Dalton in 1808 published his work under the name
Dalton's Atomic Theory.
The fundamental particles in an atom are electrons,
protons and neutrons. Electrons are negatively charged
particles, protons are positively charged particles and
neutrons are neutral particles.
Protons and neutrons are found in the nucleus of an
atom. Electrons move around the nucleus in regions
called shells or energy levels.
The atomic number of an element is the number of protons
in the nucleus of an atom; it determines the identity of
The mass number is the sum of the number of protons and
the number of neutrons in the nucleus.
Isotopes are atoms of same element that have the same
atomic number' (number of protons) but different number
Substances which give off invisible rays that affect
photographic plates in the same way as light does are
called radio-active substances and the phenomenon is
termed as Radio-activity. The radiation from
Radioactive, substances are of three types (a) Alpha
particles or helium nuclei (b) Beta particles or
electrons and (c) gamma rays or high energy x-rays.
According to Rutherford atomic model, an atom consists
of small dense, and positively "charged centre, called
nucleus, which is surrounded by electrons at relatively
greater distance from it.
Neil Bohr proposed a model of the atom in which the
electrons in a hydrogen atom moves around the nucleus in
fixed orbits. The line spectrum is only produced when an
electron jumps from higher orbit to lower orbit.
An electronic configuration is a particular distribution
of electrons among the different shells of an atom.