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Atoms
Building Blocks of Matter
Beginning of Atomic Theory
• What Is an Element? Around 440 BCE, a
Greek philosopher named Democritus
thought that you would eventually end up
with a particle that could not be cut. He
called this particle an atom.
Philosophers Debate:
• Aristotle, another Greek philosopher,
disagreed with Democritus’s ideas. He
believed that you would never end up with
a particle that could not be cut. Democritus
was right, though: Matter is made of
particles, which we call atoms. An atom is
the smallest particle into which an element
can be divided and still be the same
substance.
John Dalton: A new view
Dalton’s Theory
• Dalton’s Theory John Dalton published his
atomic theory in 1803. His theory stated that all
substances are made of atoms. Atoms are small
particles that cannot be created, divided, or
destroyed. Atoms of the same element are
exactly alike, and atoms of different elements
are different. Atoms join with other atoms to
make new substances.
• Not Quite Correct The atomic theory was then
changed to describe the atom more correctly.
J.J. Thompson:
• Negatively Charged Particles Thomson
experimented with a cathode-ray tube like the
one shown on the next slide. He discovered
negatively charged particles that are now known
as electrons.
• Like Plums in Pudding After learning that
atoms contain electrons, Thomson proposed a
new model of the atom. Thomson thought that
electrons were mixed throughout an atom, like
plums in a pudding.
Ernest Rutherford
• Negatively Charged Particles In 1909, Ernest
Rutherford aimed a beam of small, positively
charged particles at a thin sheet of gold foil. The
next slide shows his experiment.
• Surprising Results Rutherford expected the
particles to pass right through the gold in a
straight line. To Rutherford’s great surprise,
some of the particles were deflected.
Where are the Electrons?
• Far from the Nucleus Rutherford proposed that in the center of the
atom is a tiny, positively charged part called the nucleus.
•
Bohr’s Electron Levels In 1913, Niels Bohr proposed that
electrons move around the nucleus in certain paths, or energy
levels.
•
The Modern Atomic Theory According to the current theory, there
are regions inside the atom where electrons are likely to found.
What are atoms?
• The Nucleus Protons are positively charged
particles in the nucleus. Neutrons are the
particles of the nucleus that have no electrical
charge.
• Outside the Nucleus Electrons are the
negatively charged particles in atoms. Electrons
are found around the nucleus within electron
clouds. All the structures of the atom can be
seen on the next slide.
Modern Atom: Electron Cloud Model
Electron energy level groups:
• Each “level” of the electron cloud in an atom has a set of
orbitals.
• The orbitals are three-dimensional probability “shapes” in
which electrons from that set may be found.
• The number of orbitals increases as electrons are farther
away from nucleus (they have more room there…)
• The orbitals are assigned letters: s= “sharp”, p=
“principal”, d= “diffuse”, and f= “fundamental”.
(corresponds to the azimuthal quantum number…”l”)
• The maximum number of electrons possible within an
orbital increases from s to f. s=2, p=8, d=16, f=32
s orbitals
p orbitals
d orbitals
f orbitals
How do Atoms of Elements Differ?
• Starting Simply The hydrogen atom has one proton and one
electron.
• Now for Some Neutrons The helium atom has two protons, two
neutrons, and two electrons.
•
Building Bigger Atoms For bigger atoms, simply add protons,
neutrons, and electrons.
• Protons and Atomic Number The number of protons in the
nucleus of an atom is the atomic number of that atom. All atoms of
an element have the same atomic number.
How many of each particle?
• Atomic number = the number of protons in an
atom
• Atomic mass (mass number) = the average
mass of the nucleus from all of the isotopes of a
certain element. (remember that the mass of the
electron is very small)
• To find the number of neutrons in any typical
atom, subtract the atomic number from the
atomic mass.
• Remember that in a neutral atom, the number of
electrons is equal to the number of protons.
• Link to TERC atom builder.
Isotopes
• Isotopes—atoms of the same element with different
numbers of neutrons.
• Properties of Isotopes Some isotopes are radioactive.
•
Telling Isotopes Apart You can identify each isotope of
an element by its mass number.
•
Naming Isotopes To identify a specific isotope of an
element, write the name of the element followed by a
hyphen and the mass number of the isotope.
•
Calculating the Mass of an Element The atomic mass
of an element is the weighted average of the masses of
all the naturally occurring isotopes of that element.
Hydrogen Isotopes:
Various atoms and isotopes:
Forces in Atoms:
• Four Basic Forces Four basic forces are
at work everywhere, even within the atom.
These forces are gravitational force,
electromagnetic force, strong force, and
weak force. These forces work together to
give an atom its structure and properties.