Compound

A compound is a substance formed when two or more atoms of different elements combine together in a fixed proportion by weight. Some of the common examples of a compound are water (H2O), carbon dioxide (CO2), sodium chloride (NaCl) etc. When the elements combine together, the atoms lose their individual properties and have different properties from the elements they are composed of. A chemical formula is used a quick way to show the composition of compounds.
Like, calcium oxide (CaO) is a compound made up of one atom of calcium and one atom of oxygen. Hydrogen chloride (HCl) is an atom made up of one atom of hydrogen and one atom of chloride. Similarly, water (H2O) is a compound made up of two atoms of hydrogen and one atom of oxygen.

Differences between Elements and Compounds:

Atom

An atom can be defined as the smallest particles of an element which can take part in a chemical reaction. Elements are made up of smallest particles which are known as atoms. All the atoms of the same element are same but the atoms of different elements are different. There are total 118 elements discovered till now. The atoms of these all elements are different from each other. The atoms of element hydrogen are different than the atoms of the element carbon. Some atoms are found freely in nature whereas some are not found freely in nature. Atoms of helium, neon, argon, etc are found freely in nature whereas atoms of hydrogen, sodium, oxygen etc are not found freely in nature.
About the structure of an atom, an atom is made up of three fundamental particles which are also known as sub- atomic particles. They are the proton, neutron, and electron. Proton and neutron are located in the nucleus of an atom whereas electrons revolve around the nucleus in elliptical orbits called shells. The nucleus is the centre of an atom. Protons are positively charged, electrons are negatively charged and neutrons are chargeless.

Molecule

A molecule is the smallest particle of an element or a compound which can exist freely. A molecule has the chemical properties of element or compound. Molecules of the compounds are made up of two or more than two atoms of different elements whereas the molecules of element consists of two or more atoms of the same kind. The atoms in the molecules are held together by the chemical bond. These bonds are formed as a result of the sharing or exchange of electrons among atoms. For example, a molecule of Hydrogen (H2) consists of two atoms of hydrogen and a molecule of water (H2O) contains two atoms of hydrogen (H) and one atom of oxygen (O).

Molecular Formula

The molecular formula is the true formula representing the total number of atoms of each element present in one molecule of a compound. It is the symbolic representation of the molecule of a substance. It represents the composition of the molecule. It gives information about the names of elements and a total number of atoms present in a molecule of a compound. For example, the molecular formula of carbon dioxide is CO2. About writing the molecular formula, at first the symbol of the elements is written. After this, the total number of atoms in the molecule is also written as a subscript just after the symbol of the atoms.

Change in Matter

There are three physical states of matter. They are solid, liquid and gas. The matter changes from one state to another. For example, water is a liquid state of matter, when we put the glass of water into the refrigerator, the water freezes and changes its state from liquid to solid. Similarly, when we heat the solid water i.e. ice, the water changes from solid state to liquid state and when more amount if heat is applied, then the water starts boiling and changes its state from the liquid state to gaseous state. Here, we can change the state of water repeatedly. So, it is a temporary change. When we burn the paper, it changes into ash. Here, we cannot change ash into the paper. So, it is a permanent change. All the matters can change their state. Some changes are permanent while some are temporary. Depending upon the change in matter, there are two types of change in matter which are given below,

Physical change: The temporary and reversible change in which only the physical properties of matter are changed is called physical change. In a physical change, the chemical properties of the substance remain unchanged. Melting of ice, conversion of water into ice, ice into liquid etc are some common examples of physical change. Some of the characteristics of physical change are discussed below,

• It is temporary and reversible change.
• During the physical change, new types of substances are not formed.
• There is no change in chemical properties of the substance in a physical change.

Chemical change: The permanent irreversible change in which both physical and chemical properties of matter is changed is called chemical change. Burning of wood, rusting f iron, burning of matchstick etc are the examples of chemical change. It takes place due to the chemical reactions between two or more substances. Some of the characteristics of chemical change are discussed below,

• It is a permanent change.
• It is irreversible change.
• New types of substance are formed as a result of the reaction between two or more substances.
• There is a change in physical as well as in chemical properties of substance.