Download Chapter 2 ATOMS AND ELEMENTS

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 (18761934).
• She discovered radioactivity,
the spontaneous disintegration
of some elements into smaller
pieces.
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
–
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.
•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
of New Zealand
(1871-1937).
Ernest Rutherford
Canterbury
University in
Christchurch, NZ
Rutherford laboratory
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 there are 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 atoms
6.022
x
10
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.
• See CD-ROM, Screen 2.16.
Periods in the Periodic Table
Groups 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
The Hindenburg crash,
May 1939.
Group 1A: Alkali Metals
Reaction of
potassium + H2O
Cutting sodium metal
Group 2A: Alkaline Earth Metals
Magnesium
Magnesium
oxide
Calcium Carbonate—Limestone
The Appian Way, Italy
Champagne cave carved into
chalk in France
Group 3A: B, Al, Ga, In, Tl
Aluminum
Boron halides
BF3 & BI3
Gems & Minerals
• Sapphire: Al2O3
with Fe3+ or Ti3+
impurity gives
blue whereas V3+
gives violet.
• Ruby: Al2O3 with
Cr3+ impurity
Group 4A: C, Si, Ge, Sn, Pb
Quartz, SiO2
Diamond
Group 5A: N, P, As, Sb, Bi
Ammonia, NH3
White and red
phosphorus
Phosphorus
• Phosphorus first
isolated by Brandt
from urine, 1669
Group 6A: O, S, Se, Te, Po
Sulfuric acid dripping
from snot-tite in cave
in Mexico
Sulfur from
a volcano
Group 7A:
F, Cl, Br, I, At
Group 8A:
He, Ne, Ar, Kr, Xe, Rn
• Lighter than air balloons
• “Neon” signs
XeOF4
Transition Elements
Lanthanides and actinides
Iron in air gives
iron(III) oxide
Colors of Transition Metal
Compounds
Iron
Cobalt
Nickel
Copper
Zinc