number of
electrons = total number of protons = 2 + 8 = 10
• Thus the atomic number Z = 10,
and the element with this atomic number is Neon (Ne)
(ii) • Given that electron
configuration is 2,8,2
• So number of electrons =
number of protons = 2 + 8 + 2 = 12
• Thus the atomic number Z = 12,
and the element with this atomic number is magnesium (Mg)
(iii) • Given that there are a
total of 3 shells. So the shells are K,L and M
• Also given that there are 4
electrons in the valence shell. That means K and L are completely
filled up. So number of electrons = 2 + 8 + 4 = 14
• Thus the atomic number Z = 14,
and the element with this atomic number is silicon (Si)
(iv) • Given that there are a
total of 2 shells. So the shells are K and L
• Also given that there are 3
electrons in the valence shell. That means K is completely filled up.
So number of electrons = 2 + 3 = 5
• Thus the atomic number Z = 5,
and the element with this atomic number is boron (B)
(v) • Here the total number of
shells is not given. But that is not necessary to solve the problem
• Given that number of electrons
in the second shell is twice that in the first. It is clear that
there are some electrons in the second shell
• The second shell will begin to
fill only if the first shell is completely filled up. The maximum capacity of the
first shell is 2.
• So the number of electrons in
the second shell = 2×2 = 4. The second shell is capable to
carry more than this 4. So it follows that there are no more shells
than K and L.
• So there are no more electrons
than the 2 in first shell, and the 4 in the second shell.
• Thus the atomic number Z = 2 +
4 = 6, and the element with this atomic number is carbon (C)
Solved example 4.11
Nitrogen (Z=7) and Phosphorus
(Z=15) belong to group V of the periodic table. Write the electron
configuration of these elements. Which will be more electronegative?
Why?
Solution:
• Electron configuration of
nitrogen is 2,5
• Electron configuration of
phosphorus is 2,8,5
• Both of them belong to the
same group, with nitrogen above phosphorus. We have seen that while
moving from top to bottom in a group, electropositive character
increases. That is., electronegative character decreases. So,
nitrogen, which is above, is more electronegative.We have completed the basic discussion on the Modern periodic table. We will learn more details in higher classes. In the next section, we will discuss about Non-metals.
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