1. Atoms and the Periodic Table
Fundamentals of General, Organic, and Biological Chemistry
7th Edition
Chapter Two
Atoms and the Periodic Table

2. Chapter 2 Goals Topics and Goals for the Chapter:
What is the modern theory of atomic structure?
Be able to list and explain the major assumptions of atomic theory.
How do atoms of different elements differ?
Be able to define an atom.
Be able to list the three subatomic particles and their properties.
Be able to explain the composition of different atoms according to the
number of protons, neutrons, and electrons they contain.
What are isotopes, and what is atomic weight?
Be able to define and give examples of isotopes.
Be able to explain what isotopes are and how they affect an element’s atomic weight.
Chapter 2
2013 Pearson Education, Inc.

3. Chapter 2 Goals Cont.
How is the periodic table arranged?
Be able to describe how elements are arranged in the periodic table.
Be able to name the subdivisions of the periodic table.
Be able to relate the position of an element in the periodic table to its electronic structure.
How are electrons arranged in atoms?
Be able to define shell, subshell, orbital and spin.
Be able to explain the rules for how electrons are distributed in shells, subshells and orbitals around the nucleus of an atom.
Be able to write electron configurations for any element in the periodic table.
Be able to define valence shell and valence electron.
Be able to explain how the electron configurations can help explain the chemical properties of the elements.
Understand the importance of the electron configuration of the noble gases and the octet rule.
Be able to draw electron-dot symbols for main group elements.
Chapter 2 3
2013 Pearson Education, Inc. 3

4. 2.1 Atomic Theory Atom – The smallest unit of an element.
“atom” means indivisible.
Modern Atomic Theory – Four assumptions about atoms and matter.
1. All matter is composed of atoms.
2. The atoms of a given element differ from the atoms of all other elements.
Atoms of different elements are unique!
3. Chemical compounds consist of atoms combined in specific ratios.
Only whole atoms can combine.
Examples: CO and CO2 but not C1/2O1/2 or C1/2O
4. Chemical reactions change only the way the atoms are combined in compounds; the atoms themselves are unchanged.
i.e. an atom of carbon is not going to be changed into an atom of gold!
Chapter 2
2013 Pearson Education, Inc.

5. Diameter ranges from 7.4x10-11m (H) to 5.24x10-10m (Cs)
Atoms are very small
Diameter ranges from 7.4x10-11m (H) to 5.24x10-10m (Cs)
Mass ranges from 1.67x10-24 g (H) to 3.95x10-22 g (U)
A speck of dust ~ 1016 atoms!
Chapter 2
2013 Pearson Education, Inc.

6. Atoms are composed of subatomic particles
Composition of Atoms
Atoms are composed of subatomic particles
Mass proton ≈ mass neutron
Mass electron 1/1836 that of a proton
e- mass usually ignored.
Chapter 2
2013 Pearson Education, Inc.

7. Mass of p, n and e- = really small.
Atomic Mass Units
Mass of p, n and e- = really small.
Easier to express the mass of these particles in relative terms.
Relative mass - Assign a mass to a particular atom and the masses of all other atoms are relative to this.
Atomic Mass Units (amu) = scale we use for relative mass
Basis for amu scale = atom of carbon containing 6 protons and 6 neutrons
12C = carbon with 6 protons + 6 neutrons = 12 amu
1 amu = x10-24 g or 1/12 the mass of 1 carbon-12 atom
Chapter 2
2013 Pearson Education, Inc.

8. The structure of an atom
Atoms – Composed of a nucleus and an enormous space around the nucleus.
Nucleus – Contains all of the protons and neutrons in an atom.
Contains the mass of the atom.
Outside nucleus –
Contains all of the e- in the atom.
The relative size of a nucleus in an
atom is the same as that of a pea in
the middle of a football stadium.
Chapter 2
2013 Pearson Education, Inc.

9. Nucleus = + charged. Contains protons
Outside nucleus = - charged, contains e-
The structure of the atom is determined by interplay of different forces.
Opposite electrical charges attract each other, like charges repel each other.
Protons and neutrons in the nucleus are held together by the nuclear strong force.
Chapter 2
2013 Pearson Education, Inc.

10. 2.2 Elements and Atomic Number
All atoms of a specific element must have the same number of protons.
The # of protons in an atom’s nucleus determine what element the atom is.
Atoms are neutral.
Protons are positively charged (p = +1 charge)
#p = # e- for atoms (neutral)
Atomic number (Z): The number of protons in each atom of an element.
Examples:
H Z = 1 Each atom contains 1p and 1e-
N Z = 7 Each atom contains 7p and 7e-
Chapter 2
2013 Pearson Education, Inc.

11. What subatomic particles make up the mass of an atom?
Mass number (A): The total number of protons and neutrons in an atom.
Relationship between Atomic Number (Z) and Mass Number (A)
Z = #p
A = #p + #n
A = Z + #n
Chapter 2
2013 Pearson Education, Inc.

12. Example of using Atomic Number and Mass Number in determining
atomic structure
A neutral atom contains 14 electrons and has A=30.
Give the number of protons and neutrons in the atom.
Identify the atom.
Chapter 2
2013 Pearson Education, Inc.

13. 2.3 Isotopes and Atomic Weight
All atoms of a specific element must have the same number of
protons.
Atoms of a particular element can have different numbers of neutrons.
i.e. The number of neutrons can vary in atoms of the same element.
Isotopes: Atoms of the same element, which have different # of neutrons and, therefore, different mass numbers.
Chapter 2
2013 Pearson Education, Inc.

14. A specific isotope is represented by showing its mass number (A) as a superscript
and its atomic number (Z) as a subscript in front of the atomic symbol.
For example, the symbol for tritium (hydrogen with a mass number = 3) is:
Chapter 2
2013 Pearson Education, Inc.

15. How is an atom of chlorine with 20 neutrons represented?
Example:
How is an atom of chlorine with 20 neutrons represented?
Chapter 2
2013 Pearson Education, Inc.

16. Many elements have several isotopes Example: Hydrogen has 3 isotopes
11H 21H 31H
#p:
#n:
Name: protium deuterium tritium
most abundant heavy radioactive
hydrogen
We designate an isotope by the giving mass number after the element
name or symbol.
Ex: uranium-235 or U-235
Chapter 2
2013 Pearson Education, Inc.

17. Most naturally occurring elements are mixtures of isotopes. Example:
A large sample of naturally occurring copper atoms
69.09% have mass number 63 and 30.91% have mass number 65.
A sample of copper contains both Cu-63 and Cu-65
The atomic weight of an element = the average mass of the atoms in a
large sample.
Atomic weight of copper = amu
Chapter 2
2013 Pearson Education, Inc.

18. How do we calculate atomic weight (average weight)?
How do we calculate the average weight of a class?
Suppose class = 100 students
35 students weigh 130 lbs
65 students weigh 150 lbs
What is the average weight of the class?
Can you give a ballpark figure without calculating?
Chapter 2
2013 Pearson Education, Inc.

19. Example: Naturally occurring copper 69.09% 63Cu, 30.91% 65Cu
Calculating atomic weight is essentially the same as calculating the average weight of the class.
Example: Naturally occurring copper
69.09% 63Cu, 30.91% 65Cu
Mass 63Cu = amu
Mass 65Cu = amu
Atomic weight = (69.09%) ( amu) + (30.91%) ( amu)
100
= ( amu amu)/100
= amu
= amu
Chapter 2
2013 Pearson Education, Inc.

20. How do we know atoms are real?
We can actually “see” atoms now with an instrument called a STM.
STM = scanning tunneling microscope.
Instrument uses a probe to scan the surface and provide a photo of the surface of the image.
STM Copper surface
Chapter 2
2013 Pearson Education, Inc.

21. 2.4 The Periodic Table
Chapter 2
2013 Pearson Education, Inc.

22. The table has boxes representing the known elements.
Each box tells the symbol, atomic number, and atomic weight of an element.
Chapter 2
2013 Pearson Education, Inc.

23. 7 horizontal rows = periods 18 vertical columns = groups. Groups
Beginning at the upper left corner of the periodic table, elements are arranged by increasing atomic number
7 horizontal rows = periods
18 vertical columns = groups.
Groups
The elements in a given group have similar chemical properties.
Ex: Lithium, sodium, potassium, and other elements in group 1A (or 1)
All of the above are metals with very similar reactivity.
Chlorine, bromine, iodine, and other elements in group 7A (or 17)
All of the above are nonmetals with very similar reactivity.
Chapter 2
2013 Pearson Education, Inc.

24. Groups are numbered using two methods
First Method – Most useful for us!
2 groups on far left 1A & 2A
6 groups on far right 3A – 8A
10 groups in middle 1B – 8B
Second Method
All 18 groups numbered sequentially
Lanthanides and actinides not numbered in either system.
Chapter 2
2013 Pearson Education, Inc.

25. Periods – Using Periodic Table From The Text
Different periods contain different # of elements
Period # of Elements
1 2
2 8
3 8
4 18
5 18
6 32
7 32
Note: 14 elements following lanthanum (lanthanides)
14 elements following actinium (actinides)
are pulled out of periods.
Chapter 2
2013 Pearson Education, Inc.

26. Transition Metal Groups: Elements in the groups numbered 1B -8B.
The groups on the periodic table are divided into three main categories.
Main Groups: The two groups on the far left (1A and 2A) and the six on the far right (3A – 8) are the main groups.
Transition Metal Groups: Elements in the groups numbered 1B -8B.
Inner Transition Metal Groups: The 14 groups shown at the bottom of the table that are not numbered containing the Lanthanides and the Actinides.
Chapter 2
2013 Pearson Education, Inc.

27. 2.5 Some Characteristics of Different Groups
The characteristics of the elements show periodicity (a repeating rise-and-fall pattern).
Ex: Plot of atomic radius vs. atomic number
A graph of atomic size versus atomic number shows a periodic rise-and-fall pattern.
The maxima occur for atoms of the group 1A elements.
The minima occur for atoms of the group 7A elements.
Chapter 2
2013 Pearson Education, Inc.

28. Similarities in Groups
Group 1A or 1 – Alkali Metals
Li, Na, K, Rb, Cs, and Fr
Shiny soft metals
Low melting points
Na = 97.81ºC, K = 63.25ºC, Fr = 27ºC
React rapidly with water to form flammable H2 gas and alkaline or basic solutions.
Solid sodium kept under oil to prevent exposure to water.
Very unstable in nature
Not found in pure state naturally.
Chapter 2
2013 Pearson Education, Inc.

29. Group 2A or 2 – Alkaline Earth Metals Be, Mg, Ca, Sr, Ba, and Ra
Lustrous, silvery metals
Melting points higher than alkali metals
Ca = 839ºC, Mg = 648ºC
Less reactive to water than group 1A alkali metals.
React with O2
Like 1A group, never found in nature in pure form.
Chapter 2
2013 Pearson Education, Inc.

30. Colorful, nonmetals, corrosive
Group 7A or 17 - Halogens
F, Cl, Br, I, and At
Colorful, nonmetals, corrosive
Found in nature only in combination with other elements.
Ex: In combination with sodium in sodium chloride (NaCl)
Chapter 2
2013 Pearson Education, Inc.

31. Group 8A or 18 - Noble gases: He, Ne, Ar, Kr, Xe, and Rn
Colorless gases
Inert - Very low chemical reactivity.
Neighboring elements often behave similarly as well, even if not in the same group.
Chapter 2
2013 Pearson Education, Inc.

32. 2.6 Electronic Structure of Atoms
Why do particular elements share similar properties, reactivities?
Not much knowledge of “why” before 1920s.
Now we know – Properties of elements are determined by the
arrangement of electrons in their atoms.
Our understanding of the electronic structure of atoms is based
on a quantum mechanical model.
Chapter 2
2013 Pearson Education, Inc.

33. Quantum Mechanical Model
Developed by Schrödinger in 1926
Assumes electrons have both particle-like and wave-like properties.
Wave-like Properties
Atoms can radiate e- in the form of light at particular wavelengths.
Particle-like Properties
We can describe electrons as having particular amounts of energy and having restricted locations. The electrons in an atom are grouped around the nucleus into shells, roughly like the layers in an onion.
Chapter 2
2013 Pearson Education, Inc.

34. Quantum Mechanical Model Cont.
Electrons not completely free to move around an atom.
Restricted to certain locations depending on their energy.
Different e- have different energies and different locations.
Energies of e- are “quantized” (They can only have certain values).
i.e. stairs vs. a ramp
Chapter 2
2013 Pearson Education, Inc.

35. Quantum Mechanical Model Cont.
Location of every electron is given by designating an “address”
Shell subshell orbital
town street house number
Shells
A group of e- according to energy – like layers in an onion.
The farther a shell is from the nucleus, the larger it is, the more
electrons it can hold, and the higher the energies of those electrons.
The smallest shell closest to the nucleus is labeled shell 1, the next one is
shell 2, etc.
Chapter 2
2013 Pearson Education, Inc.

36. Within shells electrons are further grouped into subshells.
4 types of subshells: s, p, d and f
Subshell energy s < p < d < f
Electrons grouped according to shape of region of space they occupy.
A shell has a number of subshells equal to its shell number.
The first shell has only an s subshell; the second shell has an s and a p subshell;
the third shell has an s, a p, and a d subshell, etc.
CHE121 - Mainly concerned with s and p subshells.
Chapter 2
2013 Pearson Education, Inc.

37. Within each subshell, electrons are further grouped into orbitals.
Orbital = a region of space within an atom where the specific electrons are more
likely to be found.
Each different type of subshell has a different number of orbitals.
s = 1 orbital, p = 3 orbitals, d = 5 orbitals, f = 7 orbitals
Notice: increase by 2 orbitals s → p → d → f
Chapter 2
2013 Pearson Education, Inc.

38. Each orbital can hold a maximum of only two electrons.
Each electron in an orbital has a different property known as spin.
Electron has a clockwise or a counter-clockwise spin.
Chapter 2
2013 Pearson Education, Inc.

39. What do the different orbitals look like?
Orbitals in s subshells are spherical (a)
Orbitals in p subshells are roughly dumbbell shaped (b) and aligned with x, y and z axes.
Chapter 2
2013 Pearson Education, Inc.

40. How do we determine the number of e- in an atom and what element the atom is?
Ex:
How many e- are in an atom in which the 1s, 2s and 2p subshells are filled and there are 6 electrons in the third shell?
Name the element.
Chapter 2
2013 Pearson Education, Inc.

41. 2.7 Electron Configurations
We need more information than simply the total number of electrons
in each shell and subshell – We need to know the arrangement of electrons.
Electron configuration = Exact arrangement of e- in an atom.
Rules for predicting an atom’s electron configuration:
Rule #1
Electrons occupy the lowest energy orbitals available first.
Chapter 2
2013 Pearson Education, Inc.

42. Order of orbital energy levels:
Electrons fill orbitals from the lowest-energy orbitals upward.
Lower numbered shells fill before higher numbered shells at first.
Some overlap in energy levels occurs starting with shell 3 and 4.
Chapter 2 42
Chapter 2
2013 Pearson Education, Inc.

43. Nice diagram for remembering the order of the energy levels in an atom.
Chapter 2
2013 Pearson Education, Inc.

44. ↑↓ up and down arrows indicate opposite spin
Rule #2
Each orbital can hold only two electrons, which must be of opposite spin.
↑↓ up and down arrows indicate opposite spin
Rule#3
Two or more orbitals with the same energy (three p orbitals or five d orbitals) are
each half filled by 1 e- before 2nd e- is added.
Chapter 2
2013 Pearson Education, Inc.

45. We can represent any electron configuration by:
a. Writing the symbol for the occupied subshell.
b. Adding a superscript to indicate the # of e- in that subshell.
Ex:
Li = 3 e- = 1s22s1
S = 16 e- = 1s22s22p63s23p4
Chapter 2
2013 Pearson Education, Inc.

46. Rules 2 and 3 come into play.
We can also represent e- configurations by writing orbital-filling diagrams.
Orbital filling diagrams give us more information regarding where the e- are.
Rules 2 and 3 come into play.
Ex:
Li ↑↓ ↑
1s 2s
S ↑↓ ↑↓ ↑↓ ↑↓ ↑↓ ↑↓ ↑↓ ↑ ↑
1s s p s p
Chapter 2 46
Chapter 2
2013 Pearson Education, Inc.

47. Do you see a relationship between the e- configuration for an element
Figure: 03-05UN04T03
Title:
Electron configurations
Caption:
The table shows the electron configurations of the first twenty elements.
Notes:
Keywords:
electron configurations
Do you see a relationship between the e- configuration for an element
and the element’s location on the periodic table?
Chapter 2
2013 Pearson Education, Inc.

48. The relationship between the electron configuration for an element
and it’s location on and the periodic table:
Group 1A elements – H, Li, Na
Group 2A elements – Be, Mg, Ca
Some electron “jargon”:
Single e- in an orbital = unpaired e-
Two e- in an orbital = paired e-
Chapter 2
2013 Pearson Education, Inc.

49. Condensed Electron Configurations
A shorthand using noble gas configurations is very useful for large atoms.
Shorthand notation = condensed e- configuration
Takes advantage of the fact that noble gases (group 18) are very stable and at
the end of a row.
[noble gas] = e- configuration of noble gas
[Ne] = e- configuration of neon
We always use the noble gas preceding the atom we are looking at and build
on that configuration.
Chapter 2 49
Chapter 2
2013 Pearson Education, Inc.

50. condensed electron configuration for an atom:
Example of developing electron configuration, orbital-filling diagram and
condensed electron configuration for an atom:
Chapter 2 50
Chapter 2
2013 Pearson Education, Inc.

51. Write the e- configurations, orbital-filling diagrams and condensed e- configurations
for the following:
H (Z=1)
He (Z=2)
Li (Z=3)
Be (Z=4)
Chapter 2
2013 Pearson Education, Inc.

52. B (Z=5) to Ne (Z=10)
Chapter 2
2013 Pearson Education, Inc.

53. configuration for chlorine. What is the first step?
Ex:
Write the electron configuration, orbital-filling diagram and condensed e-
configuration for chlorine.
What is the first step?
Chapter 2
2013 Pearson Education, Inc.

54. 2.8 Electron Configurations and the Periodic Table
The electron configuration of elemental atoms and their placement on the
periodic table coincide with one another.
Chapter 2
2013 Pearson Education, Inc.

55. The electron configuration determines an atom’s reactivity.
Placement on the periodic table is a result of reactivity similarities.
The periodic table is divided into blocks to reflect the subshell filled last.
Groups 1A and 2A
s subshell filled last, s-block elements
Groups 3A – 8A
p subshell filled last, p-block elements
Transition Metals Groups 1B – 8B
d subshell filled last, d-block elements
Inner Transition Metals
f subshell filled last, f-block elements
The periodic table gives us the order of electron filling!
Chapter 2
2013 Pearson Education, Inc.

56. Valence shell: Outermost, highest energy shell of an atom.
Why do elements in a given group of the periodic table have similar properties?
Valence shell: Outermost, highest energy shell of an atom.
Valence electrons: An electron in an outermost shell of an atom.
These electrons are loosely held, they are most important in determining
an element’s properties.
Only s and p orbitals are considered valence electrons for the main group elements.
Note: All atoms in a group have the same # and type of valence electrons!
Chapter 2
2013 Pearson Education, Inc.

57. 2013 Pearson Education, Inc. Figure: 03-06UN00T04 Title:
Valence shell electron configurations
Caption:
Valence-shell electron configurations for group 1A, 2A, 7A, and 8A elements.
Notes:
Keywords:
electron configurations, group 1A, group 2A, group 7A, group 8A
Chapter 2
2013 Pearson Education, Inc.

58. Ex: Write the e- configuration for gallium.
Which e- are the valence electrons?
Chapter 2
2013 Pearson Education, Inc.

59. 2.9 Electron-Dot Symbols Electron-Dot Symbols
Dots are placed around the atomic symbol to indicate the # of valence electrons present.
Atoms in the same group have the same # of valence electrons and similar e- dot symbols.
Chapter 2 59
Chapter 2
2013 Pearson Education, Inc.

60. How are Electron-Dot Symbols generated?
Each atom has ns and np orbitals (exception is He with only ns e-)
4 orbitals = 8 valence e-
Use 4 sides around atom to place e-
Place e- singly first, then pair the electrons.
Chapter 2 60
Chapter 2
2013 Pearson Education, Inc.

61. Application – Atoms and Light
What is the relationship between light and atoms?
Light – Really energy
The shorter the wavelength (λ), the higher the energy of the light
Chapter 2
2013 Pearson Education, Inc.

62. Visible light is a very small part of the electromagnetic spectrum
Visible light λ= 400nm – 800nm
Visible light is a very small part of the electromagnetic spectrum
We can’t see the other wavelengths of light (gamma, X-rays, UV, IR)
Chapter 2
2013 Pearson Education, Inc.

63. energy state (an excited state).
What happens when a beam of electromagnetic energy collides with an atom?
An electron in the lowest energy orbitals (ground state) is kicked up to a higher
energy state (an excited state).
Excited states don’t last long – unstable
e- drops back down to ground state releasing energy.
If the energy falls in the range of visible light, we see the result.
Neon lights – Noble gas atoms are excited by an electric discharge
Neon – gives red light, Krypton gives white light
Fireworks – metal atoms excited by heat
Sr = red, Ba = green, Cu = blue
Clinical labs – Concentration of metals in body fluids determined by same
Principle.
Chapter 2
2013 Pearson Education, Inc.

64. Chapter Summary
An atom is the smallest unit of an element that maintains the properties of the element.
Atoms are made up of protons, neutrons, and electrons. Protons have a positive charge, neutrons are neutral, and electrons have a negative charge.
Protons and neutrons are present in a dense, positively charged region called the nucleus. Electrons are a relatively large distance away from the nucleus.
The number of protons an element contains is called the atomic number (Z). The total number of protons plus neutrons in an atom is called the mass number (A).
Atoms with identical numbers of protons and electrons but different numbers of neutrons are called isotopes.
Chapter 2
2013 Pearson Education, Inc.

65. Chapter Summary Cont.
The atomic weight of an element is the weighted average mass of the element’s naturally occurring isotopes measured in atomic mass units (amu).
Elements are organized into the periodic table, consisting of 7 rows, or periods, and 18 columns, or groups.
Elements in the same group have the same number of valence electrons in their outermost shell.
An atom is the smallest unit of an element that maintains the properties of the element.
Electrons in an atom are grouped into layers, or shells. In each shell, electrons are grouped into subshells, and each subshell into orbitals.
Chapter 2
2013 Pearson Education, Inc.

66. Chapter Summary Cont.
s orbitals are spherical, and p orbitals are dumbbell shaped.
Each orbital can hold 2 electrons. Each shell can hold a number of electrons equal to 2 times the shell number squared. The first shell can hold 2, the second shell can hold 8, the third shell can hold 18, and so on.
The electron configuration of an element is predicted by assigning the element’s electrons into orbitals, beginning with the lowest-energy orbital.
Chapter 2 66
Chapter 2
2013 Pearson Education, Inc.