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THE ATOM

NUCLEAR MODEL OF THE ATOM

• An atom is an electrically neutral particle

• Composed of protons, neutrons, and electrons. 

• Atoms are spherical in shape

• Have a tiny, dense nucleus of positive charge surrounded by one or more negatively charged electrons 

• Nucleus contains 99.7 % of the mass of an atom.

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BASIC STRUCTURE OF THE ATOM

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SUBATOMIC PARTICLES

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SUBATOMIC PARTICLES

• Most of the atom consists of fast-moving electrons traveling through the space around the nucleus.• Atoms are neutral in charge• The number of protons = the number of electrons  

• Atomic Number: • The number of Protons in the nucleus • determines the identity of an atom• No two types of atoms have the same number of protons

• An Element consists of atoms with the same number of protons. 

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ATOMIC NUMBER(Z): THE NUMBER OF

PROTONS IN THE NUCLEUS OF EACH ATOM OF THE ELEMENT

• Atomic Number = Protons• Elements are arranged in the periodic table

from left to right in order of increasing atomic number.• Atomic number identifies the element

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ISOTOPES AND MASS NUMBER

• Atoms can have different mass.• Naturally occurring elements are a mixture of

atoms that have different numbers of neutrons. • Atoms with the same number of protons, but

different number of neutrons are called Isotopes.

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HYDROGEN HAS THREE ISOTOPES:

• Protium 1 proton 1 electron 0 neutrons• Deuterium 1 proton 1 electron 1 neutron• Tritium 1 proton 1 electron 2 neutrons

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ISOTOPES AND MASS NUMBER

• Isotopes: atoms of the same element that have different masses. • Same number of protons• Different number of neutrons • Different Mass• Atoms of different isotopes have different

masses so identity is given by name and name and mass.mass.

• Mass Number = protons and neutrons

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ISOTOPE (NUCLEAR) SYMBOLS:

Consists of three parts1. the symbol of the element2. the atomic number of the element 3. the mass number of the specific isotope.

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NUCLEAR SYMBOL•Read as Helium-4

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ISOTOPE NAMES

Element Name-Mass NumberHelium-4Potassium-39Hydrogen-3

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NEUTRONS = MASS NUMBER – ATOMIC NUMBER

Atomic Number: _________Mass Number: __________# of Protons: _________# of Electrons: _______# of Neutrons: ________Name of Isotope: ____________ 

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EXAMPLE

Atomic Number: _________Mass Number: __________# of Protons: _________# of Electrons: _______# of Neutrons: ________Name of Isotope: ____________

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EXAMPLE

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Atomic Number: _________Mass Number: __________# of Protons: _________# of Electrons: _______# of Neutrons: ________Isotope Name: _________

 

DRAW THE ISOTOPE SYMBOL FOR CALCIUM WITH 21 NEUTRONS.

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COMPARING POTASSIUM ISOTOPES

Potassium-41• protons = ________• electrons = ______• neutrons = _______

Potassium-40• protons = ________• electrons = ______• neutrons = _______

Potassium-39• Protons = __________• Electrons = _________• Neutrons = __________ 

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RELATIVE ATOMIC MASSES

 • Atomic masses measured in grams are very small. • Example: An atom of Oxygen-16 has a mass of 2.657 x 19-23grams.

• It is most convenient to measure mass in relative atomic masses.

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TO SET UP A RELATIVE SCALE OF ATOMIC MASSES:

1. One atom is chosen and assigned a relative mass value,

2. All the other masses are expressed in relation to this defined standard.

 • Carbon-12 is the chosen standard.• A single atom of C-12 is assigned a mass

of exactly 12 atomic mass units(u).

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AN AMU

• One amu is exactly 1/12th of the mass of a carbon-12 atom or 1.660 5402 x 10-24grams.

• The atomic mass of a carbon-12 atom is exactly 12 u. • Atomic Mass: The mass of an atom expressed in

atomic mass units.

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AVERAGE ATOMIC MASS

• Atomic Masses given for the elements on the periodic table are weighted averages for the naturally occurring mixtures of isotopes, called average atomic mass.

• Average Atomic Mass depends on mass and relative abundance of the isotopes.

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TO CALCULATE AVERAGE ATOMIC MASS:

1. Atomic mass x relative abundance 2. Add results.

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EXAMPLE PROBLEM:

Naturally occurring copper consists of 69.17% Cu-63, mass of 62.939 598u and 30.83% Cu-65, mass of 64.927 793u• 0.6917 x 62.939 598u = 43.535u• 0.3083 x 64.927 793u = 20.017

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