CHAPTER 4 ATOMIC
STRUCTURE
• Aristotle (Greek)
• Thought all substances were built from
either fire, earth, air, water
• Thought that atoms of a liquid are smooth
and round
• Thought atoms of a solid were rough and
prickly
• Dalton’s Atomic Theory (1766)
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Evidence for atoms
Measured the masses of elements
Compounds have fixed composition
All matter is made up of individual particles
called atoms, which cannot be divided
• 1 All elements are composed of atoms
• 2 All atoms of the same element have the same
mass, and atoms of different elements have
different masses
• 3 compounds contain atoms of more than 1
element
• 4 in a particular compound, atoms of different
elements always combine the same way
• Evidence for subatomic particles
– Mass and charge are spread throughout the
atom
– Earnest Rutherford- (1871-1937)
• Alpha particles---fast moving positive charged
• All the atoms positive charge is in the nucleus
• The nucleus is the positively charged mass located
in the center of the atom
• Ques.1-5 pg. 105
SECTION 4-2
• Properties of subatomic particles
• Protons-- + charged particles, varies among
elements
• Each nucleus contains at least one
positively charged particle
• +charge = proton
• Electron
• - charge 1• Found outside the nucleus in an electron
cloud or (shell)
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Neutron
No charge
Found in the nucleus
Mass almost = to a proton
Protons, electrons and neutrons can be
distinguished by mass, location in an atom
and the charge
ATOMIC NUMBER AD MASS
NUMBER
• The atoms of any element contain the same
number of protons
• Example there is one proton in the
nucleus of every hydrogen atom. There
hydrogen has an atomic number of 1
• ATOMIC NUMBER = NUMBER OF
PROTONS
• Atoms of different elements have different
numbers of protons
• Sulfur= atomic #= 16 because it has 16
protons in the nucleus
• Each positive charge is balanced by a
negative charge SO hydrogen has 1 proton
and 1 electron sulfur 16 protons and 16
electrons
• MASS NUMBER= the sum of the protons
and neutrons in the nucleus of the atom
• Ex Aluminum= 13 protons+14 neutrons= an
atomic mass of 27
• Number of neutrons= mass#-atomic number
• Ex. Aluminum 27-13= 14 (#of neutrons)
ISOTOPES
• When an atom does not have the same
number of neutrons
• Same atomic number but different mass #’s
• Ex. Oxygen-16, 17, and 18
• All oxygen atoms have 8 protons, but some
have 9 or 10 neutrons
• Ques. 1-7 pg. 112
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Energy levels
1 one orbital max 2 electrons
2 four orbitals max 8 electrons
3 nine orbitals max 18 electrons
4 sixteen orbitals max 32 electrons
• Electron configuration—arraignment of
electrons in the orbitals of an atom
• Lowest levels get filled before higher
energy levels----inner to outer
• Stable electron configuration is the one in
which the electrons are in orbitals with the
lowest possible energies (ground state)
• Ques. 1-5 pg. 118
4-3 Modern Atomic Theory
• Niels Bohr-1885-1962 Danish physicist
focused on electrons in the electron cloud
• Energy levels
• Each electron has a fixed amount of energy
• Electrons closer to the nucleus have less
energy than further away from the nucleus
• They move in a less than predicable way
• Atomic orbitals
– The electron cloud represents all the orbitals in
an atom
– An orbital is a region of space around the
nucleus figure 15 page 117 (copy into notes)
– Draw in your notes the Thomson, Rutherford,
Bohr, and electron cloud models (pg 115)
• An electron in an atom can move between
energy levels when the atoms gain or lose
energy
• The size of the jump between orbitals
determines the amount of energy gained or
lost
• Evidence for movement from one level to
another is light---light is a form of energy----heat is another cause of movement of
electrons from one orbital to another
• Electron cloud-how electrons behave in
their orbitals
• Electrons move like planets in the solar system
around the nucleus, they move in a less than
predicable way
• When electrons move to a higher energy
level they are considered in an excited state
• Ex. He, Ne, Ar Kr, Xe find out the names
of these elements
• Page 118 questions 1-5