Transcript Atomic Structure Electrons in Atoms

ELECTRONS IN THE ATOM
UNIT 4
OBJECTIVES
1. Explain how atomic emission spectra
can be used to identify elements
2. Describe Bohr’s model of the atom.
3. Describe the Quantum Mechanical
model of the atom
4. Write elements’ electron
configurations.
HOW DO WE KNOW WHAT THE
STARS ARE MADE OF?
ATOMIC EMISSION SPECTRA
• When an element is heated, its atoms absorb energy
and become excited
• To become stable again, these excited and unstable
atoms then release the energy as light
• If this light is passed through a prism the element’s
atomic emission spectrum is produced
ATOMIC EMISSION SPECTRA
• An element’s atomic emission spectrum is the set
of wavelengths (colors) of light given off when
atoms of that element are excited (e.g. heated)
• Each element’s emission spectrum is unique and
can be used to identify the element
– It is the element’s “fingerprint”
HOW DO WE KNOW WHAT THE
STARS ARE MADE OF?
• Scientist analyze the light from a star using
spectroscopes (similar to powerful prisms)
• Match the frequencies of light to the known
spectra of the elements
• Stars are made of the same stuff as the rest
of the Universe: 73% hydrogen, 25%
helium, and the last 2% is all the other
elements
LIGHT
• Visible light is a type of electromagnetic radiation
• All other electromagnetic radiation is invisible
• Electromagnetic (EM) radiation is energy that
travels through space in the form of
electromagnetic waves
• The electromagnetic spectrum encompasses all
forms of electromagnetic radiations
increasing energy
BOHR’S MODEL OF THE ATOM
• Bohr studied the emission spectrum of
hydrogen and developed his model of the
atom
• The Bohr model describes the atom as a
small, positively charged nucleus
surrounded by electrons that travel in
circular orbits around it
THE BOHR MODEL OF THE
ATOM
• Each orbit or “ring” has
a distinct energy levels
or quantum number (n)
– the bigger the number
the higher the energy
• Electrons in smaller
orbits closer to the
nucleus have less
energy than electrons
found in larger orbits
farther from the nucleus
BOHR’S ATOM CONTINUED
• The lowest energy state of an atom is its ground state
• When an atom gains energy (through heating for example) it
is in an excited state
• in an excited state the electron absorbs the energy & jumps
to higher energy level
• when it falls back down to its ground state it releases excess
energy in the form of light
BOHR MODEL CONTINUED
• Because electrons jump between orbitals that have
specific energy levels only certain colors can be
given off
• This is how Bohr explained hydrogen’s emission
spectrum
color of light
Transition
emitted
n = 3 to n = 2
red
n = 4 to n = 2
blue-green
n = 5 to n = 2
blue
n = 6 to n = 2
violet
Wait!
• Bohr’s model explained the emission
spectrum of Hydrogen, but it did not explain
the emissions of any other element!
THE QUANTUM MECHANICAL MODEL
OF THE ATOM
1. Electrons behave like
waves
2. It is impossible to know the
exact location or the
velocity of an electron in an
atom
•
(they don’t travel in circular
orbits around the nucleus)
3. Although it’s impossible to
describe the exact location
or describe how they are
moving, the model describe
the probability that
electrons will be found in
ATOMIC ORBITALS
• An atomic orbital is a
three-dimensional
pocket of space around
the nucleus that the
electron is most likely
to be found
• An electron has a
90% chance of
being found within
that space
• That is the best we
can do!
ATOMIC ORBITALS
ORGANIZATION OF ATOMIC ORBITALS
value:
description
:
1. Principal energy
level (n)
2. Energy
Sublevel
n = 1-7
-(n) indicates
relative size and
energy of orbital
-As (n) increases
so do energy and
size
s, p. d, f
-sublevels are
labeled
according to
shape:
s: spherical
p: dumbbell
d/f: varied
3. Orbitals
1, 3, 5, 7
-each sublevel
has a certain
number of
orbitals:
s=1
p =3
d =5
f=7
-each orbital
can hold two
electrons
QuickTime™ and a
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are needed to see this picture.
ELECTRON CONFIGURATION
• An atoms electron configuration is the way an
atom’s electrons are distributed among the
orbitals of an atom
• The most state stable electron configuration
is an atom’s ground state
– Ground state: all electrons are in the lowest
possible energy state
• Electron configuration represented by writing
symbol for the orbital and a superscript to
indicate the number of electrons in the orbital
Li: 1s2 2s1
increasing
energy
Each orbital can hold two electrons
4d
5s
4p
Energy
3d
4s
3p
3s
2p
2s
1s


  
Hydrogen
Beryllium
Nitrogen
Oxygen
Fluorine
Helium
Lithium
Boron
Carbon
Neon
2
1
3
4
5
6
7
8
10
9
He
Ne
Be
Li
O
H
C
N
B
F
4.003
1.008
6.941
9.012
10.81
12.01
14.01
16.00
20.18
19.00
The Pauli Exclusion Principle
• The two electrons in an orbital must
spin in opposite directions 


  

  
1s
2s
2p
3s
3p
4s
3d
HUND’S RULE
• Negatively charged electrons repel each other, so:
– Electrons won’t pair up unless they have to
– Once there is one electron in every orbital…the
pairing will begin!
1.
  
1s 2s
Add an electron:
2.
2p
   
1s 2s
2p
Add an electron:
3.
    
1s 2s
Add an electron:
4.
2p
    
1s 2s
2p
DRAW THE ORBITAL DIAGRAM
AND WRITE THE ELECTRON
CONFIGURATION FOR:
• Carbon
• Helium
• Potassium
ELECTRON CONFIGURATION
• The periodic table can be divided into four distinct
blocks based on valence electron configuration
• electron configuration explain the recurrence of
physical and chemical properties
SHORTHAND (NOBLE GAS)
NOTATION
• Shows electron filling starting from
previous noble gas:
– Na: 1s22s22p63s1
– Noble gas configuration: [Ne]3s1
WRITE THE FOLLOWING
ELECTRON CONFIGURATIONS IN
NOBLE GAS NOTATION:
• Fluorine
• Titanium
• Beryllium