Describe the experiments of Thomson and Rutherford explain how
they contributed to our present understanding of atomic structure.
Describe the mass and charge differences among electrons, protons
and neutrons. Define atomic number, mass number and isotopes in
terms of the number of electrons, protons and neutrons.
Objectives
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The Modern Atom Today we define the atom as the smallest
particle of an element that retains the chemical properties of that
element. All atoms consist of two regions: The nucleus is a very
small region located near the center of an atom and contains
positively charged particles called protons.
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Surrounding the nucleus is a region occupied by negatively
charged particles called electrons. This region is very large
compared to the size of the nucleus. Protons, electrons and
neutrons are called subatomic particles.
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The Electron
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The Nucleus
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Atomic Nucleus Except for the hydrogen atom, all atomic nuclei
are made up of two kinds of particles, protons and neutrons. A
proton has a positive charge equal in magnitude to the negative
charge of an electron. Atoms are electrically neutral. A neutron
which has no charge.
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Atomic Number Atomic Number (Z) represents the number of
protons in the nucleus of each atom of that element. Atoms of the
same element have the same number of protons. Atoms of different
elements have different number of protons.
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Atomic Number On the periodic table, the elements are placed in
order of increasing atomic number. At the top left of the table is
hydrogen, H, which has an atomic number of 1. All atoms of the
element hydrogen have one proton in the nucleus.
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Next is helium, which has two protons, and therefore an atomic
number of two. This is followed by lithium with an atomic number of
three three proton in the nucleus. The atomic number for each
element is listed on the periodic table above the elements
symbol.
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The atomic number identifies an element. Because all atoms are
neutral (no charge), we know the atomic number is equal to the
number of electrons. # of protons = # of electrons Example: Silver
(Ag) has an atomic number of 47. Therefore, silver has 47 protons
and 47 electrons in each silver atom.
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Isotopes The simplest atoms are those of hydrogen. All hydrogen
atoms contain one proton. However, hydrogen atoms can contain
different number of neutrons. Three types of hydrogen atoms are
known with a neutron count of 0, 1 or 2.
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Isotopes The most common type of hydrogen atom is called
protium. It accounts for 99.985% of the hydrogen atoms found on
earth. The nucleus of protium consist of one proton and zero
neutrons.
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Isotopes Another type of hydrogen atom is deuterium. This
accounts for 0.015% of hydrogen atoms. The nucleus of deuterium
consist of one proton and one neutron.
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Isotopes The third type of hydrogen atom is tritium. It exists
in very small amounts in nature. The nucleus of tritium consist of
one proton and two neutrons.
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Isotopes Isotopes are atoms of the same element that have
different masses. Protium, deuterium and tritium are all isotopes
of hydrogen. The isotopes of an element all have the same number of
protons and electrons but different number of neutrons.
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Mass Number Mass Number represents the total number of protons
and neutrons in the nucleus of an isotope. Identifying an isotope
requires knowing both the atomic number and mass number of the
element.
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Mass Number Calculating the mass number: Mass number = atomic
number + number of neutrons Tritium: Atomic # = 1 (1 proton) # of
neutrons = 2 Mass number = 3
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Mass Numbers for Hydrogen
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Homework Worksheet: Atomic Number (B7)
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Isotopes Isotopes of an element are usually identified by
specifying their mass number. There are two common methods: 1)The
mass number is written with a hyphen after the name of the element.
hydrogen-3 represents tritium
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Isotopes 2) The second method shows the elements symbol written
with the atomic and mass numbers. hydrogen-3 represents tritium is
written as: H 3 1
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Isotopes Example: Uranium-235 contains 92 protons and 143
neutrons. Mass number = atomic number + number of neutrons 235
(protons + neutrons) = 92 protons + 143 neutrons
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Isotopes Examples of hydrogen and helium:
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Sample Problem How many protons, electrons and neutrons are
there in an atom of chlorine-37?