1. Chapter 2: Atoms and Elements
Chem 103: Chapter 2
Chapter 2: Atoms and Elements
Copyright 2006, David R. Anderson

2. The Atomic Theory of Matter: Dalton's postulates
Atomic Structure
Subatomic particles: protons, neutrons, and electrons
Atomic number and mass number
Isotopes
Atomic weight and isotopic abundances
The mole and molar masses
The Periodic Table: Classification of the elements
rows (periods) and columns (groups)
main group, transition, and inner transition elements
metals, nonmetals, and metalloids
alkali metals, alkaline earth metals, chalcogens, halogens, and noble gases

3. ~400 BC Democritus: “atomos” (indivisible)
I. The Atomic Theory of Matter
Dalton’s postulates
~400 BC Democritus: “atomos” (indivisible)
-but Plato & Aristotle argued against it
-ignored for > 2000 years
1803 John Dalton proposed:
(> 100 years before the discovery of the nucleus, protons, and electrons)
1. All matter is composed of tiny, indestructible (indivisible)
particles called atoms.
2. All atoms of a given element are the same; atoms of different
elements are different.
3. A compound is composed of atoms in a fixed number ratio.
4. Chemical reactions involve only exchanges of atoms; atoms are
neither created, destroyed, nor transmuted.

4. Dalton’s theory explained: the Law of Conservation of Mass:
I. The Atomic Theory of Matter
Dalton’s theory explained:
the Law of Conservation of Mass:
2HgO  2Hg + O2
the Law of Definite Proportions:
CO2
and predicted:
the Law of Multiple Proportions:
e.g., CO2 O/C = 2.67
CO O/C = 1.33
no loss or change in atoms,
 no loss or gain of mass
always same number (same proportion)
of same atoms (same masses)
(simple whole number ratio)

5. nucleus: protons and neutrons > 99.95% of mass of atom
II. Atomic Structure
nucleus: protons and neutrons
> 99.95% of mass of atom
electron cloud: < 0.05% of mass
Analogies: size: 73 million Cu atoms = diameter of a penny
1 million C atoms = thickness of a sheet of paper
emptiness: nucleus/electron cloud = pinhead/baseball stadium
mass: if matter were composed of only nuclear material,
a matchbox full would weigh 25 billion tons

6. i.e., protons, neutrons ~ 2000 times heavier than electrons
Atomic Structure
Subatomic particles: protons, neutrons, and electrons
where charge =  1019 C
1 u =  1024 g
i.e., protons, neutrons ~ 2000 times heavier than electrons

7. Z atomic number = number of protons in the atom (identity)
Atomic Structure
Atomic number and mass number
atomic number = number of protons in the atom (identity)
mass number = number of protons + neutrons
notation: AX
X = symbol for the element
Z = atomic number = number of protons
A = mass number = number of protons + neutrons
 number of neutrons = A  Z
e.g., How many neutrons are in 12C, 206Pb?
e.g., What is the mass number of phosphorus with 16 neutrons? Z
redundant

8. = atoms with the same atomic number (same number of protons)
Atomic Structure
C. Isotopes
= atoms with the same atomic number (same number of protons)
but different mass (different number of neutrons)
12C 13C 1H H H 79Br 81Br
deuterium tritium
D. Atomic weight and isotopic abundance
atomic “weight” = weighted average of all naturally occurring isotopes
e.g., Naturally occurring carbon is
98.892% 12C ( amu)
1.108% 13C ( amu)
 average atomic mass =
(  ) + (  ) = amu
“atomic weight”

9. Boron is 19.91% 10B (10.0129 u) and 80.09% 11B (11.0093 u).
Atomic Structure
Atomic weight and isotopic abundances
Boron is 19.91% 10B ( u) and 80.09% 11B ( u).
What is the average atomic mass of boron?
Chlorine occurs as two isotopes, 35Cl ( u) and 37Cl ( u). Its atomic weight is u. What are the abundances of 35Cl and 37Cl?

10. mole = number of atoms of 12C in exactly 12 g of 12C
Atomic Structure
The mole and molar masses
mole = number of atoms of 12C in exactly 12 g of 12C
i.e., 1 atom of 12C has a mass of 12 amu
1 mole of 12C has a mass of 12 g
find: 1 mol =  (N, Avogadro’s number)
huge number:
1 mol of marbles would cover the earth 3 miles deep!
1 mol of dollar bills would be enough to pay each of the 6 billion
people on earth $3 million per second for 100 years!
1 mol of Cu =  1023 atoms of Cu
1 mol of NH3 =  1023 molecules of NH3
1 mol of M&M’s =  1023 M&M’s

11. molar mass = mass, in g, of one mole of a substance
Atomic Structure
The mole and molar masses
molar mass = mass, in g, of one mole of a substance
molar mass of C = g/mol (atomic weight)
molar mass of Ag = g/mol
Using molar masses to convert between mass, moles, and number of particles:
e.g., A 1-carat diamond has a mass of mg. How many moles of carbon
are in a 1-carat diamond? How many atoms?
e.g., A reaction requires mol of Na. How many grams is this?

12. The Periodic Table: Classification of the Elements

13. chemical and physical properties vary in a periodic fashion
The Periodic Table: Classification of the Elements
A. periods (rows)
chemical and physical properties vary in a periodic fashion
groups (columns): families of elements
elements in a group have similar properties
B. main group (representative) elements
groups 1A - 8A
transition elements
groups 1B - 8B
inner transition elements
lanthanides (rare earth elements)
actinides

14. lustrous, malleable, ductile, electrically conductive nonmetals
The Periodic Table: Classification of the Elements
C. metals
lustrous, malleable, ductile, electrically conductive
nonmetals
not lustrous, malleable, ductile, nor electrically conductive
metalloids (semimetals, semiconductors)
in between
D. named groups
group 1A: alkali metals (form alkaline solutions in water)
group 2A: alkaline earth metals (")
group 6A: chalcogens (copper-forming ) [not often used]
group 7A: halogens (salt-forming)
group 8A: noble gases (inert gases; unreactive)