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ATOM: The Building Block of
Universe
Dr
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Each element in the periodic table has a unique atomic structure, and all atoms for
a given element have the same number of protons
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The Bohr model is often referred to as the planetary model
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For most practical purposes in electronics, the Bohr model suffices
and is commonly used because it is easy to visualize
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❖ The nucleus consists of positively charged
particles called protons and uncharged
particles called neutrons
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❖ Each type of atom has a certain number of
electrons and protons that distinguishes it
from the atoms of all other elements
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Atomic Number
• All elements are arranged in the periodic table of the elements in
order according to their atomic number
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For example, hydrogen has an atomic number of 1 and helium has an atomic number of
2
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Atomic Number

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Atomic Mass
• The atomic mass of a single atom is simply its total mass and is
typically expressed in atomic mass units or amu
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• By definition, an atom of carbon with six neutrons/ six protons,
carbon-12, has an atomic mass of 12 amu
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• In general, though, an atom's atomic mass will be very close to its
mass number but will have some deviation in the decimal places
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• Electrons near the nucleus have less
energy than those in more distant orbits
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Therefore, electrons
must orbit only at discrete distances
from the nucleus
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• In an atom, the orbits are grouped into
energy levels known as shells
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• Each shell has a fixed maximum number
of electrons
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Illustration of the Bohr model of the silicon atom
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The maximum number of electrons
that can exist in the
Inner most shell (shell 1) is
Ne = 2n2 = 2(1)2 = 2

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How to Calculate Maximum Number of
Electrons in Each Orbit: Few more examples
The maximum number of electrons that can exist in shell 2 is
Ne = 2n2 = 2(2)2 = 2(4) = 8
The maximum number of electrons that can exist in shell 3 is
Ne = 2n2 = 2(3)2 = 2(9) = 18
The maximum number of electrons that can exist in shell 4 is
Ne = 2n2 = 2(4)2 = 2(16) = 32

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Writing Electron Configurations
Electron configurations have a standard notation that tells you the principle energy levels
and sublevels that electrons occupy
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In this case, there
are two electrons in an s orbital with the principle energy level of one
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The electron configuration for Lithium is:

1s2 2s1
Lithium, containing three electrons, has two electrons occupying an s orbital at the first
energy level, and one electron occupying an s orbital at the second energy level
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At each preceding element,
pay attention to the energy
level and block it represents
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• This is because the force of attraction between the positively charged nucleus
and the negatively charged electron decreases with increasing distance from the
nucleus
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This outermost shell is known as the
valence shell, and electrons in this shell are called valence electrons
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• When a valence electron gains sufficient energy from an external source, it can
break free from its atom
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Ionization
• When an atom absorbs energy, the valence electrons can
easily jump to higher energy shells
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• The departure of a valence electron leaves a previously
neutral atom with an excess of positive charge (more
protons than electrons)
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Ionization: Positive & Negative
positive ion

negative ion

For example,
When a neutral hydrogen atom
loses its valence electron and
becomes a positive ion, it is
designated H+
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For some nonmetallic materials
such as chlorine, a free electron
can be captured by the neutral
atom, forming a negative ion
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The chlorine ion is designated as
Cl-

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