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Category : 9th Class

Atoms and Molecules

Synopsis

• The simplest form of matter which can neither be split into nor built up from two or more dissimilar substances is called an element.

• An atom of an element is denoted by the symbol of that element.

• An atom of an element also denotes the atomic mass possessed by that element.

• Each atom of an element is characterised by its atomic number and mass number.

• Atomic weight of an element in grams is called gram atomic weight or gram atom.

• The smallest part of a substance that can exist independently is called a molecule.

• An aggregation of two or more atoms of the same or different elements is called a molecule.

• Molecules are characterised by their molecular weights.

• Law of Conservation of Mass: In a chemical reaction, the sum of the masses of the reactants and the products remains constant.

• Law of Definite Proportions: A chemical compound always consists of the same elements which combine in the same fixed ratio by weight.

• Law of Multiple Proportions: When two elements combine to form two or more compounds, the different weights of one element which combine with a fixed weight of the other bear a simple integral ratio.

• Molecular weight of a compound expressed in grams is called gram molecular weight or gram molecule.

• A mole is the quantity of substance that contains the same number of particles as are present in 12 g of carbon (or) mole is the collection of Avogadro's number of particles.

• Avogadro's number is \[6.023\times 1023\]. It is denoted by N.

• Empirical formula is the ratio of atoms of various elements in a compound and it is derived from percentage composition.

• Molecular formula represents the actual number of atoms of each element in a compound.

• Molecular formula = (empirical formula) \[\times \] n, where W is a small whole number.

• Number of moles

\[=\frac{Weight\,\,of\,\,asubs\tan ce\,\,in\,\,grams}{Grammolecular\,\,weight}=\frac{Number\,\,of\,\,molecules}{Avogadro's\,\,number}\]

• Number of moles of a gaseous substance

\[=\frac{Volume\,\,of\,\,the\,\,gas\,\,at\,\,STD\,\,}{Grammolar\,\,volume\,\,}\]

Number of molecules = Number of moles \[\times \] Avogadro’s number

• Weight of a molecules \[\frac{Weight\,\,of\,\,the\,\,substace}{Grannikecykar\,\,weight}\times Avogadro'snumber\]

Atomic weight of gaseous element =\[\frac{Molecular\,\,weight}{Atomicity}\]

• Weight of the substance = Number of mole \[\times \] gram molecular weight