Download 2:1 Foundations of Atomic Theory In the 1790s the study of matter

Intro to Chemistry and Physics/Pre-AP Chem
UNIT 2 Atoms: The Building Blocks of Matter
2:1 Foundations of Atomic Theory
In the 1790s the study of matter was
revolutionized by the quantitative analysis of
chemical reactions, which had been greatly
improved by more accurate balances.
This led to several laws that are very important:
1. THE LAW OF CONSERVATION OF MASS states
mass is neither created nor destroyed during
ordinary chemical reactions or physical
changes. (This will be the basis for balancing
equations.)
Ex.-
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Intro to Chemistry and Physics/Pre-AP Chem
UNIT 2 Atoms: The Building Blocks of Matter
2. THE LAW OF DEFINITE PROPORTIONS states
that a chemical compound contains the same
number of elements in exactly the same
proportions by mass regardless of the size of
the sample or source of the compound.
Ex.- Sodium chloride [NaCl] (table salt) always
consists of 39.34% by mass of the element
sodium [Na] and 60.66% by mass of the
element chlorine [Cl].
3. THE LAW OF MULTIPLE PROPORTIONS states
if two or more different compounds are
composed of the same two elements, then the
ratio of the masses of the second element
combined with a certain mass of the first
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Intro to Chemistry and Physics/Pre-AP Chem
UNIT 2 Atoms: The Building Blocks of Matter
element is always a ratio of small whole
numbers.
Ex.- The elements carbon and oxygen form two
compounds, carbon dioxide [CO2] and carbon
monoxide [CO]—the ratio of the masses of
oxygen in these two compounds is always 2 to
1.
2:2 Structure of the Atom
In the late 1800s, experiments showed that
atoms are actually composed of several basic
types of smaller particles and that the number
and arrangement of these particles within an
atom determine that atom’s chemical
properties.
ATOM the smallest particle of an element that
retains the chemical properties of that element
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Intro to Chemistry and Physics/Pre-AP Chem
UNIT 2 Atoms: The Building Blocks of Matter
The atom consists of two regions: the nucleus
and the electron cloud. Three subatomic
particles are protons, neutrons and electrons.
(Note: protons and neutrons can be further
divided into QUARKS).
NUCLEUS contains the positively charged
PROTONS and neutral NEUTRONS. ELECTRONS
are negatively charged. Atoms are neutral (not
charged), so an atom must have the same
number of electrons as protons, though
neutrons can differ.
Super important concept:
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Intro to Chemistry and Physics/Pre-AP Chem
UNIT 2 Atoms: The Building Blocks of Matter
The table above shows that a proton and
neutron are approximately the same size and
are 2000 times larger than an electron. If a
large football stadium were an atom, this
would make the nucleus (including the protons
and neutrons) about the size of a marble. The
electrons would be similar to dust particles
floating about the stadium. This means that in
our current model of the atom, the atom is
mostly empty space.
2:3 Counting Atoms
As atoms are generally too small to be
measured individually in experiments, it is
important to know how to analyze atoms
quantitatively by knowing fundamental
properties of the atoms of each element. You
can use these properties to count the number
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Intro to Chemistry and Physics/Pre-AP Chem
UNIT 2 Atoms: The Building Blocks of Matter
of atoms in an element in a sample with a
known mass.
Atoms of different elements have different
numbers of protons; Atoms of the same
element have the same number of protons.
The
ATOMIC NUMBER (Z) of an element is the
number of protons of each atom of that
element, so it identifies an element.
The atomic number can be found above an
element’s symbol on the periodic table.
Elements are arranged in the Periodic Table in
increasing atomic number.
Ex.- Silver [Ag] has an atomic number of 47.
This means that a silver atom has 47 protons,
and since all atoms are neutral, 47 electrons.
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Intro to Chemistry and Physics/Pre-AP Chem
UNIT 2 Atoms: The Building Blocks of Matter
HOWEVER, many naturally occurring elements
have different numbers of neutrons and are
called ISOTOPES. Most elements consist of
mixtures of isotopes.
ISOTOPES are atoms of the same element that
have different masses.
Ex.- Three types of hydrogen atoms are known.
All only have one proton. The most common
type accounts for 99.9885% of the hydrogen
atoms found on Earth and is called protium,
which has no neutrons and one electron.
Deuterium with one proton, one neutron, one
electron and accounts for 0.0115% of hydrogen
atoms. The third is tritium with two neutrons,
and it is radioactive. It exists in very small
amounts in nature but can be artificially
prepared.
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Intro to Chemistry and Physics/Pre-AP Chem
UNIT 2 Atoms: The Building Blocks of Matter
The identification of an isotope requires
knowing both the name and atomic number of
the element and the mass of the isotope. The
MASS NUMBER is the total number of protons
and neutrons that make up the nucleus of an
isotope.
The three isotopes of hydrogen mentioned
above have mass numbers of 1, 2 and 3.
Isotopes are usually identified by specifying
their mass number as shown with helium
below. For example, the uranium isotope that
is used as fuel for nuclear power plants has a
mass number of 235 and is known as uranium235 in hyphen notation. Isotopes can also be
written with nuclear symbols in which the
superscript indicates the mass number
(protons+neutrons) and the subscript indicates
the atomic number (number of protons).
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Intro to Chemistry and Physics/Pre-AP Chem
UNIT 2 Atoms: The Building Blocks of Matter
Ex.So, to find the number of neutrons:
Mass number-atomic number=number of
neutrons
235-92 (can be found/matched on periodic
table)=143
Isotopes of Hydrogen and Helium
ISOTOPE
ATOMIC Number
(hypen
NUMBE of
notation)
R (# of electrons
protons)
Hydrogen-1 1
1
(protium)
Hydrogen-2 1
1
(deuterium)
Hydrogen-3 1
1
(tritium)
Helium-3
2
2
Helium-4
2
2
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Number MASS
of
NUMBER
neutrons (protons +
neutrons)
0
1
1
2
2
3
1
2
3
4