Matter

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From the earliest times, man has always wondered about the ultimate nature of matter. He has searched to establish the basic constituents of all the substances.

Any substance which can have a certain mass is called as matter.

= Atoms and molecules = The smallest unit of any matter is known as atom which cannot be subdivided by chemical methods. Atom is a Greek word which means indivisible as Greek conclude atom as the smallest particles of any matter which cannot be further divided. But now we know that each atom consist of three fundamental particles, protons, neutrons, and electron. The mass of electrons are negligible hence the total mass of an atom is because of the mass of protons and neutrons.

The energy levels of electrons in an atom are called as shell, as the number of shell increases the energy of electrons increases and the difference between two energy levels decreases.



Atoms of most of the elements do not exist independently. They either form molecules or ions. An atom having a charge on it is called an ion. When an atom loses or gains charge, it gets converted into a positively or negatively charged ion respectively. Ions may consist of single charged atoms or a group of atoms that have a net charge on them. Molecules are formed by combination of atoms or ions by chemically bonding to each other or held by attractive forces due to the charges.

[[image:http://images.tutorvista.com/content/atoms-molecules/molecule-formation.jpeg width="268" height="24" align="absmiddle" caption="formation of molecule"]]
A molecule is smallest particle of an element or compound, which is capable of independent existence.
 * If the molecule is made up of atoms of the same element it is said to be homo-atomic. Examples: O2, N2
 * If the molecule is made up of atoms of the different elements it is said to be hetero-atomic. Examples: CO2, H2O, SO3, HCl



Depending upon the number of atoms in one molecule of the element it can be classified as:



= Ions and valence electrons =

Since both protons and neutrons located at the center of atom in nucleus; hence complete mass of an atom is concentrated at its centered. Out of these three fundamental particles; electrons are negatively charged and protons are positively charged particles while neutrons are neutral in nature. Hence the equal number of electrons and protons is responsible for the electrical neutrality of an atom. If the number of protons and electrons are not equal in an atom, it gets some charge and termed as ion.

Ion with negative charge is known as anion and with positive charge termed as cation. The number of protons in an atom is called as atomic number. Since the atomic number is equals to the number of electrons in an atom, hence atomic number is the key factor for identify the chemical and physical properties of atom. In all chemical reactions; the electrons present in outermost shell take part also known as valence electrons.

=Symbol= A short hand representation of an element is called symbol. It represents the following: > > >
 * Name of the element
 * One atom of the element
 * One mole of atoms. It represents 6.023 x 1023 atoms of the element.
 * A definite mass of the element

Difference between symbol of an element and formula of an element

 * Symbol of an element represents the name of the element. It also represents one atom of the element.
 * Example: H represents hydrogen and C represents carbon
 * A formula of an element represents the number of atoms in the molecule of the compound. One molecule of hydrogen element contains two atoms of hydrogen; therefore the formula of hydrogen is H2.
 * 2H represents two separate atoms of hydrogen, whereas H2 represents 1 molecule of hydrogen similarly the molecular formula of oxygen element and chlorine element is O2 and Cl2.

=Laws of chemical combination= i) Law of Conservation of Mass: The law states that during any physical or chemical change, the total mass of the product remains equal to the total mass of the reactants.

ii) Law of Constant Composition: The law states that a chemical compound always contains same elements combined together in the same proportion by mass.

iii) Law of Multiple Proportions: The law states that when two elements combine with each other to form two or more compounds, the masses of one of the elements, which combine with fixed mass of the other, bear a simple whole number ratio to one another.

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