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Models of the Atom
A review of the Evolution of the
Model of the Atom
Chapter 13-1
The Different Models of the
Atom
Thomson
Model
 In
the XIX,
Thomson described
the atom as a ball
of positive charge
with a number of
electrons.
 Said the atom was
like plum pudding.
Rutherford
Model
 In
the Early XX,
Rutherford showed
that the most of the
atom’s mass is
concentrated in a
small, positively
charged nucleus.
 Atom is mostly
empty space.
Bohr
Model
 After
Rutherford,
Bohr proposed that
electrons travel in
definite orbits
around the nucleus.
Quantum
Mechanical
Model
 Modern
atomic
theory describes
the electronic
structure of the
atom as the
probability of
finding electrons
within certain
regions in space.
Energy Levels/Shells
Energy
Level
Quantum
 The
energy level of an
electron is the region
around the nucleus
where the electron is
likely to be moving.
 It
is the amount of
energy required to move
an electron from its
present energy level to
the next higher one.
Electron Configuration
 What
is it?
 The
configuration is
described by 4
quantum numbers.
 Just
like a point can
be described by the
(x,y) coordinates
 An
electron can be
described by 4
quantum numbers.
the atom, electrons
Configuration and the nucleus
interact to make the
most stable
arrangement possible.
 The way in which
electrons arranged
around the nuclei are
called electron
configurations.
 Electron
 In
 Principal
 Each
Principal quantum
number refers to a
Quantum
numbers (n) energy level in an atom.
 Look at the periodic
 Also known
table.
as Principal
 Along the left side
energy level.
you will see numbers
from 1 to 7.
 These represent the
energy levels of
electrons for that row
of atoms. n=1, n=2 ,
n=3 …
Principal
energy Level
Number
Type of sublevel
of
sublevels
n=1
1
1s (1 orbital)
n=2
2
2s (1orbital) 2p (3 orbitals)
n=3
3
3s (1 orbital) 3p (3orbitals)
3d (5 orbitals)
n=4
4
4s (1 orbital) 4p (3 orbitals)
4d (5 orbitals) 4f (7 orbitals)
Electron Arrangement in
Atoms

The Three Rules
that tell you how to
find the electron
configuration of
atoms

Aufbau Principle

Pauli exclusion
Principle

Hund’s rule
 Aufbau
Principle
 Pauli
Principle
 Electrons
enter
orbitals of lowest
energy first.
 An atomic orbital
may describe at
most two electrons
 To
occupy the
same orbital, the
two electrons must
have opposite spins
 Hund’s
Rule
 When
electrons occupy
orbitals of equal energy,
one electron enters
each orbital until all the
orbitals contain one
electron. Only after all
the orbitals are filled will
electrons pair-up.
• This applies to the p, d and f
sublevels.
Electrons enter orbitals
of lowest energy first.
n=1
n=2
n=3
n=4
And so on
Notation for Quantum Numbers
Principle Quantum Number (n)
1s
2
Number of Electrons
Angular Momentum
In other words
This would the
second electron
1s
The first energy
level
2
the s sublevel of the 1st
energy level
 An
atomic
orbital
may at
most
describe
two
electrons.
 See pg
365 figs
13-4 and
13-5
• s orbital can hold 2 electrons
– it has 1 orbital
• p orbitals can hold 6
electrons – each of the 3
orbitals can hold 2 electrons
• d orbitals can hold 10
electrons – each of the 5
orbitals can hold 2 electrons
• f orbitals can hold 14
electrons – each of the 7
orbitals can hold 2 electrons
electron  Only after all the
orbitals
are
filled
enters each
will electrons pairorbital one
up.
at a time
 For example:
until all the
 Carbon has 6 electron
orbitals
so it’s configuration is:
1s2
2s2 p2
contain one
electron.
n=1
 An
and it fills
in this
order
n=2
THE ULTIMATE TOOL
Sample Test Question
 Write
the complete electron
configuration for each atom.
a.
b.
Argon
Carbon
Answers
1S2 2S2 2P6
a.
Argon
a.
2
Carbon 1S
2S2
3S2 3P6
2P2