Atoms , Molecules And Compounds

Atoms

         An element is formed of many similar units called atoms. An atom is defined as the smallest of an element which participates in the chemical combinations with other elements. An atom, inturn is formed of 3 types of particles. These three particles are electrons, protons and neutrons.

            Protons and neutrons occur at the centre of the atom and they together called the nucleus of the atom.The protons are positively charged and the neutrons have no electrical charges. Hence the nucleus is positively charged.

            The electrons (e) are negatively charged. They are revolving around the nucleus in specific orbits called shells. There is a limit for the number of electrons present in the orbits. The first shell contains only 2 electrons, the second shell contains only 8 electrons, the third shell contains only 18 electrons and the fourth shell contains 32 electrons. The number of electrons in each orbit can be represented by the formula 2xn2. Here’n’ represents the serial number of the shell from the nucleus

Examples of a few atoms.

Atomic Number

               The number of protons or the number of electrons present in an atom represents the atomic number of an element. Each element has a specific atomic number.

Atomic Weight

               The sum total of weights of protons, neutrons and electrons of an atom is called atomic weight. The electron has negligible mass, protons and neutrons have mass. Thus 99.98% of the mass of an atom is concentrated in the nucleus.

               The atomic weight of the atom is approximately equal to the number of its protons and neutrons.The atomic weights range from 1 to 256 .

Examples

Stable and Unstable Atoms

               In some atoms the outer shell is complete by the presence of full number of electrons. These atoms are called stable atoms. Eg. Helium. In helium the outer shell has complete set of two electrons. The atoms which do not have the complete set of electrons in the outer shell are called unstable atoms. Eg. Hydrogen has only one electron instead of two in the outer shell. The unstable atoms become stable either by gaining electrons or losing electrons. The atom which gains electron is called electronegative atom, and the atom which loses electron is called electropositive atom because it loses one electron. The tendency of atoms to gain or lose electrons is the basis of all chemical reactions in which atoms of different unite with each other.

 

Examples of stable and unstable atoms

Molecules

            Two or more atoms unite together to form a molecule . A molecule may be formed by the union of similar or dissimilar atoms .

Difference between Atoms and Molecules

Compounds

           The molecules combine together to form compounds. compounds are two types. They are inorganic compounds and organic compounds.

Example of compound

Inorganic compounds

        Inorganic compounds do not contain C-H groups .  Eg. NaCl, NaOH, etc.

Organic compounds

       The organic compounds contain C-H groups .Eg. Glucosephosphate, ATP, ADP, etc.