Unit 2

Lesson 2.1

The Concept of Atom in

Ancient Times
Objective
At the end describe the ideas of
of the 1 the ancient Greeks on
lesson, you the atom.
should be
able to:
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The Atom
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The Subatomic Particles

● Protons – positively charged

● Electrons – negatively charged

● Neutrons – neutrally charged

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What Did People Thought of Then?

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Early days of the Atomic Theory

● 5th Century BC

● Greek Philosophers hypothesized about the

composition of matter

● Some ideas were accepted than the others

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Democritus
• Everything is made up of
small indivisible particles
known as atomos.

Democritus
(460 BC–370 BC)
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Leucippus
● Teacher of
Democritus

● Pioneer of the ideas

of composition and
change in matter

Leucippus
(5th century BCE)
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Atomism
● Matter is composed of atomos and void.

● The atomos may combine to form clusters in the void.

● Change can happen when some atoms collide with

each other in the void.
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But not everyone agreed...

• Aristotle thought the the idea
of the atom goes against ex
nihilo and it limits the power
of the gods.
• Aristotle’s nature elements

Aristotle
(384 – 322 BC)
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Atomism Revisited
● The idea of the atom was given importance during the
Renaissance.
● John Dalton formulated his Atomic Theory based on
the atomos.
● Other researches and experiments were conducted
afterwards.
Key Points

Scientific inquiries are very hard to answer in the

1 ancient era because of the lack of appropriate tools.

Ancient Greek philosophers were the first to suggest

2 the concept of the atom.

Democritus and Leucippus were the first Greek

3 scholars who believed in atomism and pioneered the
study of the composition of and changes in matter.
Key Points

According to atomism, nature is composed of two

4 basic principles: atom and void.

The idea of atomism was set aside because of the

5 ideas of prominent natural philosophers like Plato
about nature elements.

However, the Greeks’ concept of atoms and even

6 Aristotle’s arguments were rediscovered and revived
after being proven to be true.
Lesson 2.2

Discovery of the Subatomic

Particles
 point out the main ideas in
the discovery of the
1 structure of the atom and
its subatomic particles;
Objective cite the discovery of JJ
sAt the end Thomson, Ernest
of the Rutherford and James
lesson, you
2 Chadwick to the
should be understanding of the
able to: structurethe
of the atom; and
describe nuclear
model of the atom and the
3 location of its major
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The Atomos of Democritus

● solid

● indestructible

● collides with each other

● can form clusters

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Modification of Democritus’ Atomic Model

● There are smaller subatomic particles.

● Two types of the smaller particles are charged.

● Two types of the smaller particles are found at the

center.

● The particles are very small and the atom is mostly

space.
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Subatomic Particles
● Protons - positively charged and found at the nucleus.
Discovered by Ernest Rutherford.

● Neutrons - neutral and found at the nucleus.

Discovered by James Chadwick.

● Electrons - negatively charged and move around the

nucleus. Discovered by JJ Thomson.
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Dalton’s Solid Sphere Model

● The atom is indestructible.

● The atom is a solid particle.

● Atoms of the same element have the same

properties.

● The atom can combine to form compounds.

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Dalton’s Solid Sphere Model

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Thomson’s Plum Pudding Model

● The atom is divisible into charged particles.

● The atom is made of a massive mass with a positive

charge.

● Electrons are negatively charged particles that are

embedded on the body of the atom.
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Thomson’s Plum Pudding Model

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Rutherford’s Nuclear Model

● The atom is composed of positive and negatively
charged particles.

● The nucleus is at the center of the atom and this is

where the mass of the atom is concentrated.
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Rutherford’s Nuclear Model

● Protons are the positively charged particles and are
found at the nucleus.

● Electrons are the smaller negatively charged particles

and they move around the nucleus.

● The atom is made up of mostly space.

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Rutherford’s Nuclear Model

Key Points

The development of the atomic structure started with

the idea presented by Democritus that all matter is
1 made up of small indivisible particles called atomos.
This idea is widely known in ancient Greece as
atomism.
A more precise description of the atom was presented
by John Dalton later on. In his solid sphere model,
2 he imagined an atom to be extremely small and
indivisible.
Key Points

JJ Thomson’s cathode ray tube experiment allowed

him to discover the electron. He then proposed the
3 plum pudding model where an atom is presented as
a uniform positively charged sphere embedded with
electrons.
The plum-pudding model of JJ Thomson was
challenged when a physicist named Ernest
4 Rutherford performed a gold foil experiment. The
results of his experiment lead to the discovery of the
nucleus, a dense central core with a positive charge.
Key Points

Ernest Rutherford then developed the nuclear model

5 of an atom, in which all the positive charges are in the
nucleus while the electrons are orbiting around it.

James Chadwick proved the existence of neutrons

which are neutral particles in the nucleus of the atom
6 by bombarding alpha particles on beryllium. His
discovery of neutrons solved the mystery on how
protons clump themselves together in the nucleus.
Lesson 2.3

Bohr Model of the Atom

 point out the main ideas
in the discovery of the
1 structure of the atom and
Objective its subatomic particles;
sAt the end
cite the discovery of Niels
of the
Bohr to the understanding
lesson, you 2 of the structure of the
should be
able to: atom; and
describe the Bohr’s model
3 of the atom.
 Learn about It!

The Atomic Models so far...

● Dalton’s Solid Sphere Model

● Thomson’s Plum Pudding Model

● Rutherford’s Nuclear Model

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The Problems of the Nuclear Model

● The eventual crash by the electrons due to:

○ emitted radiation
○ continuous centripetal acceleration
○ increasing electrostatic force
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Another model was conceptualized...

Niels Bohr proposed his

model for the atom
which will answer
questions about
stability and spectral
data.
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Bohr Model of the Atom

● The model aims to explain the stability of the atom:

○ presence of fixed circular orbits or energy levels

○ ground state will have the electrons in constant
energy
○ light energy can be absorbed and be emitted by
electrons
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The orbits are

quantized energy
levels.

There is a specific
amount of energy
required for each
distance and this
energy increases when
.
going farther away
from the nucleus.
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Electrons can Interact with Energy

● Electrons are originally at its ground state.

● Electrons can absorb energy and will become excited.

Electrons jump to higher energy levels when excited.

● Electrons can then relax and go back to its ground

state. This will give off energy that has a specific
wavelength and color.
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The emitted light from

relaxation can be
detected resulting to
spectral lines.

These will give the

distinct color that we
see when metals are
heated or when light
is being reflected by
matter
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The Bohr Model Made Use of Quantum

Principles and Equations

For the energy of the

electron in a specific orbit

For the energy absorbed or

emitted by the electron
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Bohr Model Identified the Presence of

Orbitals
● The Balmer series of spectral lines was able to identify
the presence of orbitals. The spectroscopic data were
described as follows.
○ sharp – s orbital
○ principal – p orbital
○ diffused – d orbital
○ fundamental – f orbital
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There were still some unanswered

questions...
● The model cannot explain phenomena observed when
there are more electrons.

● The model cannot explain paired spectral lines given by

atoms with more than one electrons.
Key Points

The principles of quantum chemistry established

1new
concepts that founded
In Bohr’s atomic Bohr’s
model,atomic model.
the electrons are
positioned in
fixed orbits and revolve around the nucleus.
2These
orbits are also called energy levels because
they have
fixed energies.
Key Points
Bohr’s atom is stable. The electrons in the
allowed
3 orbits do not radiate energy, which prevents the
electrons from being pulled by the nucleus.
Electrons can move between energy levels. The
energy level at which the electron normally
resides is called the ground state. In a process
4 called promotion or excitation, an electron
moves to a higher energy level, called the
excited state, when sufficient energy is
acquired.
Key Points
The electron goes back to its ground state by
relaxation, and energy is released in the form of
light.
5
This explains why metals give off characteristic
color
when heated.
Bohr’s atomic model cannot explain the spectral
6 characteristics of larger atoms.
Lesson 2.4

Quantum Mechanical Model

of the Atom
 point out the main ideas
in the discovery of the
1 structure of the atom and
Objective its subatomic particles;
sAt the end
cite the discovery of Erwin
of the Schrödinger to the
lesson, you 2 understanding of the
should be
able to: structure of the atom; and
describe the quantum
3 mechanical model of the
atom.
 Learn about It!

Problem with the Bohr Model

● Electrons are not found in just one location.

● Electrons are scattered in an area.
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The Development of Quantum Theory

Maxwell, Planck and Einstein developed the theory due
to their work regarding the dual nature of light.

James Clerk Maxwell Max Planck Albert Einstein

(1831–1879) (1858–1947) (1879–1955)
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What is Light?
● Light is both a wave and a particle.
● Light can be reflected and refracted.
● Light can be described by the electromagnetic
spectrum.
● Light can undergo interference.
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Wave-Particle Duality of Matter

● Louie de Broglie proved that electrons can act as
a wave and as a particle.
● Electrons travel around the nucleus in a wavelike
manner.
● Heisenberg postulated his Uncertainty Principle.
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Wave-Particle Duality of Matter

Louis de Broglie Werner Heisenberg

(1892–1987) (1901–1976)
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Quantum Mechanical Model of the Atom

● Proposed by Erwin Schrodinger using the
Schrodinger Equation and his Wave Functions
● Electrons can be found in electron clouds called
orbitals
● Location of electrons is described using
probability.
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The Orbitals
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Quantum Numbers to Describe the

Electrons
● Principal Quantum Number (n) – gives the energy level
● Azimuthal Quantum Number (l)– gives the orbital
● Magnetic Quantum Number (ml) - spatial orientation of
the orbital
● Spin Quantum Number (ms) - rotation of the electron
along its axis
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In the quantum mechanical model, an electron orbiting

the nucleus will have a unique set of four quantum
numbers. The set of quantum number acts as a sort of
address for each electron.
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Principal Quantum Number (n)

• It refers to the distance of the electron from the
nucleus.
• It is also known as the shell.
• The values of n are 1, 2, 3, 4, and so on
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• The maximum
number of
electrons a shell
contains, is
computed by
• where n is the
principal
quantum
number.
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Azimuthal Quantum Number (l)

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The Orbitals
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Magnetic Quantum Number ( )

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Electron Spin Quantum Number ( )

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Key Points
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Electron Configuration
• The electron configuration is simply a description of
how electrons are distributed in an atom. An electron
configuration follows the spdf notation.
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Aufbau Principle
• Atomic orbitals are
filled from the
lowest energy to the
highest energy as
shown in the
diagram :
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• The order of atomic orbitals in increasing energy is

1s, 2s, 2p, 3s, 3p, 4s, 3d, 4p, 5s, 4d, 5p, 6s, 4f, 5d, 6p, 7s,
5d, 6p, 7s, 5f, 6d, 7p and so on.
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Hund’s Rule
• It states that every orbital in the same subshell must
be filled singly before being paired. It also states that
all electrons in singly occupied orbitals should have a
parallel spin.
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Pauli’s Exclusion Principle

• It states that no two electrons can have the exact same
set of quantum numbers. Electrons in the same orbital
must have opposite spins.
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Writing Electron Configuration of Neutral

Atoms
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• Neon
 Check Your Understanding

Analyze the electron distribution for the

following elements:
a. lithium = 3 electrons
b. carbon = 6 electrons
c. oxygen = 8 electrons
d. neon = 10 electrons
e. magnesium = 12 electrons
Check Your Understanding

Given the
information,
try to
distribute
the
electrons in
the atoms.
Check Your Understanding
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Electron configuration
• Hydrogen 1s1
• Helium 1s2
• Lithium
• Beryllium
• Boron
• Carbon
• Nitrogen
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• The quantum mechanical model of the atom is the

most realistic model at this time.
• There are still some concerns but it is already a model
that can explain many of the observed phenomena
regarding the structure of the atom.
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• Arrangement of orbitals by increasing energy

s, p, d, f
• Maximum number of electrons per orbital
s = 2, p = 6, d = 10, f = 14
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• Orbitals present per energy level

1 = s,
2 = s, p
3 = s, p, d,
4, 5, 6, 7 = s, p, d, f
Key Points
Since Bohr’s atomic model cannot explain the
spectral
characteristics of larger atoms, it was replaced by
the
1
quantum mechanical model of the atom where
electrons are found in specific regions rather than
on
fixedde
orbits.
Broglie’s wave-particle duality sees
2electrons as
waves rather than definite particles.
Key Points
Heisenberg’s uncertainty principle states
that it is
3 impossible to know the position and momentum
of an
electron in an atom at any one time.
In the quantum mechanical model of the atom,
the
electrons are confined in a region of space called
4 orbitals. Electrons are most likely to be found in
darker
areas near the nucleus.
 Key Points
The orbitals are mathematical solutions of the
Schrödinger equation, which uses the principles
of de
Broglie’s wave-particle duality and Heisenberg’s
5 uncertainty principle. The square of the wave
function represents the probability at which
the The quantum mechanical model describes
electrons can be found.
electrons
using the four quantum numbers, the
principal
6 quantum number, the azimuthal quantum
number,