Atoms and Elements:The
Nature of Matter
“It’s easier to break an atom than a prejudice” A. Einstein
Gold
Mercury
Chemists are interested in the nature of matter and
how this is related to its atoms and molecules.
Nature of Matter, cont’d
CHEMICAL COMPOUNDS are
composed of atoms and so can be
decomposed to those atoms.
The red compound is
composed of
• nickel (Ni) (silver)
• carbon (C) (black)
• hydrogen (H) (white)
• oxygen (O) (red)
• nitrogen (N) (blue)
Nature of Matter, cont’d
A MOLECULE is the smallest unit of
a compound that retains the chemical
characteristics of the compound.
Composition of molecules is given by a
MOLECULAR FORMULA
H2O
C8H10N4O2 - caffeine
Nature of Matter, cont’d
STATES OF MATTER
• SOLIDS — have rigid shape, fixed
volume. External shape can reflect the
atomic and molecular arrangement.
–Reasonably well understood.
• LIQUIDS — have no fixed shape and
may not fill a container completely.
–Not well understood.
• GASES — expand to fill their
container.
–Good theoretical understanding.
Nature of Matter, cont’d
Kinetic Nature of Matter
Matter consists of atoms and molecules
in motion.
Nature of Matter, cont’d
• An atom is the smallest particle of
an element that has the chemical
properties of the element.
Copper
atoms on
silica
surface.
See CDROM
Screen 1.4
Distance across = 1.8 nanometer (1.8 x 10-9 m)
Nature of Matter, cont’d
ATOMIC COMPOSITION
• Protons
–
–
–
+ electrical charge
mass = 1.672623 x 10-24 g
relative mass = 1.007 atomic
mass units (amu)
• Electrons
–
–
negative electrical charge
relative mass = 0.0005 amu
• Neutrons
–
–
no electrical charge
mass = 1.009 amu
The Atom
An atom consists of a
• nucleus
– (of protons and neutrons)
• electrons in space about the nucleus.
Electron cloud
Nucleus
ATOMS AND ELEMENTS
Radioactivity
• One of the pieces of evidence for the
fact that atoms are made of smaller
particles came from the work of
Marie Curie (1876-1934).
• She discovered radioactivity, the
spontaneous disintegration of some
elements into smaller pieces.
ATOM
COMPOSITION
The atom is mostly
empty space
•protons and neutrons in
the nucleus.
•the number of electrons is equal to the
number of protons.
•electrons in space around the nucleus.
•Atoms are extremely small: One teaspoon
of water has 3 times as many atoms as the
Atlantic Ocean has teaspoons of water.
The modern view of the atom was developed by
Ernest Rutherford (1871-1937).
Screen 2.9
Atomic Number, Z
All atoms of the same element
have the same number of
protons in the nucleus, Z
13
Al
26.981
Atomic number
Atom symbol
Atomic weight
Atomic Weight
• This tells us the mass of one atom of an
element relative to one atom of another
element.
• OR — the mass of 1000 atoms of one relative
to 1000 atoms of another.
• For example, an O atom is approximately 16
times heavier than an H atom.
• Define one element as the standard against
which all others are measured
• Standard = carbon
Mass Number, A
• C atom with 6 protons and 6 neutrons
is the mass standard
• = 12 atomic mass units
• Mass Number (A)
= # protons + # neutrons
• A boron atom can have
A = 5 p + 5 n = 10 amu
A
10
Z
5
B
Isotopes
• Atoms of the same element (same Z)
but different mass number (A).
• Boron-10 (10B) has 5 p and 5 n
• Boron-11 (11B) has 5 p and 6 n
11B
10B
Isotopes &
Their Uses
Bone scans with
radioactive
technetium-99.
Isotopes & Their Uses
The tritium content of ground water is
used to discover the source of the water,
for example, in municipal water or the
source of the steam from a volcano.
Masses of Isotopes
determined with a mass spectrometer
11B
Isotopes
10B
• Because of the existence of isotopes, the
mass of a collection of atoms has an average
value.
• Average mass = ATOMIC WEIGHT
• Boron is 20% 10B and 80% 11B. That is, 11B is
80 percent abundant on earth.
• For boron atomic weight
= 0.20 (10 amu) + 0.80 (11 amu) = 10.8 amu
Isotopes & Atomic Weight
• Because of the existence of isotopes, the mass of a
collection of atoms has an average value.
• 6Li = 7.5% abundant and 7Li = 92.5%
– Atomic weight of Li = ______________
•
28Si
= 92.23%, 29Si = 4.67%, 30Si = 3.10%
– Atomic weight of Si = ______________
Counting Atoms
Mg burns in air (O2) to
produce white
magnesium oxide, MgO.
How can we figure out how
much oxide is produced
from a given mass of Mg?
Counting Atoms
Chemistry is a quantitative
science—we need a
“counting unit.”
MOLE
1 mole is the amount of
substance that contains as
many particles (atoms,
molecules) as C atoms in
12.0 g of 12C.
Particles in a Mole
Avogadro’s Number
Amedeo Avogadro
1776-1856
6.02214199 x
23
10
There is Avogadro’s number of
particles in a mole of any substance.
Molar Mass
1 mol of 12C
= 12.00 g of C
= 6.022 x 1023 atoms
of C
12.00 g of 12C is its
MOLAR MASS
Taking into account all
of the isotopes of C,
the molar mass of C is
12.011 g/mol
One-mole Amounts
PROBLEM: What amount of Mg
is represented by 0.200 g? How
many atoms?
Mg has a molar mass of 24.3050 g/mol.
1 mol
0.200 g •
= 8.23 x 10-3 mol
24.31 g
How many atoms in this piece of Mg?
23
6.022
x
10
atoms
8.23 x 10-3 mol •
1 mol
= 4.95 x 1021 atoms Mg
Periodic Table
• Dmitri Mendeleev developed the
modern periodic table. Argued
that element properties are
periodic functions of their “atomic
weights”.
• We now know that element
properties are periodic functions
of their ATOMIC NUMBERS.
Periods in the Periodic Table
Regions of the Periodic Table
Element Abundance
C
O
Al
Si
Fe
http://www.webelements.com/webelements/elements/text/Si/geol.html
Hydrogen
Shuttle main engines use H2 and O2
What was the cause of the
Hindenburg fire while landing in
New Jersey in May 1939 ?
Group 1A: Alkali Metals
What are the characteristic properties of a metal ?
Reaction of
potassium + H2O
Cutting sodium metal
Solids at room temperature, react with water
Group 2A: Alkaline Earth Metals
Magnesium
Magnesium
oxide
Occur naturally only in compounds,except for Be
they also react with water. Mg and Ca are the most abundant
Calcium Carbonate—Limestone
The Appian Way, Italy
Champagne cave carved into
chalk in France
Group 3A: B, Al, Ga, In, Tl
Aluminum, the most
abundant metal in the
earth’s crust
Boron halides
BF3 & BI3
Gems & Minerals
What determines the colors of precious stones ?
• Sapphire: Al2O3
with Fe3+ or Ti3+
impurity gives
blue whereas V3+
gives violet.
• Ruby: Al2O3 with
Cr3+ impurity
Colors of Transition Metal
Compounds
Iron
Cobalt
Nickel
Copper
Zinc
Relative Densities of the Elements
Transition Elements
They are all metals, Ag, Au and Pt are the less reactive
Structural materials, paints, catalytic converters, batteries
They play important biological roles, e.g., Fe.
Lanthanides and actinides
Iron in air gives
iron(III) oxide
Glenn Seaborg
(1912-1999 )
• Discovered 8
new elements.
• Only living
person for
whom an
element was
named.
What is a synchrotron ?
How does it work?
Group 4A: C, Si, Ge, Sn, Pb
Quartz, SiO2
Diamond
Allotropes: distinct forms of nonmetals,
also called allomorphic forms
Diamond
Graphite — layer
structure of
carbon atoms
reflects
physical
properties.
“Buckyballs”
C60:buckminsterfullerene
Group 5A: N, P, As, Sb, Bi
How does nature break atmospheric N2 and transform it into
compounds such as chlorophyll, proteins and DNA ?
Ammonia, NH3
White and red
phosphorus
Phosphorus
• Phosphorus is essential
to life, important
constituent of bones and
teeth, first isolated by
Brandt from urine, 1669.
Both white and red P
ignite spontaneously in
air to form P4O10, which
then transforms into
H3PO4
Group 6A: O, S, Se, Te, Po
What allotropes of oxygen do you know ?
What is sulfuric acid most commonly used for ?
Sulfuric acid dripping from
snot-tite in cave in Mexico
Sulfur from
a volcano
Group 7A:halogens
F, Cl, Br, I, At
salt
Group 8A:
He, Ne, Ar, Kr, Xe, Rn
Gases at room temperature
Until 1962 they were considered chemically unreactive
• Lighter than air balloons
• “Neon” signs
XeOF4
Summary
Historical experiments that motivated the
development of the modern view of matter
(Thompson, Rutherford, M. Curie).
Description of elementary particles
(electrons, protons, neutrons).
Isotopes, Atomic number (Z), Mass number (A),
Atomic Weight.
Molar mass, Avogadro’s number.
Periodic Table, groups, periods, metals,
metalloids, nonmetals, halogens, noble
gases, transition elements.