Transcript Introductory Chemistry, 2nd Edition Nivaldo Tro

Introductory Chemistry, 2nd Edition
Nivaldo Tro
Chapter 9
Electrons in Atoms
and the
Periodic Table
Why do Blimps Float?
Because they are filled with a
gas less dense than air
Early blimps used hydrogen
gas; hydrogen’s flammability
led to the Hindenburg disaster
Blimps now use helium, a
nonflammable gas – in fact it
doesn’t undergo any chemical
reactions
This chapter investigates
models of the atom we use to
explain the differences in the
properties of the elements
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Electromagnetic Radiation
Light is one of the forms of
energy
Light is one type of a more
general form of energy
called electromagnetic
radiation
Electromagnetic radiation
travels in waves
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Characteristics of a Wave
Wavelength = distance from peak to peak
Amplitude = height of the peak
Frequency = the number of wave peaks that
pass in a given time
Speed = rate the waves travel
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Particles of Light
Scientists in the early 20th century showed
that electromagnetic radiation was composed
of particles we call photons
– Max Planck and Albert Einstein
– photons are particles of light energy
Each wavelength of light has photons that
have a different amount of energy
– the longer the wavelength, the lower the
energy of the photons
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The Electromagnetic Spectrum
Light passed through a prism is separated into
all its colors = continuous spectrum; colors
blend into each other
Color of light is determined by its wavelength
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Electromagnetic Spectrum
Visible light is a very small portion of the
electromagnetic spectrum
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Light’s Relationship to Matter
Atoms can absorb energy, but
they must eventually release it
When atoms emit energy, it is
released in the form of light =
emission spectrum
Atoms don’t absorb or emit all
colors, only very specific
wavelengths; the spectrum of
wavelengths can be used to
identify the element
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Emission Spectrum or Line Spectrum
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Line Spectra = specific wavelengths are
emitted; characteristic of atoms
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The Bohr Model of the Atom
Nuclear Model of atom does not explain how
atom can gain or lose energy
Neils Bohr developed a model to explain how
structure of the atom changes when it
undergoes energy transitions
Bohr postulated that energy of the atom was
quantized, and that the amount of energy in the
atom was related to the electron’s position in
the atom
– quantized means that the atom could only have very
specific amounts of energy
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Bohr Model of Atom: Electron Orbits
In the Bohr Model, electrons travel in orbits
or energy levels around the nucleus
The farther the electron is from the nucleus
the more energy it has
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The Bohr Model of the Atom:
Orbits and Energy
Each orbit (energy level) has a
specific amount of energy
Energy of each orbit is
symbolized by n, with values of
1, 2, 3 etc; the higher the value
the farther it is from the
nucleus and the more energy
an electron in that orbit has
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The Bohr Model of the Atom:
Energy Transitions
Electrons can move from
a lower to a higher
(farther from nucleus)
energy level by absorbing
energy
When the electron moves
from a higher to a lower
(closer to nucleus)
energy level, energy is
emitted from the atom as
a photon of light
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The Bohr Model of the Atom
Ground and Excited States
Ground state – atoms with their electrons in
the lowest energy level possible; this lowest
energy state is the most stable.
Excited state – a higher energy state;
electrons jump to higher energy levels by
absorbing energy
Atom is less stable in an excited state; it will
release the extra energy to return to the
ground state
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Electron Energy Levels:
Energy Level
3rd
2nd
1st
How many e fit?
18 electrons
8 electrons
2 electrons
(2n2)
2 x 32
2 x 22
2 x 12
Each energy level has a maximum # of
electrons it can hold.
H has one electron; it is in the 1st energy level.
H
Bohr model
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Bohr Model for Atom
Electrons fill the Lowest energy levels first
C
Bohr Model for C with 6 electrons
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The Bohr Model of the Atom
Success and Failure
The Bohr Model very accurately
predicts the spectrum of hydrogen
with its one electron
It is inadequate when applied to atoms
with many electrons
A better theory was needed
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The Quantum-Mechanical Model
Orbitals
Erwin Schrödinger used mathematics to
predict probability of finding an electron at a
certain location in the atom
Result is a map of regions in the atom that
have a particular probability for finding the
electron
Orbital = a region with a very high probability
of finding the electron when it has a
particular amount of energy
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The Quantum-Mechanical Model
Each principal energy level or shell has one or
more subshells
– # of subshells same as the principal quantum
number or shell
The subshells are often represented as a
letter
– s, p, d, f
Each kind of subshell has orbitals with a
particular shape
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Shells & Subshells
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Probability Maps & Orbital Shape
s orbitals are spherical
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Probability Maps & Orbital Shape
p orbitals
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Subshells and Orbitals
The subshells of a principal shell have
slightly different energies
– the subshells in a shell of H all have the same
energy, but for multielectron atoms the subshells
have different energies
– s<p<d<f
Each subshell contains one or more orbitals
–
–
–
–
s subshells have 1 orbital
p subshells have 3 orbitals
d subshells have 5 orbitals
f subshells have 7 orbitals
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The Quantum Mechanical Model
Energy Transitions
As in Bohr Model, atoms gain or lose
energy as electron moves between
orbitals in different energy shells and
subshells
The ground state of the electron is the
lowest energy orbital it can occupy
Excited state = when an electron moves
to a higher energy orbital
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The Bohr Model vs.
The Quantum Mechanical Model
Both the Bohr and Quantum
Mechanical models predict the
spectrum of hydrogen very accurately
Only the Quantum Mechanical model
predicts the spectra of multielectron
atoms
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Electron Configurations
Electron configuration = distribution of
electrons into the various energy shells
and subshells in an atom in its ground
state
Each energy shell and subshell has a
maximum number of electrons it can
hold
– s = 2, p = 6, d = 10, f = 14
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Writing Electron Configurations
We place electrons in the energy
shells and orbitals in order of
energy, from low energy up: Aufbau
Principle (order of filling of orbitals)
The d and f orbitals overlap into the
higher energy levels
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7s
6s
Energy
5s
4s
6p
5p
6
d
5d
5f
4f
4d
4p
3d
3p
3s
2p
Relative Energy of Orbitals
in the Quantum Mechanical Model
2s
1s
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Order of Subshell Filling
in Ground State Electron Configurations
Start by drawing a diagram
putting each energy shell on
a row and listing the subshells,
(s, p, d, f), for that shell in
order of energy, (left-to-right)
next, draw arrows through
the diagonals, looping back
to the next diagonal
each time
1s
2s
2p
3s
3p
3d
4s
4p
4d
4f
5s
5p
5d
5f
6s
6p
6d
7s
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Filling the Orbitals in a Subshell
with Electrons
Energy shells fill from lowest energy to high
Subshells fill from lowest energy to high
–s→p→d→f
A single orbital can hold a maximum of 2
electrons (Pauli’s exclusion principle); orbitals
that are in the same subshell have the same
energy
When filling orbitals that have the same energy,
place one electron in each before completing
pairs (Hund’s rule)
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Electron Configuration of Atoms in
their Ground State
Electron configuration = order of filling with
electrons; number of electrons in that subshell
written as a superscript
Kr = 36 electrons = 1s22s22p63s23p64s23d104p6
Shorthand way: use the symbol of the previous
noble gas in brackets to represent all the inner
electrons, then just write the last set
Rb = 37 electrons = 1s22s22p63s23p64s23d104p65s1 =
[Kr]5s1
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Electron Configurations
how many electrons
in that orbital
2
2
3
Nitrogen: 1s 2s 2p
energy level
orbital
(atomic number = 7)
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Example – Write the Ground State
Orbital Diagram and Electron
Configuration of Magnesium.
1. Determine the atomic number of the
element from the Periodic Table
– This gives the number of protons and
electrons in the atom
Mg, Z = 12, so Mg has 12 protons and 12
electrons
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Example – Write the Ground State
Orbital Diagram and Electron
Configuration of Magnesium.
2. Draw 9 boxes to represent the first 3
energy levels s and p orbitals
1s
2s
2p
3s
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Example – Write the Ground State
Orbital Diagram and Electron
Configuration of Magnesium.
3. Add one electron to each box in a set,
then pair the electrons before going to
the next set until you use all the
electrons
•
When pairing, put in opposite arrows


1s
2s
  
2p

3s
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Example – Write the Ground State
Orbital Diagram and Electron
Configuration of Magnesium.
4. Use the diagram to write the electron
configuration
– Write the number of electrons in each set
as a superscript next to the name of the
orbital set
1s22s22p63s2 = [Ne]3s2

1s

2s
  
2p

3s
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Valence Electrons
Valence electrons = electrons in all the
subshells with the highest principal
energy shell (outermost shell)
Core electrons = in lower energy shells
Valence electrons responsible for both
chemical and physical properties of
atoms.
Valence electrons responsible for
chemical reactions
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Valence Electrons
Rb = 37 electrons =
1s22s22p63s23p64s23d104p65s1
The highest principal energy shell of Rb that
contains electrons is the 5th, therefore Rb
has 1 valence electron and 36 core electrons
Kr = 36 electrons = 1s22s22p63s23p64s23d104p6
The highest principal energy shell of Kr that
contains electrons is the 4th, therefore Kr has
8 valence electrons and 28 core electrons
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How many valence electrons
does each atom have?
carbon: 1s22s22p2
chlorine: 1s22s22p63s23p5
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How many valence electrons
does each atom have?
carbon: 1s22s22p2 = 4
chlorine: 1s22s22p63s23p5 = 7
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Electron Configurations and
the Periodic Table
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Electron Configurations from
the Periodic Table
Elements in the same period (row) have
valence electrons in the same principal
energy shell
The number of valence electrons increases
by one as you progress across the period
Elements in the same group (column) have
the same number of valence electrons and
they are in the same kind of subshell
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Electron Configuration & the
Periodic Table
Elements in the same column have
similar chemical and physical
properties because their valence shell
electron configuration is the same
The number of valence electrons for
the main group elements is the same
as the group number
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The Explanatory Power of
the Quantum-Mechanical Model
The properties of the elements are
largely determined by the number of
valence electrons they contain
Since elements in the same column
have the same number of valence
electrons, they show similar properties
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The Noble Gas
Electron Configuration
The noble gases have 8 valence
electrons
– except for He, which has only 2 electrons
Noble gases are especially unreactive
– He and Ne are practically inert
Reason noble gases are unreactive is
that the electron configuration of the
noble gases is especially stable
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