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STUDYING ATOMS
Ancient Greeks
 Ancient Greeks believed that all matter
consisted of extremely small particles that
could not be divided.
 Atoms comes from the Greek word atomos,
which means uncut or indivisible.
Dalton’s Atomic Theory
 Dalton proposed the theory that all matter is
made up of individual particles called atoms,
which cannot be divided.
Dalton’s Atomic Theory
 Main points:
 All elements are composed of atoms
 All atoms of the same element have the same
mass
 Compounds contain atoms of more than one
element
 Atoms of different elements always combine in
the same way
Dalton’s Atomic Theory
Thomson’s Model
 Thomson used a cathode ray tube to discover
electrons.
 It is called the plum pudding model.
 Protons had already been discovered by this
time.
Thomson’s Atomic Theory
Rutherford’s Atomic Theory
• Rutherford used the Gold Foil Experiment to
discover the nucleus.
• The nucleus is a dense, positively charged
mass located in the center of the atom.
• According to Rutherford’s model, all of an
atom’s positive charge is concentrated in its
nucleus.
Rutherford’s Model
STRUCTURE OF AN ATOM
Structure of an Atom
• Protons, electrons, and neutrons are
subatomic particles.
• Protons are positive and weigh 1 amu.
• Neutrons are neutral and weigh 1 amu.
• Electrons are negative and weigh 1/1836 amu.
Structure of an Atom
 Protons and neutrons are large and in the
nucleus of an atom.
 Electrons are small and are outside the
nucleus.
Atomic Number
 The atomic number of an element equals the
number of protons and electrons in an atom
of that element.
 Atoms of different elements have different
numbers of protons.
Mass Number
 The mass number of an atom is the sum of
the protons and neutrons.
 To find neutrons:
 Mass number – Atomic number = # of
neutrons
Isotopes
 Isotopes of an element have the same atomic
number but different mass numbers because
they have different numbers of neutrons.
 EX: O-16 and O-17
MODERN ATOMIC THEORY
Bohr’s Model
 Bohr focused on energy levels.
 Energy levels are the possible energies that
electrons can have in an atom.
 An electron in an atom can move from one
energy level to another when the atom gains
or loses energy.
Bohr’s Model
Energy Levels, Orbitals, and
Electrons
Energy Level
Max # of electrons
1
2
2
8
3
18
4
32
Electron Configurations
 The most stable electron configuration is the
one in which the electrons are in orbitals with
the lowest possible energies.
 The ground state is the lowest possible
energy where the atom is most stable.
Electron Cloud Model
• An electron cloud is a visual model of the
most likely locations for electrons in an atom.
• The cloud is denser where the probability of
finding an electron is high.
• Scientists use the electron cloud model to
describe the possible locations of electrons
around the nucleus.
Electron Cloud
 The electron cloud represents all the orbitals
in an atom.
 An orbital is a region of space around the
nucleus where an electron is likely to be
found.
Electron Cloud Model