# Introduction to Atomic Structure

The smallest unit of material, atom, consists of three particles. They are

1. Electron,
2. Proton, and
3. Neutron.

Electrons are negatively charged particles of magnitude 1.6 × 10-19 C (Coulomb), whereas protons are positively charged particles of equal magnitude. Neutrons are neutral particles, having no charge. Proton and neutron are the nucleons bounded by nuclear forces into atomic nuclei. A neutral atom has the same number of electrons and protons. Neutrons and protons have very similar masses, roughly equal to 1 a.m.u. each. Mass of an electron is negligible on that of proton or neutron.

Whereas protons are positively charged particles of equal magnitude. Neutrons are neutral particles, having no charge. Proton and neutron are the nucleons bounded by nuclear forces into atomic nuclei. A neutral atom has the same number of electrons and protons. Neutrons and protons have very similar masses, roughly equal to 1 a.m.u. each. Mass of an electron is negligible on that of proton or neutron.

The following are the basic terminology related to characteristics of an atom:

1. Atomic Number: A chemical element is characterized by the number of protons in the nucleus. Atomic number (Z) is the number of protons found in the nucleus of an atom, identical to the number electrons for an electrically neutral atom. The atomic number ranges in integral units from 1 for hydrogen to 92 for uranium, the highest of the naturally occurring elements.

2. Atomic Weight: Atomic weight of an element is the average relative weight of its single atom as compared to the weight of one atom of carbon-12 which taken to be 12. All atoms having different atomic weights but belonging to the same element are called isotopes.

3. Atomic Mass Unit: Atomic mass unit (a.m.u) is a unit of mass, equal to 1.6605 × 10-27 kg. One mol of an atom is the amount of mass in grams equal to the atomic mass in a.m.u. of the atoms. Thus, one mole of carbon has a mass of 12 g.

4. Avogadro’s Number: Avogadro’s number is formally defined as the number of carbon-12 atoms in 12 grams of unbound carbon-12 in its rest-energy electronic state. Thus, it is the reciprocal of atomic mass unit:

NA = 1/( 1.6605 × 10-27)

= 6.0223 × 1026 /k.mole

= 6.0223 × 1023 /mole

The number of atoms in one mole of a substance is called the Avogadro’s number (NA).