CHEM101
Chapter 2
Early Philosophy of Matter
Atoms and Elements
• Some early philosophers believed that matter had an
ultimate, tiny, indivisible particle.
Leucippus and Democritus
• Other philosophers believed that matter was infinitely
divisible.
Plato and Aristotle
• In the late 17th century, observations about nature were
made that could not easily be explained by the infinitely
divisible matter concept.
Scanning Tunneling Microscope
Instructor: Dr. Saravanan Rajendrasozhan
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Tro, Principles of Chemistry: A Molecular Approach
Law of Conservation of Mass
Antoine Lavoisier (1743–1794)
• In a chemical reaction, matter is neither created nor
•
destroyed.
Total mass of the materials you have before the
reaction must equal the total mass of the materials you
have at the end.
total mass of reactants = total mass of products
7.7 g Na
+ 11.9 g Cl2
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Law of Definite Proportions
Joseph Proust (1754–1826)
• All samples of a given compound, regardless of their
source or how they were prepared, have the same
proportions of their constituent elements.
→ 19.6 g NaCl
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CHEM101
Law of Multiple Proportions
John Dalton (1766–1844)
• When two elements (call them A and B) form two
•
•
•
different compounds, the masses of B that combine
with 1 g of A can be expressed as a ratio of small whole
numbers.
Carbon combines with oxygen to form two different
compounds, carbon monoxide (CO) and carbon dioxide
(CO2).
CO contains 1.33 g of oxygen for every 1 g of carbon.
CO2 contains 2.67 g of oxygen for every 1 g of carbon.
Dalton’s Atomic Theory
Proposed by Dalton
1) Each element is composed of tiny, indestructible
particles called atoms.
2) All atoms of a given element have the same mass and
other properties that distinguish them from atoms of
other elements.
3) Atoms combine in simple, whole-number ratios to
form molecules of compounds.
4) In a chemical reaction, atoms of one element cannot
change into atoms of another element.
They simply rearrange the way they are attached.
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Is the statement Correct or Incorrect?
Is the statement Correct or Incorrect?
(according to Dalton’s Atomic Theory)
(according to Dalton’s Atomic Theory)
• Copper atoms can combine with zinc atoms to make gold
•
atoms–Incorrect; According to Dalton, atoms of one
element cannot turn into atoms of another element by a
chemical reaction. He knew this because if atoms could
change it would change the total mass and violate the Law
of Conservation of Mass.
Water is composed of many identical molecules that have
one oxygen atom and two hydrogen atoms–Correct;
According to Dalton, atoms combine together in
compounds in small whole-number ratios, so that you could
describe a compound by describing the number of atoms of
each element in a molecule. He used this idea to explain
why compounds obey the Law of Definite Proportions.
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• Some carbon atoms weigh more than other carbon atoms–
Incorrect; According to Dalton, all atoms of an element are
identical.
• Since the mass ratio of Fe:O in hematite is 1.5 times larger
than the Fe:O ratio in litharge, there must be 1.5 Fe atoms
in hematite and 1 Fe atom in litharge–Incorrect; According
to Dalton, atoms must combine in small whole-number
ratios. If you could combine fractions of atoms, that would
mean the atom is breakable and Dalton’s first premise
would be incorrect. You can get the mass Fe:Fe ratio to be
1.5 if the formula for hematite is Fe2O3 and litharge is FeO.
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CHEM101
Thomson’s experiment
•
Glass (in which almost all air is evacuated) tube
containing metal electrodes.
•
When connected to a high-voltage power supply, a
glowing area is seen emanating from the cathode.
•
Cathode ray may be composed of tiny particles with
an electrical charge.
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Thomson’s Plum Pudding Model of Atom
Thomson’s experiment
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Millikan’s Oil Drop Experiment
• The mass of the atom is due to the mass of the
electrons within it.
• Later experiments prove that this model is wrong.
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CHEM101
Millikan’s Oil Drop Experiment
Radioactivity
Henri Becquerel & Marie Curie (1980s)
•
•
•
•
• Certain elements constantly emit small, energetic
Electrons are particles found in all atoms.
particles and rays.
• These energetic particles could penetrate matter.
• Rutherford discovered that there were three different
Cathode rays are streams of electrons.
The electron has a charge of −1.60 × 1019 C.
kinds of emissions.
The electron has a mass of 9.1 × 10−28 g.
alpha, α, particles with a mass 4× H atom and +
charge
beta, β, particles with a mass ~1/2000 H atom and –
charge
gamma, γ, rays that are energy rays, not particles
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Thomson’s Nuclear Model of Atom
1) The atom contains a tiny dense center called the
2)
Nuclear Atom
3)
.
.
.
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4)
nucleus.
The amount of space taken by the nucleus is only
about 1/10 trillionth the volume of the atom.
The nucleus has essentially the entire mass of the
atom.
The electrons weigh so little they give practically
no mass to the atom.
The nucleus is positively charged.
The amount of positive charge balances the
negative charge of the electrons.
The electrons are dispersed in the empty space of the
atom surrounding the nucleus.
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CHEM101
Structure of Atom
Structure of Atom
• Nucleus had a particle that had the same amount of
charge as an electron but opposite sign.
• These particles are called protons.
• Rutherford and Chadwick proposed that there was
another particle in the nucleus—it is called a neutron.
• Neutrons have no charge and a mass of 1 amu.
charge = +1.60 × 1019 C
mass = 1.67262 × 10−24 g
• The neutral atom must have equal numbers of protons
and electrons.
Some questions:
Why the protons did not repel each other in nucleus?
How beryllium atom (contain 4 protons) weighs 9 amu?
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Elements
• Each element has a unique number of protons in its
nucleus.
• The number of protons in the nucleus of an atom is called
the atomic number.
The elements are arranged on the Periodic Table in
order of their atomic numbers.
• Each element has a unique name and symbol.
symbol either one or two letters
one capital letter or one capital letter and one
lowercase
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CHEM101
Periodic Table of Elements
Periodic Table of Elements
Some symbols are one capital letter, like C, S, and I.
Some symbols are two letters, and the second is lower
case, like Br and Sr
Some others come from the Latin name of the element,
like Au for gold (aurum) and Cu for copper (cuprum)
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Structure of Nucleus & Isotopes
Isotopes
• Nucleus contain protons and neutrons. It is responsible
• All isotopes of an element are chemically identical.
for the atomic mass.
• Same element could have atoms with different masses,
which he called isotopes.
Two isotopes of chlorine found in nature:
Cl with 35 amu mass
undergo the exact same chemical reactions
• All isotopes of an element have the same number of
protons.
• Isotopes of an element have different numbers of
neutrons.
Cl with 37 amu mass
• Isotopes are identified by their mass numbers.
protons + neutrons
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CHEM101
How many protons, electrons, and neutrons are in
an atom of
?
Isotopes
Number of Number of A, Mass
Protons
Neutrons Number
Symbol
Ne-20 or
20 Ne
10
Percent
Natural
Abundance
10
10
20
90.48%
21 Ne
Ne-21 or 10
10
11
21
0.27%
22 Ne
Ne-22 or 10
10
12
22
9.25%
The observed mass is a weighted average of the weights
of all the naturally occurring atoms.
Conceptual
Plan:
25
Relationships:
Solution:
7
42
96
55
27
# p+
atomic & mass
numbers
# e-
# n0
Z = 24 = # p+
# e− = # p+ = 24
A = Z + # n0
52 = 24 + # n0
28 = # n0
for most stable isotopes, n0 ≥ p+
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Practice: Complete the table
Atomic
Mass
Protons Neutrons Electrons Number Number
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Check:
atomic
number
in neutral atom, # p+ = # e−
mass number = # p+ + # n0
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Practice: Complete the table
6
symbol
symbol
Mass of Ne = (20/100)x90.48 + (21/100)x0.27 + (22/100)x9.25
= 18.096 + 0.0867 + 2.035
= 20.1877
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therefore, A = 52, Z = 24
# p+, # e−, # n0
Given:
Find:
133
Atomic
Symbol
Atomic
Mass
Protons Neutrons Electrons Number Number
6
7
6
6
13
42
54
42
42
96
13
14
13
13
27
55
78
55
55
133
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Atomic
Symbol
28
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CHEM101
Charged Atoms
Ions and Compounds
• Number of protons in the atom does not change in a
• Ions behave much differently than the neutral atom.
chemical reaction (because the number of protons
determines the element).
• However, many reactions involve transferring electrons
from one atom to another.
• When atoms gain or lose electrons, they acquire a
charge.
• Charged particles are called ions.
• When atoms gain electrons, they become negatively
charged ions, called anions.
• When atoms lose electrons, they become positively
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must be equal amounts of charge from cations and
anions in them.
Neutral compound: Na Cl (charge: Nuetral)
Cation: Na+ (charge: +1)
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Cations
• Nonmetals form anions.
• For each negative charge, the ion has 1 more electron
than the neutral atom.
F = 9 p+ and 9 e−, F─ = 9 p+ and 10 e−
P = 15 p+ and 15 e−, P3─ = 15 p+ and 18 e−
• Anions are named by changing the ending of the
name to -ide.
Oxygen
• Since compounds like table salt (NaCl) are neutral, there
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Anions
Fluorine
Sodium ions (Na+, cations) found in table salt (NaCl)
are very nonreactive and stable.
Anion: Cl- (charge: -1)
charged ions, called cations.
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Sodium metal (Na atoms) is highly reactive and quite
unstable.
F + 1e− → F─
Fluoride ion
O + 2e− → O2─
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Oxide ion
• Metals form cations.
• For each positive charge, the ion has 1 less electron than
the neutral atom.
Na atom = 11 p+ and 11 e−, Na+ ion = 11 p+ and 10 e−
Ca atom = 20 p+ and 20 e−, Ca2+ ion = 20 p+ and 18 e−
• Cations are named the same as the metal.
sodium
Na → Na+ + 1e−
sodium ion
calcium
Ca →
calcium ion
Ca2+ +
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2e−
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CHEM101
Practice: Complete the table
Atomic
Number
Protons
Electrons
16
Practice: Complete the table
Ion
Charge
Ion
Symbol
18
12
Atomic
Number
Protons
Electrons
Ion
Charge
Ion
Symbol
16
16
18
2−
S2−
12
12
10
2+
Mg2 +
13
13
10
3+
Al3 +
35
35
36
1−
Br −
2+
3+
Al
36
1−
Number of electrons (in ion)= (Number of Proton) - (Charge)
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Periodic Table
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Periodic Pattern
• Elements are ordered by atomic number (Z)
• Periodic Law—When the elements are arranged in
order of increasing atomic number, certain sets of
properties recur (repeat) periodically.
• Elements with similar properties in the same column
• Used to predict properties of undiscovered elements.
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CHEM101
Metals
• Solids at room temperature (except Hg)
• Reflective surface
•
•
•
•
•
•
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Sulfur, S(s)
Nonmetals
• Found in all three states (solid, liquid,
electricity
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Metalloids
• Show some properties
gas)
• Poor conductors of heat and
shiny
Conduct heat & electricity
Malleable
can be shaped
Ductile
drawn or pulled into wires
Lose electrons and form CATIONS in
reactions
About 75% of the elements are metals.
Lower left on the periodic table
Bromine, Br2(l)
of metals and some of
nonmetals
• Also known as
• Solids are brittle (easily breakable).
• Gain electrons in reactions to
semiconductors
become ANIONS
• Upper right on the periodic table
Properties of Silicon
Shiny
Conducts electricity
Does not conduct heat well
Brittle
Chlorine, Cl2(g)
except H
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CHEM101
Patterns in Metallic Character
Patterns in Metallic Character
= Metal
= Metalloid
= Nonmetal
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Modern Periodic Table
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Modern Periodic Table
• Elements with similar chemical and physical properties
are in the same column.
• Columns are called Groups or Families.
designated by a number and letter at top
• Rows are called Periods.
Main Group = Representative Elements = “A” groups
Transition Elements = “B” groups (all metals)
Bottom Rows = Inner Transition Elements = Rare
Earth Elements
(Metals & really belong in Periods 6 & 7)
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CHEM101
Important Groups in Periodic Table
= Alkali Metals
= Halogens
= Alkali Earth Metals
= Lanthanides
= Noble Gases
= Actinides
= Transition Metals
Important Groups - Hydrogen
• Nonmetal
• Colorless, diatomic gas
very low melting point and density
• Reacts with nonmetals to form molecular
compounds
HCl is acidic gas
H2O is a liquid
• Reacts with metals to form hydrides
metal hydrides react with water to form H2
• HX dissolves in water to form acids
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Important Groups – Alkalai Metals
• Group 1A = Alkali Metals
• Hydrogen usually placed here, though it doesn’t really
belong.
• Soft, low melting points, low density.
• Very reactive, never find uncombined in nature.
• Tend to form water-soluble compounds, therefore salt
is crystallized from seawater then molten salt is
electrolyzed
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Important Groups – Alkalai Earth Metals
• Group 2A = Alkali Earth Metals
• Harder, higher melting, and denser than alkali metals
Mg alloys used as structural materials
• Reactive, but less than corresponding alkali metal
• Form stable, insoluble oxides from which they are
normally extracted
oxides are basic = alkaline earth
• Reactivity with water to form H2 →
colorless solutions
• React with water to form basic (alkaline) solutions and
Be = none; Mg = steam; Ca, Sr, Ba = cold water
H2 (release lot of heat)
2 Na + 2 H2O → 2 NaOH + H2
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CHEM101
Important Groups – Halogens
•
•
•
•
•
•
Important Groups – Noble Gases
• Group 8A = Noble Gases
• All are gases at room temperature
Group 7A = Halogens
Nonmetals
F2 and Cl2 gases; Br2 liquid; I2 solid
Very low melting and boiling points
• Very unreactive, practically inert
• Very hard to remove electron from the noble gas or
All diatomic
Very reactive
Cl2, Br2 react slowly with water
Br2 + H2O → HBr + HOBr
give an electron to noble gas
Very hard to produce ions
• React with metals to form ionic compounds
• HX (Hydrogen-Nonmetal) all acids
HF weak < HCl < HBr < HI
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Ion Charge and Periodic Table
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Ion Charge and Periodic Table
• Charge on an ion can be determined from an element’s
position on the Periodic Table.
• Metals always form positively charged cations.
For many main group metals,
Charge = Group number
• Nonmetals form negatively charged anions.
For nonmetals,
Charge = group number − 8
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•
•
•
•
•
Potassium cation
Sulfide anion
Calcium cation
Bromide anion
Aluminum cation
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K+
S2−
Ca2+
Br−
Al3+
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CHEM101
Calculate the number of atoms in 2.45 mol of Cu
Counting Atoms by Moles
• If we can find the mass of a particular number of atoms,
we can convert the mass of atoms into number of atoms
in the sample.
• The number of atoms we will use is 6.022 x 1023 and we
call this a mole.
1 mole = 6.022 × 1023 things
Given:
Find:
2.45 mol Cu
atoms Cu
Conceptual
Plan:
mol Cu
atoms Cu
1 mol = 6.022 × 1023 atoms
Relationships:
like 1 dozen = 12 things
Solution:
= 602200000000000000000000 Cu atoms
Check:
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Tro, Principles of Chemistry: A Molecular Approach
A silver ring contains 1.1 × 1022 silver atoms.
How many moles of silver are in the ring?
1.1 × 1022 atoms Ag
moles Ag
Given:
Find:
Conceptual
Plan:
Relationships:
atoms Ag
mol Ag
1 mol = 6.022 × 1023 atoms
Solution:
Check:
Since atoms are small, the large number of
atoms makes sense.
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Mole & Mass
• Mass of one mole of atoms is called the molar mass.
• Mass of 1 mole of atoms = Mass number in gram (g)
Mass of 1 mole of C (Molar mass of C) = 12.01 g.
• 1 Mole C = Number of atoms in 12.01 g of C.
• The number of particles in 1 mole is called Avogadro’s
Number = 6.0221421 × 1023.
1 mole of C = 12.01 g = 6.022 × 1023 atoms
Since the number of atoms given is less than
Avogadro’s number, the answer makes sense.
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CHEM101
Calculate the moles of carbon in
0.0265 g of pencil lead
Mole & Mass Relationships
Given: 0.0265 g C
Find: mol C
Conceptual
Plan:
gC
mol C
Relationships: 1 mol C = 12.01 g
Solution:
1 mole
carbon
12.01 g
1 mole
sulfur
32.06 g
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Check:
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Since the given amount is much less than
1 mol C, the number makes sense.
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How many copper atoms are in
a penny weighing 3.10 g
Given:
Find:
Conceptual
Plan:
Relationships:
3.10 g Cu
atoms Cu
g Cu
mol Cu
atoms Cu
1 mol Cu = 63.55 g, 1 mol = 6.022 × 1023
Solution:
Check: Since the given amount is much less than
1 mol Cu, the number makes sense.
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