Page 142 - Chemistry ICSE Class IX

130 ICSE Chemistry – 9
Ans. The completed statement is
In its modi ed form, the periodic table is an organisation of all the known elements, arranged in order of
increasing atomic number. Elements that lie in the same group have similar GNGEVTQPKE EQPſIWTCVKQP, have
the same number of valence electrons. The table is particularly useful for predicting the outcome of chemical
reactions between the same or different elements since an atom of any element tends to react so as to convert its
electronic structure to that of a nearby noble gas.
Q.13. Which element has
(a) an atomic mass of 14.
(b) an atomic number of 16.
(c) two energy shells, both of which are lled with electrons.
(d) the electronic structure 2, 8, 2.
(e) a total of three energy shells, with four electrons in its valence shell.
(f) a total of two energy shells, with three electrons in its valence shell.
(g) twice as many electrons in its second shell as in its rst.
Ans. (a) The element having atomic mass of 14 is nitrogen.
(b) The element having atomic number of 16 is sulphur.
(c) Neon has both of its electron/energy shells completely lled.
(d) The element having electronic structure 2, 8, 2 is magnesium (atomic no. 12).
(e) The element having three electronic shells and four electrons in its valence shell belongs to 3rd period and IVA
group of the long form periodic table (Böhr). Thus, the element is silicon. Silicon has electronic con guration
of 2, 8, 4.
(f) The element having two energy shells and three electrons in its valence shell belongs to 2nd period and IIIA
group of the long form periodic table (Böhr). Thus, the element is boron, (2, 3).
(g) The element has two shells. The rst shell can accommodate 2 electrons. So, the second shell has 4 electrons.
Thus, the electronic con guration of the element is 2, 4 (atomic number = 2 + 4 = 6). From the periodic table,
this element is carbon.
Q.14. Name two elements you would expect to show same kind of chemical reactivity as magnesium. What is the basis
for your choices?
Ans. The elements which are expected to show the same kind of chemical reactivity as magnesium are calcium and
strontium, as they belong to the same group.
Q.15. What is the name and the chemical symbol of the alkaline earth metal with the smallest atomic number?
Ans. The alkaline earth metal with the smallest atomic number is Beryllium (Be). Its atomic number is 4.

ASSIGNMENT 3

Modern periodic law, Long form periodic table and its uses

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ϱ͘ /Ŷ ǁŚŝĐŚ ƉĂƌƚ ŽĨ ƚŚĞ ƉĞƌŝŽĚŝĐ ƚĂďůĞ ĂƌĞ ;ĂͿ ŵĞƚĂůƐ ;ďͿ ŶŽŶŵĞƚĂůƐ ůŽĐĂƚĞĚ͍
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ϳ͘ tŚĂƚ ŝƐ ƚŚĞ ƐŝŐŶŝĮĐĂŶĐĞ ŽĨ ƚŚĞ ĂƚŽŵŝĐ ŶƵŵďĞƌ ŝŶ ƚŚĞ ŵŽĚĞƌŶ ƉĞƌŝŽĚŝĐ ĐůĂƐƐŝĮĐĂƟŽŶ͍
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ϵ͘ DĞŶƟŽŶ ƚǁŽ ƵƐĞƐ ŽĨ ƚŚĞ ŵŽĚĞƌŶ ƉĞƌŝŽĚŝĐ ƚĂďůĞ͘
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ϭϭ͘ ,Žǁ ĚŽĞƐ ƚŚĞ ĞůĞĐƚƌŽŶŝĐ ĚŝƐƚƌŝďƵƟŽŶ ŝŶ ĂƚŽŵƐ ĐŚĂŶŐĞ ŝŶ Ă ƉĞƌŝŽĚ ĨƌŽŵ ůĞŌ ƚŽ ƌŝŐŚƚ͍