Download The Atom: Idea to Theory

Important Vocabulary for Ch. 3
1. Atomic mass:
Round the average atomic mass on
the periodic table to the nearest whole number unless the
mass is given in name.
2. Anion: A negative ion (ex. O2-)
3. Cation: A positive ion (ex. Sr2+)
4. Isotopes: Atoms of the same element with different
numbers of neutrons and therefore, different masses
5. Ions: Atoms of the same element with different numbers
of electrons, denoted by a positive or negative superscript.
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Protons = atomic number
Neutrons = mass – protons
Electrons = protons if no charge
Electrons = protons + the charge if anion
Electrons = protons – the charge if cation
Example:
3
Li
6.941
lithium
Li has 3 protons (atomic #)
4 neutrons (7-3)
3 electrons (no charge)
Li+ has 3 protons (atomic #)
4 neutrons (7-3)
2 electrons (3 protons -1 (charge is +1))
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More examples:
35
Br
79.904
bromine
56
Ba
137.33
barium
Br has 35 protons (atomic #)
45 neutrons (80-35)
35 electrons (no charge)
Br- has 35 protons (atomic #)
45 neutrons
36 electrons (35 protons +1 (charge is -1))
Ba2+ has 56 protons (atomic #)
81 neutrons (137-56)
54 electrons (56 protons - 2 (charge is +2))
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56
Ba
137.33
barium
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The Atom: Idea to Theory
• Democritus (~ 400 BC) called nature’s
basic particle an atom
• Atom comes from Greek word meaning
“indivisible”
• 1808: Dalton proposed a theory with
several statements which were later
verified, but his “model” of an atom was
that of a sphere.
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Modern Atomic Theory
• Democritus = Idea about an atom
• Dalton = Scientific theory which could be tested
• Dalton’s concepts which have “held true:”
– All matter is composed of atoms
– Atoms of any one element differ in properties from
atoms of another element
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John Dalton
• Reinstated atomic theory into
western science, composed 4
postulates and 3 laws to explain the
atom as he found them to be, and
explained that the atom is the
smallest amount of an element to
retain its basic properties.
Dalton’s Atomic Theory
1. All matter is composed of extremely small
particles called atoms.
2. Atoms of a given element are identical in size,
mass and other properties.
3. Atoms cannot be divided, created or destroyed.
4. Atoms of different elements combine in simple
whole-number ratios to form chemical
compounds.
5. In chemical reactions, atoms are combined,
separated or rearranged.
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• Law of Conservation of Mass :
Mass is neither created nor destroyed during
ordinary physical or chemical changes.
•Law of Definite Proportions:
A chemical compound contains the same
elements in exactly the same proportions by mass
regardless of the size of the sample.
•Law of Multiple Proportions:
If two or more different compounds are made of the
same two elements, then the ratio of the masses of the 2nd
element combined with a certain mass of the 1st element is
always a ratio of small whole numbers.
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• Atomic Number
– Number of protons or electrons in an element
– Identifies the element
• Atomic Mass
–
–
–
–
–
Nucleus contains most of the mass of an atom.
Protons and neutrons are each ~ 1.67 x 10-24 g.
Electrons are each ~ 9.11 x 10-28 g.
Use atomic mass unit (amu) instead of gram.
The mass of one proton is ~ 1 amu.
• Mass Number
– The sum of the number of protons and number of
neutrons in the nucleus
– Is approximately equal to the average atomic mass
shown on periodic table.
– Number of neutrons = mass number – atomic number
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• Isotopes
– Atoms of the same element with different numbers of
neutrons
– Have the same number of protons
– Example: Carbon-12 and Carbon-14
• Radioactive Isotopes
– Unstable in nature
– Atoms with a neutron: proton ratio of > 1.5:1 are
unstable.
– Can be used to date fossils and rocks
– The time it takes for half of the radioactive atoms in a
piece of the fossil to change to another element is its
half-life.
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