Page 128 - Chemistry ICSE Class IX
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116 ICSE Chemistry – 9
Let us say it Again
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Gases conduct electricity only under very low pressure (10 atm) and high potential (10,000 V).
Cathode rays. Cathode rays are produced when an electrical potential of about 10,000 volts is applied across a gas under
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very low pressure (about 10 atm).
Cathode rays consist of negatively charged particles called electrons. These rays get deflected towards the positive plate
when placed in an electric field.
Electrons. Electrons are an essential constituents of all matter (shown by J. J. Thomson’s experiment).
Positive rays. Goldstein showed the presence of positively charged particles, the nature of which depends upon the gas
used in the discharge tube.
Proton. Proton is a positively charged particle having mass equal to 1 atomic mass unit. (= 1.676 × 10 –27 kg).
Neutron. Neutron is an electrically neutral particle having mass equal to that of a proton.
Nucleus. Rutherford showed the presence of a small, heavy, positively charged core at the centre of the atom. This core
is called nucleus.
Atomic number. Atomic number (Z) of an element is equal to the number of protons inside the nucleus of its atom.
Mass number. Mass number (A) of an atom is equal to the sum of the number of protons and neutrons present inside
the nucleus.
Orbits. According to Böhr, an atom is similar to our solar system. Just as the planets revolve around the Sun; in an atom,
electrons revolve around the nucleus in certain definite circular paths. These circular paths are called orbits or energy
shells.
Orbits or shells in an atom are denoted either by integral numbers, such as 1, 2, 3, 4..., or by the letters K, L, M and N.
Valency. The combining capacity of an element is termed as its valency.
Isotopes. Isotopes are the atoms of the same element possessing different mass numbers but the same atomic number.
Isobars. Atoms of different elements which have the same mass number and different atomic number are called isobars.
Octet rule. During any chemical reaction, atoms of all the elements tend to gain stability by acquiring electronic
configuration of the nearest noble gas element. This is called the octet rule.
