1. Introductory Chemistry, 2nd Edition Nivaldo Tro
Chapter 9
Electrons in Atoms
and the
Periodic Table

2. Tro's Introductory Chemistry, Chapter 9
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
Tro's Introductory Chemistry, Chapter 9

3. 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
Tro's Introductory Chemistry, Chapter 9

4. 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
Tro's Introductory Chemistry, Chapter 9

5. Tro's Introductory Chemistry, Chapter 9
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
Tro's Introductory Chemistry, Chapter 9

6. 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
Tro's Introductory Chemistry, Chapter 9

7. Electromagnetic Spectrum
Visible light is a very small portion of the electromagnetic spectrum
Tro's Introductory Chemistry, Chapter 9

8. 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
Tro's Introductory Chemistry, Chapter 9

9. Emission Spectrum or Line Spectrum
Tro's Introductory Chemistry, Chapter 9

10. Tro's Introductory Chemistry, Chapter 9
Line Spectra = specific wavelengths are emitted; characteristic of atoms
Tro's Introductory Chemistry, Chapter 9

11. 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
Tro's Introductory Chemistry, Chapter 9

12. 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

13. 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
Tro's Introductory Chemistry, Chapter 9

14. 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
Tro's Introductory Chemistry, Chapter 9

15. 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
Tro's Introductory Chemistry, Chapter 9

16. Electron Energy Levels:
Energy Level How many e fit? (2n2)
3rd electrons x 32
2nd electrons x 22
1st electrons 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
Tro's Introductory Chemistry, Chapter 9

17. Bohr Model for Atom Electrons fill the Lowest energy levels first
Bohr Model for C with 6 electrons
Tro's Introductory Chemistry, Chapter 9

18. 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
Tro's Introductory Chemistry, Chapter 9

19. 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
Tro's Introductory Chemistry, Chapter 9

20. 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
Tro's Introductory Chemistry, Chapter 9

21. Tro's Introductory Chemistry, Chapter 9
Shells & Subshells
Tro's Introductory Chemistry, Chapter 9

22. Probability Maps & Orbital Shape s orbitals are spherical
Tro's Introductory Chemistry, Chapter 9

23. Probability Maps & Orbital Shape p orbitals
Tro's Introductory Chemistry, Chapter 9

24. 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
Tro's Introductory Chemistry, Chapter 9

25. 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
Tro's Introductory Chemistry, Chapter 9

26. 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
Tro's Introductory Chemistry, Chapter 9

27. 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
Tro's Introductory Chemistry, Chapter 9

28. 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
Tro's Introductory Chemistry, Chapter 9

29. Tro's Introductory Chemistry, Chapter 95f 4s 4p 4d 4f 3s 3p 3d
Energy 2s 2p
Relative Energy of Orbitals in the Quantum Mechanical Model 1s
Tro's Introductory Chemistry, Chapter 9

30. 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) 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
Tro's Introductory Chemistry, Chapter 9

31. 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)
Tro's Introductory Chemistry, Chapter 9

32. 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
Tro's Introductory Chemistry, Chapter 9

33. Electron Configurations
how many electrons
in that orbital
Nitrogen: 1s22s22p3
energy level
orbital
(atomic number = 7)
Tro's Introductory Chemistry, Chapter 9

34. Mg, Z = 12, so Mg has 12 protons and 12 electrons
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
Tro's Introductory Chemistry, Chapter 9

35. Tro's Introductory Chemistry, Chapter 9
Example – Write the Ground State Orbital Diagram and Electron Configuration of Magnesium.
Draw 9 boxes to represent the first 3 energy levels s and p orbitals 1s 2s 2p 3s 3p
Tro's Introductory Chemistry, Chapter 9

36. Tro's Introductory Chemistry, Chapter 9
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
Tro's Introductory Chemistry, Chapter 9

37. Tro's Introductory Chemistry, Chapter 9
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 = [Ne]3s2 1s 2s 2p 3s 3p 
Tro's Introductory Chemistry, Chapter 9

38. Tro's Introductory Chemistry, Chapter 9
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
Tro's Introductory Chemistry, Chapter 9

39. Tro's Introductory Chemistry, Chapter 9
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
Tro's Introductory Chemistry, Chapter 9

40. How many valence electrons does each atom have?
carbon: 1s22s22p2
chlorine: 1s22s22p63s23p5
Tro's Introductory Chemistry, Chapter 9

41. How many valence electrons does each atom have?
carbon: 1s22s22p2 = 4
chlorine: 1s22s22p63s23p5 = 7
Tro's Introductory Chemistry, Chapter 9

42. Electron Configurations and the Periodic Table
Tro's Introductory Chemistry, Chapter 9

43. 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
Tro's Introductory Chemistry, Chapter 9

44. 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
Tro's Introductory Chemistry, Chapter 9

45. 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
Tro's Introductory Chemistry, Chapter 9

46. 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
Tro's Introductory Chemistry, Chapter 9