ATOMIC THEORY
The Classical Atomic Theory
 If a substance could be divided into smaller and smaller portions of itself,
eventually one should reach the level of particles that could not be divided
any further. These extremely small, invisible, and indivisible particles were
called atoms.
 Atoms of various shapes combine through interlocking patterns to form
the objects of the world.
 Hard and compact substances (such as diamond, iron,..) contained atoms
which were interlocked in a very tight pattern.
 In liquid substances, the atoms were thought to be held together much
more loosely. Also, they thought that liquids made up of round atoms
since they would pour so easily.
Dalton's Atomic Theory (Modern atomic theory):
• Elements are composed of small, indivisible particles called atoms.
• All atoms of element such as iron are identical in mass, size, and shape, and
show the same physical and chemical characteristics.
• Atoms of different elements ( iron & zinc) have a different mass, size, and
shape, and show different physical and chemical properties.
• Atoms of two or more elements combine together to form a compound.
The smallest particle that still has the properties of the compound is called a
molecule.
Internal Structure of the Atom
Atom is made up of three subatomic particles: protons, neutrons, and
electrons.
PROTON: It is an elementary particle with a mass of 1.67×10-24g and has the
smallest unit of positive charge. Protons will repel each other, attract
particles with negative charges, and do not interact with particles that carry
no charge.
ELECTRON: It has the lowest mass, only 1/1836 that of a proton and has a
negative charge which is equal in magnitude to that of the proton. Electrons
repel each other, attract protons, and do not interact with electrically neutral
particles.
NEUTRON: It is a subatomic particle with a mass almost equal to that of the
proton but with no electrical charge. Because of its electrically neutral
nature, this particle will neither attract nor repel positively charged protons,
negatively charged electrons, or other neutrons.
Particle
Relative mass
Relative charge
Proton
1
+1
Neutron
1
0
Electron
1/1836
-1
The Atomic Nucleus according to Rutherford Model
(Solar system model):
• Atom has central positive nucleus. The entire mass of the atom is concentrated in its
nucleus and the rest of the atom was mostly empty space
• The mass of electron is negligible compared to the mass of a proton or a neutron. This
indicated that the protons and the neutrons are located in the nucleus, while the
electrons are found in the outer regions of the atom.
• The positive charge of the nucleus is determined by the number of the protons it
contains. As protons and electrons have equal but opposite charges, it follows that in
an electrically neutral atom the number of protons must be the same as the number
of electrons.
• Rutherford proposed that the electrons orbit the nucleus in the same manner that the
earth and other planets orbit the sun. Therefore, Rutherford's model is often referred
to as the solar-system model of the atom.
Bohr Model of the Atom (electron shell model):
The electron shell model assumes that the electrons orbit around the nucleus on the
surfaces of imaginary spherical shells (levels). These electron shells are concentric about
the nucleus in the same way as the successive layers of an onion are packed together.
There are seven electron shells according to the energy level (n = 1, 2, 3, 4, 5, 6 and 7)
n is known as the principal quantum number.
An electron in a shell with a relatively low value of n is at a shorter distance from the
nucleus than an electron in a shell with a higher value of n.
Electrons in shells with low value n have a lower energy than electrons in shells with
higher value of n.
Each energy level of an atom could only accommodate a certain number of electrons.
The maximum number of electrons is given by the following formula:
Atomic Number and Nucleon Number
The nucleus of an atom always contains a whole number of protons, exactly equal
the number of electrons in the neutral atom.
Atomic number is known as the number of protons in the nucleus of an atom.
Nucleon (Mass) number is known as the sum of the numbers of protons and
neutrons.
Example 1: What is the atomic number of the element uranium which contains
92 electrons in each neutral atom?
Solution:
The number of protons must equal the number electrons in a neutral atom.
Thus, the nucleus of a uranium atom contains 92 protons. The atomic number
of uranium is 92.
Example 2:
The nucleus of an atom of fluorine contain 9 protons and 10 neutrons. What is
the atomic number and the nucleon number of fluorine?
Solution:
Atomic number = Number of protons = 9
Nucleon number = Number of protons + Number of neutrons = 9 + 10 = 19
Isotopes
Isotopes are known as atoms that have the same number of protons but a
different number of neutrons in the nucleus. Thus, isotopes have the same
atomic number but a different nucleon number.
To distinguish between the isotopes of an element, the following symbolic
representation is often used:
Electron Configuration of the Elements:
The arrangement of electrons in an atom is called the electron configuration. When
electron fill the energy levels, it fills the lowest energy level first.
Problem: Give the electron configuration for silicon (atomic number 14) according to
Bohr Model of the Atom?
Solution: Silicon, Si, atomic number 14 and hence 14 electrons. The first shell (K shell)
can accommodate 2 electrons, and the second shell (L shell) can hold 8 electrons.
That leaves 4 electrons to be accommodated in the third shell (M shell).
According to the Quantum Mechanical Model of the Atom:
The Quantum Mechanical Model assumes that each shell is subdivided into several
number of sublevels (s, p, d and f ).
There is only one s-type orbital,
there are three p-type orbitals,
there are five d-type orbitals,
there are
seven f-type orbitals.
The Distribution of Electrons in each Principal Energy Level
Energy
Level, n
Type of
Atomic Orbital
Number of
Atomic Orbitals
Maximum Number of
Electrons per Sublevel
Maximum
Total Number of
Electrons
1
2
1s
2s
2p
3s
3p
3d
4s
4p
4d
4f
1
1
3
1
3
5
1
3
5
7
2
2
6
2
6
10
2
6
10
14
2
8
3
4, 5, 6, 7
18
32
n=1
1S
n= 2
2S
2P
n=3
3S
3P
3d
n=4
4S
4P
4d
4f
n=5
5S
5P
5d
5f
n= 6
6S
6P
6d
6f
n= 7
7S
7P
7d
7f
Name
Atomic Number
Electron Configuration
Hydrogen
1
1s1
Helium
2
1s2
Lithium
3
1s2 2s1
Beryllium
4
1s2 2s2
Boron
5
1s2 2s22p1
Carbon
6
1s2 2s22p2
Nitrogen
7
1s2 2s22p3
Oxygen
8
1s2 2s22p4
Fluorine
9
1s2 2s22p5
Neon
10
1s2 2s22p6
Sodium
11
1s2 2s22p63s1
Magnesium
12
1s2 2s22p63s2
Aluminum
13
1s2 2s22p63s23p1
Silicon
14
1s2 2s22p63s23p2
Phosphorus
15
1s2 2s22p63s23p3
Sulfur
16
1s2 2s22p63s23p4
Chlorine
17
1s2 2s22p63s23p5
Argon
18
1s2 2s22p63s23p6
Potassium
19
1s2 2s22p63s23p64s1
Calcium
20
1s2 2s22p63s23p64s2