Page 111 - Chemistry ICSE Class IX

Atomic Structure and Chemical Bonding 99

Illustrative Questions

Q.1. What is the most important discovery made through the D-particle scattering experiment?
Ans. The D-particle scattering experiment proved the presence of a highly dense, positively charged core (called
nucleus) at the centre of an atom.
Q.2. How did the thickness of the gold foil affect the scattering of D-particles in Rutherford’s scattering experiment?
What conclusions can be drawn from it?
Ans. Rutherford observed that the number of D-particles bouncing back got doubled when the thickness of the gold
8
10
foil was doubled. From this he estimated that the area of the nucleus is about 10 to 10 times smaller than the
area of the atom as a whole, i.e., nucleus is very small as compared to the size of the atom.
+
Q.3. Which of the Na and He has completely lled K and L shells?
+
Ans. Na has completely lled K and L shells.
+
Sodium atom (Na) (atomic number 11) gets converted to Na by losing one electron from its outermost shell.
Na o Na + + e –
2, 8, 1 2, 8
Helium (He) has only two electrons in its K-shell.
ASSIGNMENT 2

Rutherford’s scattering experiment, Thomson, Rutherford and Böhr’s atomic models, Nuclear composition

1. Why do most of the alpha (DͿ ƉĂƌƟĐůĞƐ ƉĂƐƐ ƐƚƌĂŝŐŚƚ ƚŚƌŽƵŐŚ ƚŚĞ ŐŽůĚ ĨŽŝů͍
Ϯ͘ tŚĂƚ ŽďƐĞƌǀĂƟŽŶƐ ůĞĚ ZƵƚŚĞƌĨŽƌĚ ƚŽ ĐŽŶĐůƵĚĞ ƚŚĂƚ
;ĂͿ ŵŽƐƚ ŽĨ ƚŚĞ ƐƉĂĐĞ ŝŶƐŝĚĞ ƚŚĞ ĂƚŽŵ ŝƐ ŚŽůůŽǁ͘
;ďͿ ƚŚĞ ĐĞŶƚƌĂů ƉŽƌƟŽŶ ŽĨ ƚŚĞ ĂƚŽŵ ŝƐ ƉŽƐŝƟǀĞůLJ ĐŚĂƌŐĞĚ͘
;ĐͿ ǀŽůƵŵĞ ŽĐĐƵƉŝĞĚ ďLJ ƚŚĞ ŶƵĐůĞƵƐ ŝƐ ǀĞƌLJ ƐŵĂůů ĂƐ ĐŽŵƉĂƌĞĚ ƚŽ ƚŚĞ ƚŽƚĂů ǀŽůƵŵĞ ŽĨ ƚŚĞ ĂƚŽŵ͘
;ĚͿ ĂůŵŽƐƚ ƚŚĞ ĞŶƟƌĞ ŵĂƐƐ ŽĨ ĂŶ ĂƚŽŵ ŝƐ ĐŽŶĐĞŶƚƌĂƚĞĚ Ăƚ ŝƚƐ ĐĞŶƚƌĞ͘
ϯ͘ ,Žǁ ĚŽĞƐ ZƵƚŚĞƌĨŽƌĚ͛Ɛ ĂƚŽŵŝĐ ƐLJƐƚĞŵ ƌĞƐĞŵďůĞ ŽƵƌ ƐŽůĂƌ ƐLJƐƚĞŵ͍
ϰ͘ ƌĂǁ Ă ƉŝĐƚƵƌĞ ŽĨ ƂŚƌ͛Ɛ ŵŽĚĞů ŽĨ ƚŚĞ ĂƚŽŵ͘
ϱ͘ ĞƐĐƌŝďĞ ƚŚĞ ƂŚƌͲ ƵƌLJ ƐĐŚĞŵĞ ĨŽƌ ƚŚĞ ĚŝƐƚƌŝďƵƟŽŶ ŽĨ ĞůĞĐƚƌŽŶƐ ŝŶ ĂŶ ĂƚŽŵ͘
ϲ͘ ;ĂͿ ,Žǁ ŝƐ ƚŚĞ ŵĂƐƐ ŶƵŵďĞƌ ŽĨ ĂŶ ĞůĞŵĞŶƚ ƌĞůĂƚĞĚ ƚŽ ŝƚƐ ĂƚŽŵŝĐ ŶƵŵďĞƌ͍
;ďͿ tŚŝĐŚ ŽĨ ƚŚĞƐĞ ƚǁŽ ŝƐ ƚŚĞ ŵŽƌĞ ĨƵŶĚĂŵĞŶƚĂů ƉƌŽƉĞƌƚLJ ŽĨ ĂŶ ĞůĞŵĞŶƚ͍
ϳ͘ 'ŝǀĞŶ ďĞůŽǁ ŝƐ ƐŽŵĞ ŝŶĨŽƌŵĂƟŽŶ ƌĞŐĂƌĚŝŶŐ Ă ĨĞǁ ĐŽŵŵŽŶ ĞůĞŵĞŶƚƐ͘ ŽŵƉůĞƚĞ ƚŚĞ ƚĂďůĞ͘
Atomic Number of
Element Mass number
number protons neutrons electrons
,LJĚƌŽŐĞŶ 1 1 — — —
^ŽĚŝƵŵ — 11 — 12 —
ŚůŽƌŝŶĞ — — 17 18 —

ϴ͘ EƵĐůĞĂƌ ĐŽŵƉŽƐŝƟŽŶƐ ŽĨ ƚŚĞ ĂƚŽŵƐ ĂŶĚ ĂƌĞ ĂƐ ĨŽůůŽǁƐ͗
Ͷ ƉƌŽƚŽŶƐ ϲ͕ഩŶĞƵƚƌŽŶƐ ϲ͕ പപപ Ͷ ƉƌŽƚŽŶƐ ϲ͕ഩŶĞƵƚƌŽŶƐ ϴ
'ŝǀĞ ƚŚĞ ŵĂƐƐ ŶƵŵďĞƌƐ ŽĨ ƚŚĞ ĂƚŽŵƐ ĂŶĚ ͘

Valence Electrons and Valency

What are the valence electrons
The electrons in the outermost shell of an atom take part in all the chemical
reactions given by that element. Thus, these electrons describe the
combining capacity (or valency) of an element. That is why the electrons in
the outermost shell of an atom are called its valence electrons.