Introductory Chemistry, 2nd Edition Nivaldo Tro

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Introductory Chemistry, 2nd EditionNivaldo Tro
Chapter 9 Electrons in Atoms and the Periodic
Table
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Why do Blimps Float?

• Because they are filled with a gas less dense
  than air
• Early blimps used hydrogen gas hydrogens
  flammability led to the Hindenburg disaster
• Blimps now use helium, a nonflammable gas in
  fact it doesnt 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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Lights 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 dont 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 electrons 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 AtomOrbits 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 AtomEnergy 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 AtomGround 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 How many e fit? (2n2) 3rd
18 electrons 2 x 32 2nd
8 electrons 2 x 22 1st
2 electrons 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.
Bohr model
H
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Bohr Model for AtomElectrons fill the Lowest
energy levels first
C
Bohr Model for C with 6 electrons
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The Bohr Model of the AtomSuccess 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 ModelOrbitals

• 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 Shapes orbitals are
spherical
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Probability Maps Orbital Shapep 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 lt p lt d lt 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 ModelEnergy 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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Energy
Relative Energy of Orbitals in the Quantum
Mechanical Model
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Order of Subshell Fillingin 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)
1s 2s 2p 3s 3p 3d 4s 4p 4d 4f 5s 5p 5d 5f 6s 6p 6d
7s
next, draw arrows through the diagonals, looping
back to the next diagonal each time
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Filling the Orbitals in a Subshellwith 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 (Paulis 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 (Hunds 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
  Kr5s1

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Electron Configurations
how many electrons in that orbital

• Nitrogen 1s22s22p3

energy level
orbital
(atomic number 7)
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Example Write the Ground State Orbital Diagram
and Electron Configuration of Magnesium.

• 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.

1. Draw 9 boxes to represent the first 3 energy
   levels s and p orbitals

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Example Write the Ground State Orbital Diagram
and Electron Configuration of Magnesium.

• 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
3p
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Example Write the Ground State Orbital Diagram
and Electron Configuration of Magnesium.

• 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 Ne3s2

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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 andthe Periodic Table
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Electron Configurations fromthe 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 ofthe 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