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Electrons

An electron is a negatively charged particle present in the atoms. Charge on each electron is
– 1.6 × 10 – 19 Coulomb. These electrons surround the nucleus. Some facts about electrons in
an atom are listed and explained below,

1. If an atom is having the same number of protons and electrons, the atom is electrically
neutral as the negative charge of electrons neutralizes the positive charge of protons.
2. The electrons revolve around the nucleus in shells (also called orbits).
3. A force of attraction is excreted on negatively charged electrons by positively charged

nucleus. This force of attraction works as centripetal force required for electrons
revolution around the nucleus.
4. The electrons which are near to nucleus are tightly bound with the nucleus and it is
more difficult to pull out (remove) these electrons from the atom than those which are
far away from the nucleus.

Each subatomic has its own charge and mass show in Table 2.1. The electron charge cloud
thus constitutes almost all the volume of the atom but accounts for only a very small part of
its mass. The electrons, particularly the outer ones, determine most the properties of the atom.

And thus a basic knowledge of atomic structure is important in the study of engineering
materials.

Table 2.1 The mass and charge of the proton, neutron and electron.
Sub-atom Mass(g) Charge (Coulombs)

-9
-24
Protons 1.673 x 10 + 1.602 x 10
Neutron 1.675 x 10 0
-24
Electron 9.109 x 10 - 1.602 x 10
-28
-19

Atomic Number

The atomic number of atoms is the number of protons in the nucleus of an atom. The
number of protons define the identity of an element (i.e., in Figure 2.2: Shows an element with

6 protons is a carbon atom, and 13 protons is a aluminum atom, no matter how many neutrons
may be present). Atoms of an element that have the same atomic number but a different
number of neutrons are known as the isotopes of the element. Example, titanium ( Ti) is
22
50
composed of five stable isotope; Ti, Ti, Ti, Ti and Ti with Ti being the most abundant
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