The document summarizes the historical development of the atomic model and structure of the atom. It discusses: 1) Early scientists like Democritus, Leucippus, and Dalton who proposed ideas of atoms and atomic theory. 2) Experiments by Thomson, Rutherford, Bohr, and Chadwick that led to the discovery of subatomic particles like electrons and neutrons and the nuclear model of the atom. 3) The modern atomic model consisting of a small, dense nucleus surrounded by electrons in energy levels or shells.

1. 1
CHAPTER 4
ATOMIC STRUCTURE
LESSON 1. ATOMIC STRUCTURE
Objectives
1. Describe the basic structure of an atom.
2. Explain what holds an atom together
3. Discuss the historical foundation of modern atomic structure
Definition: An atom is the smallest particle of any element that still retains the characteristics
of that element.
Scientists and Their Contribution to the Development of the Atom
1.Democritus (460- 370 B.C.) proposed that all things are composed of tiny indivisible atoms.
To Democritus, atoms were completely solid, homogeneous, indestructible objects
2. Leucippus (45y Century BC( - a Greek philosopher who introduced the idea of indivisible
atom units called atomos which means uncut.
3.John Dalton(1766 -191) –developed the Dalton's Atomic Theory with on the following
postulates :
a) All elements are composed of atoms, which are indivisible and indestructible
particles.
b) All atoms of the same element are exactly alike; in particular, they all have the same
mass.
c) All atoms of different elements are different; in particular, they have different masses
d) Compounds are formed by the joining of atoms of two or more elements. In any
compound , the atoms of the different elements in the compound are joined in a
definite whole-number ratio, such as 1 to 1, 2 to 1, 3 to 2, etc.
4. Eugene Goldstein –he discovered the proton with the use of gas discharge tube
5. Joseph John Thomson(1856-1940) – he was credited with the discovery of electron.
6. Ernest Rutherford (11871-1973- discovered the nuclear atom; he performed the alpha
scattering experiment and concluded that the atom had a small and very dense positively
charged nucleus where the mass t=of the atom is concentrated,
7. Niels Bohr(1885-1969. – proposed that the electrons in an atom could only be in a certain
orbits or energy levels around the nucleus.
8. Werner Heisenberg (1901 -1976) – pointed out that it is impossible to know both the exact
position and momentum of an object at the same time called the Heisenberg Uncertainty
Principle
9. Erwin Schrödinger(1887-1961) – introduced the ‘wave mechanics” as a mathematical model
of an atom –viewed electrons as continuous clouds(electron cloud)
10. James Chadwick(1891-1961) – discover the neutron, a neutral atomic particle with a mass
close to a proton.

2. 2
Diagram of the Historical Development other ATOM
What does an atom look liked ? This model on the right
side is a culmination of the work of 5 eminent scientists ,
Dalton, Thomson, Rutherford, Bohr and Chadwick.
Their work basically culminates in the important
fact that an atom consists of:
 A nucleus containing protons and neutrons
surrounded by electrons
 Electrons in orbital around the nucleus.
 An atom is held together by forces of attraction
between the electrons and the protons. The neutrons
help to hold the protons together. Protons and neutrons
are believed to be made up of even smaller particles
called quarks.
Table 4.1: The Three Major Subatomic Particles
Name Symbol Charge Mass (g) Mass (amu) Location
Proton P+
+1 1.673 x 10-24
1 Nucleus
Neutron n0
0 1.675 x 10-24
1 Nucleus
Electron e-
–1 9.109 x 10-28
0.0005 Outside Nucleus
In the table, the masses of the subatomic particles are listed in two ways: grams and amu,
which stands for atomic mass units. Expressing mass in amu is much easier than using the
gram equivalent.
The table shows the mass in grams of protons and neutrons are almost exactly the same, both
protons and neutrons are said to have a mass of 1 amu. Notice that the mass of an electron is
much smaller than that of either a proton or neutron. It takes almost 2,000 electrons to equal the
mass of a single proton.
The table also shows the electrical charge associated with each subatomic particle. Matter can
be electrically charged in one of two ways: positive or negative. The proton carries one unit of

3. 3
positive charge, the electron carries one unit of negative charge, and the neutron has no charge
— it's neutral.
Scientists have discovered through observation that objects with like charges, whether positive
or negative, repel each other, and objects with unlike charges attract each other.
The atom itself has no charge. It's neutral. (Well, actually, certain atoms can gain or lose
electrons and acquire a charge. Atoms that gain a charge, either positive or negative, are
called ions.)
So how can an atom be neutral if it contains positively charged protons and negatively charged
electrons? The answer is that there are equal numbers of protons and electrons — equal
numbers of positive and negative charges — so they cancel each other out.
The last column in the table lists the location of the three subatomic particles. Protons and
neutrons are located in the nucleus, a dense central core in the middle of the atom, while the
electrons are located outside the nucleus.
Atomic Number, Mass number and Isotopes
 Atomic number = no. of protons and no. of electrons It defines the element, no two
elements can have the same atomic number.
Atomic number is often written as a subscript in front of the symbol of an element.
E.g. 6C, 92U or 53I etc
 Mass number = no. of particles in nucleus
 No. of neutrons = Mass number – Atomic number
Some elements contain different numbers of neutrons, but have the same number of protons
(these are what specify an element remember). Two atoms with the same number of protons,
but different numbers of neutrons are called isotopes.
Symbol Notation
“Symbol Notation is an appropriate way of representing the composition of a particular
atom.
Example: Helium
Li, is that for lithium symbol:
Ti, is that of titanium
The 22 is the atomic number for titanium
and 48 is its mass number. The number of
neutrons is 48 minus 22 = 26.

4. 4
Exercise: Fill in the rest of the table with your knowledge of the relationships between subatomic
particles.
Symbol
Notation
Atomic
Number
Mass Number
Number of
Protons
Number of
Neutrons
Number of
Electrons
2 2
23 11
15 16
85 37
53 74
Lesson 2 Arrangement of Electrons and their Energy Level
Objectives
1. Explain how electrons maintain electrical balance
2. Described Describe the energy levels of elements.
3. Write the electronic configuration of common elements
Maintaining electrical balance
Each basic element has a certain number of electrons and protons, which distinguished
each element from all other basic elements. In most elements, the number of electrons is
equal to the number of protons. This maintains an electrical balance in the structure of atoms
since protons and electrons have equal, but opposite electrostatic fields.
ENERGY LEVEL
The electrons surrounding a n atom are located in regions around the nucleus called “energy
levels”.• An energy level represents the 3-dimensional space surrounding the nucleus where
electrons are most likely to be.
Pictured here is an atom of copper, which is much more complex
than either an atom of hydrogen or helium.
The copper atom has 29 protons in its nucleus with 29 electrons
orbiting the nucleus. Notice that in the copper atom, the electrons
are arranged in several layers called shells. This is to graphically
represent that the electrons are at different energy levels within the
atom. The energy of an electron is restricted to a few particular
energy levels. The energy is said to be quantized, meaning that it
cannot vary continuously over a range, but instead is limited to
certain values. These energy levels or shells follow a very
predictable pattern. The closest shell to the nucleus can have up to 2 electrons. The second

5. 5
shell from the nucleus can have up to 8 electrons. The third shell can have up to 18 electrons.
The fourth shell can have up to 32 electrons, and so on. Atoms can have this many electrons,
but they do not have to have this many electrons in each shell. The greater distance between
the electrons in the outer shells and the protons in the nucleus mean the outer shell electrons
experience less of a force of attraction to the nucleus than do the electron in the inner shells.
out the outer shell of an atom called the valence shell.
1. Principal Energy Levels
The electrons in an atom are located at specified distances from the nucleus and are found to
have different amounts of energy. Only seven (7) main --principal energy levels, or electron
shells are necessary to account for the electrons of the known elements. The main or principal
energy levels (n) are numbered,starting with n = 1 as the energy level nearest to the nucleus
and going to n = 7 . The energy levels are also
identified by the letters K, L, M, N, O, P, and Q, with K equivalent to the first energy level, L
equal to the second level etc.
• In looking at a diagram of an atom, note that the nucleus is
fairly centrally located. The energy levels are built up from the
level closest to the nucleus outward. This process of filling in
the electrons from the first, lowest energy level to the second,
slightly higher energy level to the third, even higher energy level
is called filling the electrons in by the Aufbau Principle.
The Aufbau principle, very simply stated, is: start at the lowest
energy level and build up to th higher enrgy level only after the
lowest are filled.
The electrons in the energy level farthest from the nucleus are
called valence electrons.
The maximum number of electrons that can occupy a specific
energy level can be calculated by using the
formula 2n2, where n is the number of the principal energy
level. Thus, the third energy level (3) can hold 18 electrons.
[ 2(3)2 = 18]

6. 6
Energy Sublevels
In each principal energy level the electrons are seen to differ in how they carve out space
around the nucleus. The four different patterns are called sublevels. These sublevels contain
the orbitals in which the electrons are located. These four sublevels are identified by the letters
s, p, d, and f. Thus there are sorbitals, p orbitals, d orbitals, and f orbitals. Each orbital type has
a particular spatial arrangement or shape.
An electron spins on its own axis in one of only two directions---clockwise or counterclockwise.
As a result, only two electrons can occupy the same orbital, one spinning clockwise and the
other spinning counterclockwise. When an orbital contains a pair of electrons, the electrons are
said to be paired.
Sublevel Chart
Not all principal energy levels contain each and every type of sublevel. To determine what type
of sublevels occur in an energy level, we need to know the maximum number of electrons
possible in that principal energy level, and we need to use two rules:
1. No more than two electrons can occupy one orbital.
2. An electron will occupy the lowest sublevel possible
Below is a 3-dimensional shape of sublevels,- s,p,d
Electronic Configuration Notation (ECN)
In order to understand the chemical properties of an element, you must be able to place its
electrons in the proper principal energy levels and sublevels. Our next task is to determine the
order in which electrons occupy the various principal energy levels and sublevels. This
arrangement of electrons in the principal energy levels and sublevels is called “electron
configuration notation” ECN. The chart below will help us to determine the proper order for
filling.
Points to Remember:

7. 7
 Energy Level is a specific amount of energy which an electron in an atom can possess.
The energy difference between 2 particular energy level is called the QUANTUM of
energy associated with the transition between the 2 levels
 SHELL is the set of all orbitals having the same n-value.
 SUBSHELL is a set of orbitals of the same type.
 ORBITAL is the actual region of space occupied by an electron in a particular
energy level.
Energy Level Diagrams
& it can also written in...↓↓↓↓
The following notation illustrates how to read ECN:

8. 8
Write the ECN for Oxygen, Z = 8
Write the ECN for Chlorine, Z = 17
Orbital Notation (ON)
Orbital filling diagrams called orbital notation (ON) illustrate the distribution of electrons. Only
those electrons in the outermost principal energy level need to be shown unless a “d” or “f”
sublevel is partiallyfilled, then that sublevel must be shown. All orbital diagrams are shown for
the elements “ground state”,the lowest energy possible. The symbol r e p r e s ents an orbital
containing a pair of electrons spinning in opposite directions--one clockwise and one
counterclockwise. In order to write correct orbital notation you must use the elements electron
configuration notation (ECN). Listed below are the electron
configuration notations
ECN and orbital notations ON for the first seven elements:

9. 9
Hund’s Rule: states that when electrons fill orbitals within a sublevel, each orbital is occupied
by a single electron before any orbital has two electrons, and all electrons in singly occupied
orbitals have the same direction of spin.
Electron Dot Notation or Electron Dot Symbol (EDN)
A third method used to show electron arrangement is the electron dot notation. This notation
uses only those electrons in the outermost principle energy level. The “s” and “p” orbitals are
arranged around the symbol for the element. To write the correct electron dot notation (symbol)
for an element you can use the orbital notation to determine how many paired and unpaired
electrons exist in the outermost level.
Example1
Example 2
Example 3
Example 4
APPLICATION:

10. 10
Direction: Complete the table below on energy level and electronic configuration ECN) and
electron dot notation(EDN)
ENERGY LEVEL AND ELCTRONIC CONFIGURATION
Name of
Element
Symbol
Notation
Shell Method Electronic
Configuration (ECN)
Electron Dot
Notation(EDN)
Arsenic
Bromine
Gold
Mercury
Tungsten