According to modern atomic theory, an atom is composed mainly of three types of particles – protons, neutrons and electrons. PROTONS are positively (1 unit) charged particles and are equal in weight to hydrogen atoms. NEUTRONS are neutral particles (no charge) and are equal in weight to protons, i.e. Hydrogen atoms. ELECTRONS are negatively (1 unit) charged particles and have negligible weight (1 / 1850th of a hydrogen atom). An atom, on the whole, is neutral and therefore, 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. Most of the atom is without any mass and is called the extra-nuclear part. Electrons are present in this part and are in continuous motion in fixed orbits (or shells) of fixed energy, around the nucleus. The distribution of electrons in the shells of any atom is called its electronic configuration. The mass of the atom (called 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.
