Atoms and Molecules Class 9 Notes Science
A chemical compound always contains same elements combined together in same proportion by mass.
A chemical compound always contains same elements combined together in same proportion by mass.
The existence of electrons in an atom was shown by J.J. Thomson in 1897 by passing electricity at high voltage through a gas at very low pressure.
A kind of matter that cannot be separated into other kinds of matter by any physical process is called a substance.
Although carbon constitutes only 0.03% of earth’s crust, yet it forms innumerable complex compounds with hydrogen, oxygen and nitrogen.
At present, more than 118 chemical elements are known. Each element has a set of characteristic properties.
A German chemist J.W. Dobereiner in 1829 classified the elements in groups of three, called triad.
The simplest definition of an acid is given on the basis of its formula and was proposed by the scientist Arrhenius.
A chemical change is one in which the identity of the original substance changes and a new substance or new substances are formed.
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