Modern atomic theory:

 According to modern atomic theory, an atom is composed mainly of three types of particles called protons, neutrons and electrons. Protons are positively charged particles and are equal in weight to hydrogen atoms. Neutrons are neutral particles and are equal in weight to protons. Electrons are negatively charged particles and have negligible weight. The theory also states that atoms of same element may possess different atomic weights. And in some cases, atoms of different elements possess same atomic weight and different properties. Atom is no longer indestructible because the mass of an atom can be changed into energy E = mc². Transmutation is the process in which atom of one element may be changed into atoms of another element. Atoms of the same element may possess different relative masses and this is called isotopes. Isotopes are atoms of the same element that differ in number of neutrons.

An atom is neutral, the number of protons and electrons in any atom is the same. Almost the entire mass of the atom is concentrated in a very small part of its total size. This part is called the nucleus and all protons and neutrons are embedded in it. The atomic mass depends upon the number of protons and neutrons present in its nucleus. The atomic mass is also called the mass number of the atom and is denoted by the symbol A. The number of protons in the nucleus of an atom or the number of electrons in its extra-nuclear part is called its atomic number and is denoted by Z. The atomic number is the sole factor that determines the identity of the atom. A mole is defined as the amount of any substance that contains as many particles as there are atoms in exactly 12 g of carbon-12. This number is known as Avogadro's number and is equal to 6.02x10²³. Avogadro's number is used as a conversion factor between the number of particles and the number of moles.