Page 48 - Chemistry ICSE Class X

36 ICSE Chemistry – 10
The ions so formed are free to move in any direction. However, when
CP GNGEVTKECN ſGNF KU CRRNKGF CETQUU VJG UQNWVKQP OGNV QH UWEJ UQNKFU
the ions move towards the respective electrodes and get discharged
by losing or gaining electron(s). Electrolysis is based on such directed
OQXGOGPV QH KQPU RTGUGPV KP VJG UQNWVKQP OGNV

–
+
Why is sodium chloride actually (Na Cl ) and not NaCl
n
+
In sodium chloride, a large number of sodium ions (Na ) and chloride ions
–
+
(Cl ) are arranged in three dimensions such that each Na is surrounded by
–
–
+
six Cl and each Cl is surrounded by six Na .
+
–
Thus, in sodium chloride, Na will have Cl as its nearest neighbour and
–
+
Cl ion has Na as its nearest neighbour. A two-dimensional arrangement
of ions in sodium chloride crystal is shown in Fig. 2.1.
There is no discrete molecule like NaCl in the crystal. Crystal of
+
–
sodium chloride is electrically neutral. So, the number of Na and Cl
Fig. 2.1 A section of two-dimensional
arrangement of ions in sodium chloride ions in sodium chloride crystal are equal. As a result, a crystal of sodium
+
–
+
–
crystal. In actual crystals, such chloride can only be described as (Na Cl ) and not as Na Cl QT 0C%N.
n
arrangement is in three dimensions. The value of n depends on the size of the sample.
Illustrative Questions
Q.1. Formula of magnesium oxide is MgO. Justify it on the basis of bond formation.
Ans. 6JG GNGEVTQPKE EQPſIWTCVKQP QH /I KU CPF QH QZ[IGP KU /CIPGUKWO CVQO /I thus gains stability
D[ NQUKPI VYQ GNGEVTQPU 6JG QZ[IGP CVQO CVVCKPU C UVCDNG GNGEVTQPKE EQPſIWTCVKQP D[ ICKPKPI VYQ
GNGEVTQPU CU UJQYP DGNQY
Mg o Mg 2+ + 2e –
(2, 8, 2) (2, 8)
–
O + 2e o O 2–
(2, 6) (2, 8)
2–
2–
2+
Mg 2+ + O o Mg O o MgO
Q.2. Write down the formula of the compound resulting from the combination of an element X (atomic no. 20) with
another element Y (atomic number 17).
Ans. 6JG GNGEVTQPKE EQPſIWTCVKQPU QH : CPF ; CTG
X (atomic number, 20) = 2, 8, 8, 2
Y (atomic number, 17) = 2, 8, 7
6JWU VQ CESWKTG C PQDNG ICU EQPſIWTCVKQP CP CVQO QH : UJQWNF NQUG GNGEVTQPU CPF CP CVQO QH ; UJQWNF ICKP
one electron. Thus, one atom of X would react with 2 atoms of Y to form a compound XY CU UJQYP DGNQY
2
X o X 2+ + 2e –
(2, 8, 8, 2) (2, 8, 8)
–
2Y + 2e o 2Y –
(2, 8, 7) (2, 8, 8)
2+
–
–
X 2+ + 2Y o X (Y ) o XY 2
2
Q.3. Give the empirical formulae and electron dot structures for the ionic compounds formed by each of the following
RCKTU QH KQPU
2+
–
+
2–
+
–
+
2–
3+
K , O ; Ca , Cl ; Na , S ; Al , F ; Na , P 3–