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Elementary Particles

The word Elementary indicates only single piece where no sub particles are present. Thus Elementary Particles means that they are not themselves made up of any sub structure or in other words they are the basic building blocks of particles and other things in the universe.

In the past (early 19th century) it is considered that the atom is the elementary particle. The meaning of atom itself is “indivisible” that is particle which cannot be further divided into sub particle.

In the late 19th century and early 20th century more sub particles like neutrons, protons, electrons are examined by the scientist and hence it was concluded that the atom itself is made of the sub particles and hence cannot be treated as the elementary. At the same time a new concept of quantum mechanics propped up in the scientific arena and it revolutionized our understanding of the particles and sub particles. It opens a new dimension known as “Quantum Mechanics”.

The elementary particles are distinguished with respect to their spin. The particles with half integer and integer spin are considered as the elementary particles. The example of the half integer spin particles is fermions and that of integer spin is boson.


Standard Model of Elementary Particles

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An Elementary Particle or fundamental particle is a particle which do not a have substructure or it is not known to be made up of smaller particles. If an elementary particle truly has no substructure, it will be one of the basic building blocks of the universe from which all other particles are made. The quarks, leptons, and gauge bosons are elementary particles known In the Standard Model.

Elementary Particle

There are six types of quarks, which are known as flavors: up, down, charm, strange, top, and bottom.
Up and down quarks are stable and have the lowest masses compared with all the other quarks. The heavier quarks are unstable and rapidly change into up and down quarks through a process of particle decay.

Standard Model of Elementary particle include followings:
  1. Six "flavors" of quarks which are as follows: bottom, top, strange, and charm, up, down.
  2. Six types of leptons which are as follows : electron, electron neutrino, muon, neutrino, muon , $\tau$ neutrino, $\tau$.
  3. (force carriers) Twelve gauge bosons the photon of electromagnetism, the three W and Z bosons of the weak force, and the eight gluons of the strong force.
In the standard model of elementary particles, the elementary particles considered are bosons and fermions. The bosons and fermions are distinguished by their spin.

The fermions obeys the “Pauli’s exclusion principle”. Each fermion has an antiparticle associated with it. The fermions are further classified into 2 broad categories or types, Quarks and Leptons. These are further divided into 12 different types.

The elementary bosons also have two types:
  1. Gauge boson
  2. Higgs boson.
The Gauge boson is further classified into three types namely
  1. Photon
  2. Gluon
  3. Z boson
  4. W boson.
The Higgs boson is currently (as of 2012 A.D.) a theoretical particle since it is not experimentally experienced and efforts are underway to determine its existence.

List of Elementary Particles

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As discussed above, in this section we will further discuss the fermions and the bosons and their different types in brief detail.
The 12 fundamental fermions are divided into Quarks and Leptons (six each).

Quarks are the fundamental constituent of the matter. The quarks combine to form a composite particles like baryons or mesons (together baryons and mesons are known as hadrons). The stable hadrons are neutrons and protons.
The Leptons are of two types:
  1. Charge leptons
  2. Neutral leptons.
The Charged leptons combines with particles like hadrons to form atoms and molecules, whereas the neutral leptons are not associated with any atoms and hence are rarely experienced.

The quarks and leptons are further divided into three generations first, second and third generation. The following table gives a brief view of these.

First Generation
Second Generation
Third Generation
Name Symbol Name
Electron e-
Muon $\mu^{-}$ tau
Electron neutrino v
Muon neutrino
tau neutrino N
e $\mu$
up quark u charm quark ctop quark T
down quark d strange quark
s bottom quark

As we know that each fermions is associated with its antiparticle and hence in the below table the fermions antiparticles are shown with respect to its generation.

Particle Generation
First Generation Second Generation
Third Generation
Name Symbol
Antielectron(positron) e+ antimuon $\mu^{+}$
electron antineutrino
muon antineutrino
tau antineutrino
up antiquark
u charm antiquark
top antiquark t
down antiquark
strange antiquark
bottom antiquark

W & Z bosons:
These are together known as weak bosons. These bosons carry force in the weak force field. These are represented by the symbols (W, Z). The W bosons can have two types W+, W- where each of them carries 1 unit charge. The W+, W- are each others anti particle. The Z boson is electrically neutral and hence it is its own antiparticle.

These bosons carry force in the strong force field. These are exchange particles between the interactions of the quarks. The Gluon is the vector boson same as the photon.

Atomic mass of an atom is the sum of the number of protons and neutrons present in the nucleus and is expressed in atomic mass units (amu).One atomic mass unit is defined as the mass of $\frac{1}{12^{th}}$ of a carbon atom (isotope : C-12).
Standard Atomic mass is the mass of an atom or molecule on a scale where the mass of a carbon-12 (12C) atom is exactly 12. Atomic mass or the mass of any atom is approximately equal to the total number of its protons and neutrons multiplied by the atomic mass unit (u).
Its value is 1.660539 × 10-24 gram since electrons are much lighter. The atoms do not differ much from this simple formula, but only by less than 1%.
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Atomic Mass is the mass of a single atom and it is usually expressed in atomic mass units (or amu). Most of the atomic mass is concentrated in the nucleus, in the protons and neutrons contained.
Average atomic mass is the mass of all atoms present in the molecule. It is given by :
Average Atomic Mass = $\sum$ (Mass of isotope $\times$ Percentage of Occurrence)To find the Average Atomic mass of an atom, we consider all of the isotopes that exist and the percentage of each type.
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Atomic Radii may be defined as the distance between the nucleus and the outermost electronic level of the atom.
As there is no exact definition for the atomic radius, a number of radii have been defined for an atom.
They are
  1. Covalent radius
  2. Crystal radius
  3. Vander Waal radius
Covalent radius : It is used to measure the atomic radii of non- metals. The atomic radius of a non- metal is calculated from the covalent bond length. In case of Homo-nuclear diatomic molecules ( type AA) like F2, Cl2,Br2 ....etc half of the covalent bond length is taken as atomic radius. For example the value of Cl - Cl bond distance is 1.98 Ao half of the distance 0.99 Ao is taken as the atomic radius of chlorine.

Crystal radius : It is otherwise called as Atomic or Metallic radius, and defined as one half of the distance between the nuclei of two adjacent metal atoms in the metallic close-packed crystal lattice. For example the internuclear distance between two adjacent Na atoms in a crystal of sodium metal is 3.80 Aoand hence the atomic radius of a Na metal is half of the distance,
i.e., 3.80 $\frac{A^{o}}{2}$ = 1.90 Ao.

Vander waal radius : The distance between the two non-bonded isolated atoms or the distance between two non-bonded atoms belonging to two adjacent molecules of an element in the solid state is called Vander Waals distance while half of this is called Vander Waals Radius.Example : The Vander Waals distance of Cl2 molecule = 3.6 Ao. Half of this value is 1.8 Ao is the Vander Waal radius of chlorine atom.
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More topics in Elementary Particles
Atomic Mass Average Atomic Mass
Atomic Radii
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