Structure of atoms10
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Transcript of Structure of atoms10
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Structure of Atoms
The History and Models of Atoms
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Studying Atoms
• Scientists have been studying atoms since the 1800s.
• If atoms are too small to be seen, how do we study them?
• Scientists use models to study atoms.• Sometimes scientists can tell what things look
like by studying how they act.– Example: Wind
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Models
• In science, a model is an idea, a picture, or an object that is built to explain how something else looks or works.
• The model might not look exactly like the object it is built to describe, but it helps people understand the way the object acts.
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History of the Atom
• Democritus, a Greek philosopher, first used the term atom (atomos) around 400 BC.
• John Dalton showed that materials were made of atoms in 1803.
• In the late 1800s we discovered that the atom has many smaller parts.
• In the late 1800s, English physicist J. J. Thomson discovered the electron.
ThomsonDalton
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More History of the Atom
• In 1901 Ernest Rutherford found discovered the nucleus, or center of the atom.
• In 1917 Niels Bohr, working as Ernest Rutherford’s student, discovered how electrons orbit the nucleus.– His model of the atom is the current base for the
modern atomic model.
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Atomic Mass Unit
• Subatomic Particles are so small that a special unit was devised to measure them.
• This is the atomic mass unit (amu).
• Atoms are almost entirely empty space.• If the nucleus were the size of a basketball,
the first electrons would be somewhere near Springfield.
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Model of an AtomThe central part of the atom is called the nucleus.
The nucleus contains positively charged protons that weigh 1 atomic mass unit.
The nucleus also contains neutrons that have no charge and weigh 1 atomic mass unit.
Electrons are extremely small units of an atom that have a negative charge and move around the atom in orbits.
Protons and electrons stay together because they attract each other.
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Model of an Atom
Neutron
Electron
Proton
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Nucleus
• The nucleus is where the protons and neutrons are located within the atom.
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Atomic Number
• The atomic number of an atom is the number of protons it has.
• If an atom has 6 protons, it is carbon.– It does not matter what else it has, if it has
6 protons then it is carbon.
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Atomic Number
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Practice Number of Protons
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Electron Cloud
• The electron cloud is the region around the nucleus occupied by electrons.
• Atoms are much, much denser near the nucleus.
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Electrons
• Electrons are also arranged in shells.• The first shell can hold a maximum of 2
electrons. All other shells can hold up to 8 electrons.
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Valence Electrons
• The outermost shell of an atom is called the valence shell.
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Protons and Electrons
• In an ATOM the number of electrons should always equal the number of protons.
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Isotopes
• An Isotope is when an atom has a different number of protons and neutrons.
• Most elements have many different isotopes. • This makes the nucleus unstable or radioactive
in many cases.
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Writing Isotopes
• To describe an atom, write the name of the atom and then write the total number of protons and neutrons that are found in it’s nucleus.
• The total number of protons and neutrons in the nucleus is called the Mass Number.
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Isotopes and Mass Number
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Practice IsotopesName Atomic # Protons Neutrons Mass Number Full Name
Carbon 6 8
Hydrogen 1 0
Lithium 3 2
Aluminum-26
8 17
Fluorine 20
12 13
Sulfur 34
2 2
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Abbreviating Mass Number
• To abbreviate mass number, write the symbol of the atom and then to the upper left write the mass number and under that, write the atomic number.