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7.1 (and a bit of 7.2)
Transcript of 7.1 (and a bit of 7.2)
The Development of a Modern Atomic Theory
Today's learning objectives:
We will learn how was our current view of the atom developed.
Over 2000 years ago...
Aristotle believed that anything could be divided infinitely and that there was NO smallest particle.
Around the same time another guy named
came along. He disagreed with Aristotle and said that if you kept dividing something eventually you would get to the smallest particle which he called "atomos" which means indivisible!
Flash forward almost 2000 years
J.D. (John Dalton)
By this time most scientists agreed that an element could not be broken down into simpler substances.
In 1808, John Dalton proposed an atomic theory that would explain how elements could combine to form compounds.
He described the atom, the smallest piece of an element as a smooth solid sphere, with no elecrical charge.
All matter is made up of atoms, which are too small to see.
Each element has its own kind of atom, with its own particular mass.
All atoms of any one element are identical.
Compounds are created when atoms of different elements combine in a specific ratio.
Atoms cannot be created, destroyed or subdivided during chemical changes.
In the 1830s, Michael Faraday showed that atoms could gain electrical charges (like when we get shocked by static electricity!) but Dalton's theory didn't account for this.
Faraday making discoveries about electricity.
So, Dalton's theory had to be revised.
Revisions to Dalton's theory
Matter must contain positive and negative charges.
Opposite charges attract, and like charges repel.
Atoms combine to form the particles of a compound because of the electrical attraction between charged atoms.
POSITIVE AND NEGATIVE CHARGES
The new theory explained how compounds were formed but no one knew how an atom became charged.
In the late 1800s J.J. Thomson discovered the negatively-charged electron.
He thought that the atom was mostly made up of positively charged matter with small negatively charged electrons scattered randomly. This was called the "raisin bun" model.
Thomson's Revision to the atomic theory
Atoms contain electrons.
The electrons have a negative charge and a very small mass.
The rest of the atom has a positive charge.
The electrons are embedded randomly in the positive part of the atom.
Electrons can be removed from, or added to, atoms to create charged atoms.
Rutherford decided to one day just bombard a thin piece of gold with small positively charged particles.
According to Thomson's model most of the particles should have been deflected slightly by the gold atoms.
But he was shocked to discover that most
of the particles passed straight through and
a few particles even passed straight back.
He thought about this for almost 2 years
before coming up with his revised atomic
I don't look it, but I am shocked!
Rutherford's Revisions to the atomic theory
The nucleus contains all of the positive charge and most of the mass of the atom.
The nucleus contains postively charged protons and uncharged neutrons.
Neutrons have the same mass as protons.
The nucleus is very small, compared with the size of the atom.
The electrons orbit the nucleus, like satellites around a planet.
The mass of an electron is 1/1800 the mass of a proton.
The size of the atom is determined by the size of the orbit of the electrons.
There is only empty space between the electrons and the nucleus.
But Rutherford already knew his theory wasn't perfect.
A theory of electromagnetic waves predicted that the orbiting electrons should emit energy which his model could not account for....
I don't look it, but I am frustrated!
Another revision had to be made...
When atoms absorb electrical or heat energy they can re-emit this energy as light.
Think of a neon light
Bohr realized he had to
make a atomic model that
could show how an atom
could absorb and emit energy.
His revised Rutherford's model with a model that included different
Bohr's Revisions to the atomic theory
Electrons are located in defined shells, which are located certain distances from the nucleus.
Electrons cannot exist between the defined shells.
Electrons can gain energy to move up to a higher shell or they can lose energy to move down to a lower shell.
Electrons are more stable (have less energy) when they are closer to the nucleus.
Atoms only emit or absorb energy when their electrons "jump" between shells.
John Dalton thinks of
Faraday's discoveries show
JJ Thomson discovers
and adds the electron.
Rutherford adds the nucleus
Bohr realizes energy is
emitted and absorbed through
Lots to remember?
Want more info?
Lots of info. Goes past Bohr and adds some new revisions!
Basic explanation up to Rutherford.