Atoms and Molecules

Atoms and molecules An atom is the smallest particle of the element that can exist independently and retain all its chemical properties. Atom can take part in chemical reaction. Atoms can exist either as molecules or as ions. Molecule is an electrically neutral group of two or more atoms chemically bonded together. The molecules of an element contain two or more atoms chemically combined together. The number of atoms constituting a molecule is known as its atomicity. Depending upon the atomicity, molecule of an element can be classified into monoatomic molecule, diatomic molecule, triatomic molecule, tetra-atomic molecule and polyatomic molecule.

The molecules of a compound contain two or more different types of atoms chemically combined together. Atoms are electrically neutral. When they gain or lose electron they are convert into charged species which is called ions. There are two types of ions: Positively charged ions and negatively charged ions. If an atom loses electrons then it becomes positive ion. Positive ions are called cations. If an atom gains electrons then it becomes negative ion. Negative ions are called anions. Valency of ions is equal to the charge on the ion. Dalton was the first scientist to use the symbols for elements in a very specific sense. The chemical formula of a compound is a symbolic representation of its composition. While writing the chemical formulae for compounds, we write the constituent elements and their valencies. Valency is the combining capacity of an element. Scientists use the relative atomic mass scale to compare the masses of different atoms of elements. Atoms of carbon-12 isotopes are assigned a relative atomic mass of 12 and the relative masses of all other atoms are obtained by comparison with the mass of a carbon-12 atom.

Laws of chemical combination Ancient Indian and Greek philosophers always wondered about the unknown and unseen form of matter. An Indian philosopher Maharishi Kanad, postulated that if we go on dividing matter (padarth), we shall get smaller and smaller particles. Antoine Lavoisier laid the foundation of chemical sciences by establishing two important laws of chemical combination. According to the law of conservation of mass, matter can neither be created nor destroyed in a chemical reaction. During a chemical reaction, the sum of the masses of the reactants and products remains unchanged. This is known as the Law of Conservation of Mass. Total mass of the reactants is equal to total mass of the products Law of constant proportions states that a chemical compound always consists of the same elements combined together in the same proportion by mass, independent of the method by which it is prepared, or the source from which it is obtained. The next problem faced by scientists was to give appropriate explanations of these laws. British scientist John Dalton provided the basic theory about the nature of matter. According to his theory, matter must be made up of minute “unit particles”, called ATOM, which take part in chemical reaction in fixed whole numbers ratio.

Mole Concept The most remarkable concept that Dalton’s atomic theory proposed was that of the atomic mass. The atomic mass of an element is the relative mass of its atom as compared with the mass of carbon-12 atom taken as 12 units. One atomic mass unit (earlier abbreviated as amu) but now written as u (unified mass) is defined as the mass unit equal to exactly one twelfth of the mass of one atom of carbon-12. The molecular mass of a substance is the sum of the atomic masses of all the atoms in a molecule of the substance. The formula unit mass of a substance is a sum of the atomic masses of all atoms in a formula unit of a compound. The word “mole” was introduced around 1896 by Wilhelm Ostwald who derived the term from the Latin word moles meaning a ‘heap’ or ‘pile. One mole is the amount of a substance that contains as many particles or entities as there are atoms in exactly 12 g of carbon-12 isotope. A group of 6.022 x 1023 particles of a substance is called a mole of that substance. This number is called the Avogadro’s number.

To Access the full content, Please Purchase