Page 77 - ICSE Chemistry 8

Take a Break!

Write/Choose the correct answer.
1. All atoms of a par cular element are iden cal in _____________ and _____________.
2. Cathode rays are produced from the _____________ in a discharge tube.

3. The anode rays consist of the posi vely charged par cles called _____________.
4. The charge to mass ra o (e/m) of a proton is fi xed. (True/False).

5. _____________ revolve in fi xed orbits around the nucleus.
6. Neutrons and protons are present outside the nucleus of an atom. (True/False).

MODERN CONCEPT OF AN ATOM
orbits or
According to the modern concept of the atom: shells
• An atom consists of three subatomic par cles—electrons,
protons and neutrons.
• There are two structural parts of an atom. P P electrons
N N
 The nucleus
 The extra-nuclear region or the empty space that
nucleus
surrounds the nucleus (protons + neutrons)

• The nucleus is posi vely charged central part of the atom. It Fig. 4.10: Modern concept of an atom
contains the protons and neutrons. The protons and neutrons
are held ghtly in the nucleus by strong nuclear forces. The posi ve charge of nucleus is due to the
protons present in it. The neutrons have no charge. The en re mass of the atom is concentrated in
the nucleus (since electrons have negligible mass).
• The extra-nuclear region of an atom consists of the electrons which revolve around the nucleus in
defi nite orbits or shells. Each orbit has a fi xed energy level. The energy increases as you go away
from the nucleus. The electrons experience an outward pull (or centrifugal force) due to their
circular mo on. This outward pull is counterbalanced by the inward a rac on between these
electrons and the protons present inside the nucleus.
• An atom is electrically neutral as it contains equal number of electrons and protons.

STRUCTURAL STABILITY OF AN ATOM

– negatively charged
A force of a rac on exists between the electrons and protons electron
present within the atom. The electrons being charged and lighter
are in constant mo on and would lose energy, and eventually fall
+
into the nucleus leading to structural collapse of the atom. However,
this does not happen. This is because the electrons revolve in fi xed
positively charged
orbits (shells) around the nucleus at a very high speed. Each orbit nucleus
is associated with a fi xed amount of energy. The electrons in the Fig. 4.11: Electron losing energy and
orbits neither lose nor gain energy un l some external force is falling into the shell

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