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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 are extremely small and occupy
the space around the nucleus.
Scientists
• Chadwich
Thomsom
Becquerel
Curie
Rutherford
Millikan
Bohr
The modern view of
was developed by
Ernest
Rutherford
the “Nuclear Atom”
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.
• Define one element as the standard
against which all others are
measured
• Standard = carbon
• For example, an O atom is
approximately 16 times heavier than
an H atom.
Mass Number, A
• Mass Number (A) = # p + # n
• C atom with 6 protons and 6 neutrons is
the mass standard
= 12 atomic mass units
• For a boron atom
A = 5p + 5n =
= 10 amu
A
10
Z
5
B
Isotopes
• Atoms of the same element
but different mass number (A).
Boron-10 has 5 p and 5 n (10B)
Boron-11 has 5 p and 6 n
(11B)
11B
10B
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
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.
• Today: element properties are
periodic functions of their
ATOMIC NUMBERS.
Periodic Table History
 Henry
Mosley-
-Grouped elements in order of
atomic number
-credited with modern periodic table
Came up with Period Law-physical and chemical properties
of elements are a periodic function
of their atomic numbers
Modern Periodic Table
 Glen
Seaborg-
-Grouped elements according to
their electron configurations
-credited with periodic table as it
looks today
Modern Periodic Table
Groups
(families) o
a vertical column i
of elements.
determined by # of
valence electrons
Modern Periodic Table
Periods
(a)
n horizontal row
of elements.
-determined by the
electron’s energy level.
NOBLE GASES
NONMETALS
METALLOIDS
METALS
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
Group 3A: B, Al, Ga, In, Tl
Aluminum
Boron halides
BF3 & BI3
Group 4A: C, Si, Ge, Sn, Pb
Quartz, SiO2
Diamond
Group 5A: N, P, As, Sb, Bi
Ammonia, NH3
White and red
phosphorus
Group 6A: O, S, Se, Te, Po
Sulfuric acid dripping
from snot-tite in cave
in Mexico
Sulfur from
a volcano
Group 7A:
HALOGENS
F, Cl, Br, I, At
Group 8A: NOBLE GASES
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
Periods
Groups/ Families
3A 4A 5A 6A 7A
“B” or
Transition Metals
Rare
Earth
Lanthanides
Actinides
8A
Halogens
Noble Gases
1
2
3
4
5
6
7
2A
Alkali
Alkaline Earth
1A
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