Page 39 - Chemistry ICSE Class X

Chemical Bonding 27
0QDNG ICUGU JCXG JKIJN[ UVCDNG GNGEVTQPKE EQPſIWTCVKQPU YJKEJ
cannot be easily changed. As a result, these elements do not show any
chemical reactivity.

#NN QVJGT GNGOGPVU JCXKPI NGUU VJCP GKIJV GNGEVTQPU KP VJGKT
outermost shell (except helium which has two electrons in its outermost
shell) show a reasonable chemical activity. For example, sodium and
chlorine are quite reactive because their outermost shells are incomplete,
i.e., Na and Cl contain less than eight electrons in their outermost
shell.

K L M
Na (atomic no. 11) 2, 8, 1 (outermost shell contains only one electron)
Cl (atomic no. 17) 2, 8, 7 (outermost shell contains seven electrons)

Thus, we see that the elements having NGUU VJCP GKIJV electrons in their
outermost shell (except helium) are chemically reactive.

Electron Dot Structures

What are electron dot structures

An American chemist, G. N. Lewis introduced a simple notation to denote
the valence electrons in an atom. This notation is called electron dot
symbol or Lewis symbol. According to this method, atom of an element
is denoted by the symbol of element alongwith some dots on it. In this
notation,
ƒ The symbol of element represents G. N. Lewis

Š the nucleus, and
Š the electrons in all the inner shells which are not involved in
bonding.

ƒ The dots on the symbol represent the number of valence electrons in
that atom.

For example, the atomic number of chlorine is 17. The electronic
distribution in an atom of chlorine is 2, 8, 7. The electron dot symbol of
chlorine is given alongside. Here,
ƒ Cl represents the nucleus of chlorine, and ten (2, 8) inner electrons. Ŗ
Ŗ Ŗ ClŖ Ŗ
ƒ Seven dots on it represent the valence electrons in an atom of chlorine. Ŗ Ŗ
Electron dot symbol of chlorine
When two similar atoms combine to form molecules, valence electrons
on the two atoms are shown differently. Dots are put on one atom, and
crosses on the other. For example, for a chlorine molecule, one can write as
shown alongside. Here, × is called the shared pair of electrons.
Ŗ
In the case of ions, the charge on an ion is also shown with the symbol.
–
For example, chloride ion (Cl YKVJ GNGEVTQPU JCU GNGEVTQPKE EQPſIWTCVKQP
2, 8, 8. It can be written as shown alongside.
ƚŽŵƐ ŽĨ Ăůů ŶŽďůĞ ŐĂƐ ĞůĞŵĞŶƚƐ
+
Sodium ion (Na YKVJ GNGEVTQPU JCU VJG EQPſIWTCVKQP +V KU ĞdžĐĞƉƚ ŚĞůŝƵŵ͕ ĐŽŶƚĂŝŶ ϴ ĞůĞĐƚƌŽŶƐ
+
written as, [Na ]. ŝŶ ƚŚĞŝƌ ŽƵƚĞƌŵŽƐƚ ƐŚĞůů͘ ƚŽŵƐ ŽĨ Ăůů
ŽƚŚĞƌ ĞůĞŵĞŶƚƐ ĐŽŶƚĂŝŶ ůĞƐƐ ƚŚĂŶ
'NGEVTQPKE EQPſIWTCVKQPU CPF GNGEVTQP FQV UVTWEVWTGU QH EGTVCKP EQOOQP ϴ ĞůĞĐƚƌŽŶƐ ŝŶ ƚŚĞŝƌ ŽƵƚĞƌŵŽƐƚ ƐŚĞůů͘
elements are given in the following table (Table 2.2).