Chemistry began with Aristotle's idea that all matter was made of four elements. Democritus later suggested that matter was made of indivisible particles called atoms. In the 1800s, John Dalton developed the atomic theory stating that all matter is made of atoms that are identical within an element. At the turn of the 20th century, scientists discovered atoms are made of even smaller parts including positively charged protons and negatively charged electrons. Rutherford discovered the nucleus at the center of the atom with electrons orbiting outside. Bohr then proposed electrons orbit in distinct energy levels like planets around the sun. Isotopes with different neutron numbers but the same element were later identified.

1. WHAT IS CHEMISTRY

2. Aristotle suggested that
everything was composed
of
4 elements: (4 Elements Theory)
Water, Air, Earth and Fire

3. DEMOCRITUS—400 BC
 suggested that matter was
composed of atoms, which
he called “atomos” meaning
“indivisible.”

4. In the dark ages,
the idea of atoms
was frowned
upon. Not much
progress was
made.
ATOMS?
What’s that?
OFF with your
HEAD!

5. In the early
1800’s, John
Dalton came
up with the
ATOMIC
THEORY.

6. Dalton’s Atomic Theory
1) All matter is made of atoms. Atoms
are indivisible and indestructible.
2) All atoms of a given element are
identical in mass, size, shape &
structure
3) Compounds are formed by a
combination of two or more different
kinds of atoms.
4) A chemical reaction is a
rearrangement of atoms.

7. Compounds
are composed
of little “balls”
called atoms,
joined together
by “bonds” to
form
molecules.

8. Through a series of
experiments around the start
of the twentieth century,
scientists discovered that
atoms are made of even
smaller parts.

9. Eugene Goldstein discovered
the proton in 1886.
The proton is positively charged
and determines the identity of an
element.

10. JOSEPH JOHN THOMSON
discovered the electron in 1897
Electrons are negatively charged
particle and have almost no mass
at all, compared to a proton.

11. He described an atom
that consisted of
negative charges
scattered throughout a
ball of positive charge—
something like raisins
or berries in a muffin.

12. • THOMSON‘S MODEL SUGGESTED
THAT ATOMS HAD NEGATIVELY
CHARGED ELECTRONS EMBEDDED IN
A POSITIVE SPHERE.
• COMPARING AND CONTRASTING HOW
IS THOMSON’S MODEL DIFFERENT
FROM DALTON’S?

13. Thomson’s Plum
Pudding Model

14. In 1911, one of
Thomson’s students,
Ernest Rutherford
discovered nucleus, a
small region at the
center of the atom

15. Rutherford’s Scattering Experiment

16. Rutherford’s Model
 an atom is mostly an empty
space.
 a small positively charged
nucleus is found at the center
of the atom
 electrons are found outside the
nucleus

19. Rutherford could not explain why the
electron didn’t fall into the nucleus and
destroy the atom.
?

20. Neils Bohr
I think I
can help!

21. In 1913, Neil's Bohr, a Danish
scientist and a student of both
Thomson and Rutherford, revised
the atomic model again.
The series of orbits in Bohr’s model
resemble planets orbiting the sun

22. According to Bohr, each energy “level”
corresponded to a different “orbit” of an
electron around the atom. (Like planets
around the sun.)
This model
is really
cool !!

23. In the 1920’s things changed!
Although Bohr’s idea of energy levels
was still accepted, his idea of planetary
orbits for electrons was rejected!
REJECT ! !

27. Frederick Soddy (1877-1956)
proposed the idea of isotopes
Isotopes are atoms of the
same element having different
masses due to varying numbers
of neutrons.
Soddy won the Nobel Prize in
Chemistry in 1921 for his work with
isotopes and radioactive materials