Page 91 - ICSE Chemistry 8

Anode rays: A stream of rays originated from the anode
Protons: Posi vely charged par cles that are found inside the nucleus

Neutrons: Electrically neutral par cles that are found inside the nucleus
Nucleus: The posi vely charged central part of an atom containing protons and neutrons

Atomic number: The number of protons present inside the nucleus of an atom
Mass number: The sum total of the number of protons and the number of neutrons present
inside the nucleus of an atom of an element

Rela ve atomic mass: The number of mes by which the average mass of one atom of an element is
heavier than 1/12th of the mass of a carbon atom
Isotopes: Atoms of the same element having same atomic number but diff erent mass
number

Electronic confi gura on: The arrangement of electrons in diff erent orbits or shells around the nucleus of
an atom
Valence shell: The outermost shell of an atom

Valence electrons: The electrons present in the valence shell of an atom
Valency: The number of electrons that an atom needs to accept, donate or share so as to
form a complete outer octet

Electroposi ve valency: The valency of a posi vely charged ion
Electronega ve valency: The valency of a nega vely charged ion

Monovalent elements: Elements with valency one
Divalent elements: Elements with valency two

Trivalent elements: Elements with valency three
Radicals: Two or more non-metals that collec vely accept or donate one or more electrons
and become nega vely or posi vely charged

Quick Review

• Around the turn of the 20th century, it was discovered that an atom was actually a cluster of various
subatomic par cles (mainly electrons, protons and neutrons) which can exist separately from each
other.
• John Dalton proposed that an atom is a solid, indestruc ble and indivisible sphere.

• In 1879, the English scien st, William Crookes showed that the rays were emi ed from the cathode
when a high voltage electric current was passed through the discharge tube.
• In 1897, the English scien st Sir J.J. Thomson proved that an atom can be split into even smaller parts.
His discovery of the electron was the fi rst step towards a detailed model of the atom.
• J.J. Thomson found that the cathode rays consisted of nega vely charged par cles. These par cles
were named as electrons.
• E. Goldstein performed an experiment in the discharge tube with a perforated cathode and found that
the anode rays consisted of posi vely charged par cles called protons.

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