1. The Atom

2. a quick note before we start…
- we have never actually seen inside an atom
(even with the world’s best microscopes)
- therefore, we must use models to explain it

3. Indirect Evidence-
Evidence you can get without actually seeing or touching an object

4. Early Models
a.) 2,500 years ago- Ancient Greeks
believed all matter was made up of four
elements:
fire
water
earth
air

6. - Democritus (Greek philosopher)
- first used the term “atom”
(atomos- “indivisible”)
believed that if you kept dividing matter
in half, you’d eventually get down to an
object that couldn’t be divided anymore

7. his theory was not widely accepted, and the idea of atoms was largely forgotten until…

8. Dalton’s Atomic Theory (1803)
b.) Early 1800’s- John Dalton
did a number of experiments
that led to the acceptance of
the idea of atoms
believed (like Democritus) that the atom
was indivisible
- came up with
Dalton’s Atomic Theory (1803)

9. 1. All elements are composed of atoms.
Atoms are indivisible and indestructible
2. Atoms of the same element are exactly
alike
3. Atoms of different elements are different
4. Compounds are formed by the joining of
atoms of two or more different elements

10. note…
1.) Dalton never tried to describe what
an atom looked like
2.) NOBODY had any idea what an
atom looked like for almost another
100 years, until…

11. c.) 1897- J.J. Thomson - disproved Dalton’s idea that
the atom was indivisible
- discovered that an atom had positive and
negative electrical charges
first person to attempt to
create a model of the atom

12. - thought that the atom had negatively-
charged particles
in a positively-charged “pudding”
(he couldn’t find any positively-charged particles) - - - - - - -
- sometimes called the “plum pudding model”

13. negatively-charged
particles - - - - - - - - -
positively-charged “pudding”

14. d.) 1908- Ernest Rutherford - found the positively-charged
particles that Thomson could
not
- guessed that the atom had a small, dense,
positively-charged center that he called the
nucleus
- also named the negatively-charged particles
electrons, and guessed that they were
scattered around the nucleus

15. - his model:
atom
nucleus
electrons

16. e.) - 1913- Niels Bohr - improved on Rutherford’s model
- placed the electrons in specific orbits or
“shells” around the nucleus
- his model:

17. shells
nucleus
electrons

18. f.) Wave Model
- currently used model
- improved on Bohr’s model
- states that the electrons remain at a
certain distance from the nucleus,
but follow no specific path

20. - electrons are moving so fast that they
appear as a cloud around the nucleus
electron cloud
model-
nucleus

21. Atomic Masses - 99.9% of the mass of an atom is in the nucleus
- because the masses of atoms are so small,
scientists had to invent a unit of measure
just for atoms:
the Atomic Mass Unit (AMU)
- 1 gram = =
600,
000
,000
,000
,000
,000
,000
,000 amu’s
(6x1023)

22. Subatomic Particles - we currently know of the existence of
over 200 subatomic particles
- the three most important are
protons,
neutrons,
and electrons

23. 1.) Protons - positively charged (+) - found in the nucleus
- are made up of particles called quarks
- are identical to all other protons, no
matter what element they are in
- have a mass of 1 amu

24. 2.) Neutrons - have no electric charge - found in the nucleus
- are also made up of quarks
- are identical to all other neutrons, no
matter what element they are in
- have a mass of 1 amu

25. 3.) Electrons - negatively charged (-) - orbit the nucleus
- have a mass of 1/1836 amu
- in a “neutral” atom, the number of
protons and the number of
electrons are the same

26. - electrons move around the nucleus
billions of times each second in what is
called an electron cloud
the distance of an electron from the
nucleus depends on how much energy
the electron has:
more energy →
farther away
less energy →
closer

27. more energy
less energy

28. - the electrons are then arranged in energy
levels or shells
- each shell surrounding the nucleus can
only hold a limited number of electrons:
1st shell-
2 electrons
2nd shell-
8 electrons
3rd shell-
8 electrons
and so on…

29. - the chemical properties of an element
depend on how many electrons are in the
various shells
- according to modern atomic theory,
electrons can move from one shell to
another

31. Things that are determined by an atom’s protons, neutrons, and electrons…
a.) atomic number
the number of protons in the nucleus
of an atom
every element has a unique atomic
number

32. the number of protons and neutrons in the nucleus
(note: you can change the number of neutrons and electrons in an atom and still have the same element, but if you change the number of protons you change the element)
b.) mass number
the number of protons and neutrons
in the nucleus
example:
carbon
number of protons- 6
usual number of neutrons- 6
mass number= 12

33. c.) isotope
two (or more) atoms of the same
element which have the same
number of protons (obviously!),
but different numbers of neutrons
example:
carbon (again)

34. Sample A:
number of protons- 6
number of neutrons- 6
this type of carbon atom is called C12 or carbon-12

35. Sample B:
number of protons- 6
number of neutrons- 8
this type of carbon atom is called C14 or carbon-14

36. C12 and C14 are both carbon atoms, but are isotopes because they have different numbers of neutrons

37. d.) atomic mass
the average mass number of all known
isotopes of a particular element
is also influenced by the relative
percentages of each isotope
example:
carbon (yet again)

38. 950 carbon-12 atoms (mass number- 12)
in a sample of 1,000 carbon atoms you might find the following-
950 carbon-12 atoms (mass number- 12)
30 carbon-13 atoms (mass number- 13)
20 carbon-14 atoms (mass number- 14)
to average them,
find the total number of protons and neutrons (the total mass),
and divide it by the number of atoms:

39. 950 atoms x 12 =
11,400 protons + neutrons
30 atoms x 13 =
390 p + n
20 atoms x 14 =
280 p + n
total-
12,070 p + n
12,070 (p +n)
÷ 1,000 atoms =
12.07
(the atomic mass of carbon)

41. Last term…
Ion-
an atom where you have changed the number of electrons (more on this later)
In Summary…
If you take an atom and change the number of
protons,
you get a whole new element
neutrons,
you get an isotope
electrons,
you get an ion