Atom Percent (at %)

It is defined as concentration specification on basis of number of moles (or atoms) of a particular element relative to the total number of moles (or atoms) of all elements within an alloy.


It is tiny basic unit of matter which is neutral and from which any substance gets constructed. It has positively charged nucleus constituting nucleons (Protons & Neutrons) surrounded by electrons revolving round the nucleus.  

Atomic Clock

It is accurate type of time device in the world, which is controlled by vibrations of an atomic or molecular system such as Cesium or Ammonia. The precision of an atomic clock depends on fact that an atom, caused to oscillate, will always vibrate at same frequency. NIST-F1, the United States standard atomic clock, is said to be so accurate that it would neither gain nor lose a second in over 30 yrs. They are used as primary standard in global navigation satellite system such as GPS.

Atomic Mass Unit (amu)

It is defined as 1/12 of mass of Carbon-12 atom. It is average of Proton & Neutron rest mass i.e. 1.67377 x 10-24 gm. The unit is used to express relative masses of various isotopes of element.

Atomic Number

The number of protons found inside the nucleus of a particular atom. The modern periodic table is structured based on this number.

Atomic Orbital

A quantum state attributed to electron around atom in quantum mechanics to describe wave characteristic of electron.  It is expressed by mathematical function which is used to calculate probability of finding any electron in any specific region around atom’s nucleus.

Atomic Packing Fraction

It is the ratio of volume of atoms per unit cell to volume of unit cell in a crystal structure.

Atomic Physics

It is study of structure of atom, arrangement of electrons around nucleus i.e. energy states, interaction of atoms with other particles and with electric and magnetic fields.

Atomic Radius

It is defined as half the distance between neighboring atoms in a crystal of a pure element.

Atomic Structure

It is study of configuration of atom which is essential for knowing many phenomenon involving electrons. Bohr’s atomic model is early model which has explained structure of atom to a great extent. Later the limitations of that model overcome by attributing dual nature to particle. The atomic structure represents the manner in which electron states are occupied.

Atomic Weight

It is the weighted average of the atomic masses of an atom’s naturally occurring isotopes. It is usually expressed in amu.


Total no. of atoms present in one molecule of an element of a substance is called as atomicity.


Prefix in metric system denoting a factor of 10-18.

Audio Frequency

It is the frequency range of periodic vibrations audible to human ear. The range is 20Hz to 20 KHz.

Audio Meter

It is an instrument for measuring hearing activity for pure tones of normally audible frequencies. 

Aufbau’s Principle

Principle which states that sub-shell of lower energy in an atom is filled up first while that of higher energy is filled up later.

Auger Effect

See Auger electron.

Auger Electron

In photoelectric effect, following expulsion of photoelectron, another electron from high energy state may occupy its location emitting X-ray photon. This photon may in turn eject electron from loosely bound outer electron shell, called auger electron and such photoelectric effect is called “Auger Effect”.


It is a luminous atmospheric phenomenon appearing as streamers or bands of light sometimes visible in the night sky in northern or southern regions of the earth. It is thought to be caused by charged particles from the sun entering the earth’s magnetic field and stimulating molecules in atmosphere.