Atoms, Molecules, and Ions
The atom is a basic unit of matter consisting of a dense, central nucleus surrounded by a cloud of negatively charged electrons. An atom is the smallest unit of an element that could part in a chemical reaction. The atomic nucleus contains a mix of positively charged protons and electrically neutral neutrons (except in the case of Hydrogen-1, which is the only stable isotope with zero neutrons). The electrons of an atom are bound to the nucleus by the electromagnetic force. Likewise, a group of atoms can remain bound to each other, forming a molecule. An atom containing an equal number of protons and electrons is electrically neutral, otherwise it has a positive or negative charge and is an ion. An atom is classified according to the number of protons and neutrons in its nucleus: the number of protons determines the chemical element, and the number of neutrons determine the isotope of the element.
The Concept of Atom first came when an Indian Thinker Maharish Kanad said that if we go on dividing all the materials present around us then a stage will come when it cannot be divided further. He named the material found at this stage as Parmanu, which is what atom is called in hindi. Later Greek philosopher Democritus coined the name Atom which in Greek means indivisible. Later French Chemist Antoine L. Lavosier (Father of Chemistry) discovered Two laws of Chemical Combination - Law of conservation of mass and Law of constant proportion, which raised the question, "What comprises the elements?" To answer this question came another British Scientist Dalton who again introduced the theory that there exists a stage where the material can no more be divided and named that stage as Atom. He also stated that Atom can neither be created nor destroyed. The principles of quantum mechanics were used to successfully model the atom.
Relative to everyday experience, atoms are minuscule objects with proportionately tiny masses. Atoms can only be observed individually using special instruments such as the scanning tunneling microscope. Over 99.9% of an atom's mass is concentrated in the nucleus, with protons and neutrons having roughly equal mass. Each element has at least one isotope with unstable nuclei that can undergo radioactive decay. This can result in a transmutation that changes the number of protons or neutrons in a nucleus. Electrons that are bound to atoms possess a set of stable energy levels, or orbitals, and can undergo transitions between them by absorbing or emitting photons that match the energy differences between the levels. The electrons determine the chemical properties of an element, and strongly influence an atom's magnetic properties.
Everything, from the computer you are on, to the fingers you are typing with, is made of atoms. These tiny particles combine, transform, and bond to create the world around us. Atoms, in turn, are made of electrons, protons and neutrons, in various combinations. The basic Bohr's Model shows their relationship.
Electrons are tiny, subatomic particles that are 'negatively' charged. Their mass is insignificant compared to that of protons and neutrons, so, for most purposes, it is ignored.
Protons are small, subatomic particles that are 'positively' charged. Their mass is not ignored in calculations of atomic mass (also known as atomic weight), but is given a value of 1 Atomic Mass Unit, or amu (also known as a dalton). The atomic number of an element is the number of protons in one atom of an element.
A neutron is a particle that has no charge. Their mass is included in calculations of atomic mass (also known as atomic weight), and, like protons, is given a value of 1 Atomic Mass Unit, or amu (also known as a dalton).
An ion is an atom possessing an electric charge. Atom(s) which contain more electrons than protons are said to be negatively charged ions or anions, while atoms possessing more protons that electrons are said to be positively charged ions or cations. Sometimes, group of atoms possessing an electric charge which react as a single unit is called a radical.