Download Lesson 4 - 5.1: Models of the atom

UNDERSTANDING THE
PROPERTIES OF
ELEMENTS
Chapter 5
5.1 Evolution of the Atomic Model
 An
atom is the smallest
particle of an element that
retains the identity of the
element
John Dalton (1808)
Dalton’s Atomic Theory
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All matter is made up of small particles called
atoms.
Atoms cannot be created nor destroyed.
All atoms of the same element are identical in
mass and size. The atoms of one element are
different is mass and size from the atoms of
other elements.
Compounds are created when atoms of different
elements link together to definite proportions.
1. Dalton’s Model of the Atom

Dalton’s atomic model – an atom is
a solid sphere

The atom contains no protons, no
electrons and no neutrons
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Analogy – compared to a billiard ball
JJ Thomson (1897)
2. Thomson’s Discovery of
Electrons
When electric current was applied to
cathode (negatively charged), a ray was
emitted and moved toward the anode
(positively charged)
 Different elements in the gas discharge
tube emitted the same kind of ray
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http://www.youtube.com/watch?v=IdTxGJj
A4Jw
Thomson concluded that:
Thomson’s model – the
atom is a positive sphere
with embedded electrons
 Net charge of zero
 No protons and no
neutrons
 Identified as the raisinbun model or plum
pudding model

Ernest Rutherford (1914)
3. Rutherford’s Discovery of
Nucleus
Rutherford’s model:
He aimed a type of radiation called alpha
particles (positive) at a thin sheet of gold foil
 He predicted based on Thomson’s raisin-bun
model that the particles would pass straight
through the gold foil
 However, a small number of alpha particles
bounced almost straight back
from the gold foil
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Nuclear model
 An
atom has a tiny, dense, positive core
called a nucleus
 The nucleus is surrounded mostly by
empty space and there are no neutrons
http://www.youtube.com/watch?v=5pZj0u_XMbc
Niels Bohr (1921)
4. Bohr’s Description of Energy
Levels
Bohr supported Rutherford’s theory but
explained it further.
 Proposed that electrons could only move
within fixed regions or energy levels
around the nucleus

Bohr --Explained that an electron must absorb a
specific amount of energy (quantum) in
order to move from one energy level to a
higher one
 Compared the idea to the steps on a
ladder which can only be at certain
distances from the ground. It is not
possible to stand in between the rungs of
a ladder.

Bohr concluded that:
Electrons could only occupy certain
energy levels
 Absorption of a specific amount of energy
will cause the electron to move to a higher
energy level
 The atom has a central positive nucleus
and negative electron in energy levels
around the nucleus.
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Bohr’s Theory
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electrons move around the
nucleus in orbits like the
planets orbiting the sun
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http://www.youtube.com/watc
h?v=wCCz20JOXXk
Other Discoveries:
Proton – positively charged particle found
in the nucleus of an atom
 Neutron – particle without a charge found
in the nucleus of an atom (by Chadwick)
 Both these particles provided the evidence
needed by Rutherford to account for the
mass of the nucleus
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In summary --
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Dalton – The atom is the smallest particle of
matter which is indestructible.
Thomson – The atom has negatively charged
electrons embedded in a mass of positively
charged material.
Rutherford – The atom has a very small dense
positively charged nucleus with electrons
moving around it.
Bohr-Rutherford – The electrons in an atom
occupy energy levels around the nucleus
Homework
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Do pg 186 # 1, 2, 3, 5, 6