General Science (Atomic Structure/Atomic and Molecular Masses)

John Dalton

1803, propounded the atomic theory, according to which an atom is indivisible, and this concept remained predominant up to the end of the 19th century.

Rutherford’s model and Bohr’s model

atoms are composed of a number of micro particles like electron, proton and neutron.

Atom

an element is that smallest particle which takes part in a chemical reaction but doesn’t exist in a free state

Molecule

an element or compound is that smallest particle which doesn’t take part in a chemical reaction but exists in a free state.

Electron

It is a negatively charged particle that rotates in various orbits around the nucleus.

Proton

it is a positively charged stable particle.

Neutron

It is a neutral unstable particle. The masses of a proton and neutron are nearly equal.

Nucleus

consists of protons and neutrons and these are collectively known as nucleons.

Rutherford Atomic Model

1911, an experiment to detect the inner composition or configuration of an atom.

Bohr Atomic Model

In 1913, introduced a quantum concept to explain the stability of an atom.

1961

the International Union of Chemists selected a new unit of expressing the atomic masses

mass spectrometer

The atomic mass can be determined by this instrument by comparing the mass of an atom with the mass of a particular atom chosen as standard.

atomic mass unit

This scale of relative masses of atoms is a scale unit and is abbreviated as a.m.u.

Avogadro

The number 6.022 × 10^23

Mole as a Volume

also related to the volume of the gaseous substance

molar volume

The volume of one mole of any substance is called

Isotopes

atoms of the same element which have the same atomic number but different mass number.

Isobars

Atoms of different elements having the same mass number but different atomic numbers

Isotones

Atoms having the same number of neutrons but different mass numbers

Atomic Number

The number of protons or electrons of an atom of the element

Mass Number

In every atom there is a small central massive part called nucleus

Shell or Orbit

The electrons revolve in the various orbits with different and definite energies

Subshell or Sub-orbit

Every shell or orbit has a number of subshells in which various orbitals are found and these are represented by the various small letters s, p, d and f

Orbital

the three-dimensional space around the nucleus of an atom where there is maximum probability of finding an electron.

Electronic Configuration

A comprehensive and proper distribution of various electrons in various shells and subshells of any atom

Valance electron and Core electron

The electron present in the outermost orbit of the atom is called valence electron whereas the electron present in inner orbit of the atom