Modern Atomic Theory

According to the modern atomic theory, an atom is composed of 3 types of particles mainly called protons, neutrons and electrons. The protons are positively charged particles and are equal in weight to the hydrogen atoms. The neutrons are neutral particles and are equal in weight to protons. The electrons are negatively charged particles and have negligible weight. The theory states that the atoms of same element can possess different atomic weights. And in some of the cases, atoms of different elements have same atomic weight and different properties. Atom is no longer indestructible as the mass of an atom can be changed in the form energy E = mc². Transmutation is the process in which the atom of one element can be changed into atoms of another element. Atoms of same element can possess different relative masses and this is called as isotopes. Isotopes are the atoms of same element which differ in number of neutrons. 

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