Page 113 - Chemistry ICSE Class IX

Atomic Structure and Chemical Bonding 101
Q.3. Is it possible to write down the electronic con guration of an atom if we know its atomic number? Why is it so?
Ans. Atomic number of an element is equal to the number of protons inside the nucleus of its atom. In an atom, the
number of electrons is equal to the number of protons (hence, equal to the atomic number). Knowing the total
number of electrons in an atom, it is possible to write the electronic con guration of an atom.
It is possible because the atom is electrically neutral and total number of positive charges are equal to the total
number of negative charges.
+
+
Q.4. Write the electronic con guration of a positively charged sodium ion (Na ). What is the atomic number of Na ?
(atomic number of sodium (Na) is 11)
Ans. Sodium (Na) has 11 electrons. Its electronic con guration is K L M
2 8 1
+
Formation of Na from Na involves loss of one electron from its atom. For it, the outermost electron is lost. So,
+
the electronic con guration of Na is K L
2 8
+
As there is no change in the composition of the nucleus, therefore, the atomic number of Na is also equal to 11.
–
Q.5. Number of valence electrons in Cl ion is
(a) 16 (b) 8 (c) 17 (d) 18
Ans. Electronic con guration of Cl atom : 2, 8, 7
–
So, Electronic con guration of Cl ion : 2, 8, 8
–
Thus, Cl ion has 8 valence electrons. So, the correct answer is (b).
Q.6. Which one of the following is correct electronic con guration of sodium?
(a) 2, 8 (b) 8, 2, 1 (c) 8, 2 (d) 2, 8, 1
Ans. Sodium (at. no. = 11) atom has 11 electrons. So, the correct electronic con guration of sodium is 2, 8, 1. Thus, the
correct answer is (d).

ASSIGNMENT 3

Valence electrons, Valency
ϭ͘ dŚĞ ĂƚŽŵŝĐ ŶƵŵďĞƌ ŽĨ ĂŶ ĞůĞŵĞŶƚ ŝƐ ϭϯ͘ &ŝŶĚ ŽƵƚ ƚŚĞ ŶƵŵďĞƌ ŽĨ ǀĂůĞŶĐĞ ĞůĞĐƚƌŽŶƐ ŝŶ ŝƚƐ ĂƚŽŵ͘
Ϯ͘ tŚĂƚ ŝƐ ƚŚĞ ǀĂůĞŶĐLJ ŽĨ ĂŶ ĞůĞŵĞŶƚ ŝŶ ƚĞƌŵƐ ŽĨ ŝƚƐ ǀĂůĞŶĐĞ ĞůĞĐƚƌŽŶƐ͍
ϯ͘ tŚŝĐŚ ŽĨ ƚŚĞ ĨŽůůŽǁŝŶŐ ŝƐ ŵŽƌĞ ƌĞĂĐƟǀĞ͍
Ϯϳ
Ϯϯ y പപŽƌപപ z [Ans. X]
Ϯϯ
ϭϭ
ϭϯ
ϭϭ
Isotopes
What are isotopes
Isotopes may be de ned as follows:
Atoms of the same element, having the same atomic number, but
different mass numbers are called isotopes of that element.
Since all isotopes of an element have the same atomic number, all the
isotopes should contain the same number of protons inside their nuclei.
Also, since, different isotopes of an element have different mass numbers,
the number of neutrons in the nuclei of isotopes of an element should be
different. So, isotopes may also be de ned as follows:
The atoms of the same element which have the same number of
protons but different number of neutrons inside their nuclei are called
isotopes of that element.
Isotopes are described by writing the mass number of that isotope as a
superscript on the left side (top-left side) of the symbol of the element. The
atomic number is written as a subscript on the left side (bottom-left side) of
the symbol. For example, an isotope of the element X is described as,