Explanation The distinction between one kind of matter and another depends purely on the nature of their atoms and molecules. For example, an atom of gold is different from an atom of silver. Matter can exist in three states: solids, liquids and gases. For example, water can exist either as solid(in the form of ice), liquid or a gas(in the form of steam). ATOMIC NATURE OF MATTER: An atom comprises of three types of particles known as protons, neutrons and electrons. The protons and neutrons are lodged in the nucleus at the center of the atom while the electrons revolve around the nucleus just like the planets go round the sun. The protons carry positive unit charge; the electron carries unit negative charge while neutron is a neutral particle without any charge. The proton and neutron are almost equal in mass. They are approximately 2000 times heavier compared to an electron. For this reason the mass of electrons is neglected in determining the mass of an atom. The size of the nucleus is very small (10-5 times) compared to the size of an atom. The mass of an atom, therefore, is concentrated in its nucleus. Another interesting thing is the fact that an atom carries same number of electrons as the number of protons in its nucleus making it electrically neutral. ELECTRONIC CONFIGURATION OF AN ATOM: The electrons revolve around the small nucleus of an atom at great speed and occupy different orbits known as shells which are generally identified by letters, K, L, M, N, O and P. The K shell for example is closest to the nucleus and is called the innermost shell, while the P shell is the farthest. The energy of an electron depends on the shell in which it is revolving. If it is in the farthest shell P, it has more energy compared to the electron in the K shell, which is closest to the nucleus. The arrangement of electrons in various shells of an atom is known as the electronic configuration of the atom. The electrons depending on their energy levels occupy different shells, but the maximum number of electrons that can occupy a given shell is determined based on Bohr's atomic theory and follows a definite relation as explained below. The K shell has the 1st energy level known as n= 1. The number of electrons in K shell is given by 2n2 or 2 x 12 = 2 . This means only two electrons can exist in the K shell that is closest to the nucleus. According to the above formula, the L shell (energy level, n = 2) can have a maximum of (2 x 22) 8 electrons. Likewise, the maximum number of electrons that can exist in the M, N, O and P shells are 18, 32, 50 and 72 respectively. Due to variation of the number of protons, neutrons in the nucleus and the electronic configuration, different elements are formed. For example, the atom of the hydrogen element comprises of a single proton in its nucleus with a single electron orbiting in its K shell. When there are two protons in its nucleus with two electrons revolving in its K shell, the element Helium is formed. Similarly, a carbon atom contains 6 protons and 6 neutrons in its nucleus with 2 electrons in its K shell and 4 electrons in its L shell. An Oxygen atom consists of 8 protons and 8 neutrons in its nucleus with 2 electrons in its K shell and 6 electrons in its L shell, while an atom of gold has 79 protons and 118 neutrons in its nucleus with 79 electrons distributed into 2, 8, 18, 32, 50 and 18 occupying K, L, M, N, O and P shells respectively. MASS NUMBER AND ATOMIC NUMBER: The total number of protons and neutrons in the nucleus of an atom is known as its mass number(A) ( it is also called as atomic weight). The mass number of an atom gives an indication of how heavy the atom is. Higher the mass number, heavier is the element and vice versa. The number of protons in the nucleus of an atom is known as the atomic number(Z). If 'E' represents an element, its atomic structure is represented as follows. The atomic number(Z) is written as subscript and its mass number(A) as superscript with the symbol of the element as Z E A. For example the oxygen element is represented as 8O16. Here O is the symbol for oxygen and 16 represents its mass number and 8 represents its atomic number. Since the number of protons and electrons are the same the atomic number gives us the number of electrons in the atom. Therefore, we can derive the electronic configuration for a given atom from this form of representation. It may be noted that the chemical nature of the atom depends on the number of electrons in the outermost shell, known as valence electrons, since they only participate in a chemical reaction , keeping the nucleus unchanged. MOLECULAR NATURE OF MATTER: Atoms generally do not exist in free state. They combine with each other to form a stable unit of matter called molecule. The atoms combine either by exchanging or sharing their electrons in their outermost shell forming a chemically bonded molecules. The molecules of the same element are identical. A molecule is the smallest unit of an element which can exist freely in nature retaining the properties of the element. A molecule, however, may comprise of two or more atoms of the same substance or of different substances. Two hydrogen atoms combine to form one molecule of hydrogen gas while two hydrogen atoms combine with one oxygen atom to form a water molecule. Matter, in general, can be classified into elements, compounds and mixtures. Elements can exist freely in nature can can be further classified into metals, nonmetals, metalloids and noble gases. A compound is a substance that is composed of two or more elements combined chemically in a definite proportion. A mixture is made up of two or more elements or compounds combined physically in any proportion, but not chemically combined. KINETIC AND POTENTIAL ENERGY: In matter the inter-molecular spacing depends on the state, whether it is in solid, liquid or gaseous form. The molecules of matter are always in a state of motion. In solids, the molecular motion is least, and in gases, it is highest. Similarly, in solids, the intra-molecular spaces are least and in gases, they are highest. The motion of molecules and the inter-molecular space increases when heat is applied. The molecules posses two types of energy:: Kinetic energy Potential energy. Due to kinetic energy, the molecules are in constant movement and due to potential energy, the molecules attract or repel with each other depending on their relative positions. By application of heat, the kinetic and potential energies increase or decrease depending on the nature of the molecule, thus, causing the change of state of matter.