Transcript Chapter 3 - SharpSchool

Chapter 3
Chemistry
The Atom
I. Early ideas

A. ___________ (450 BC)- all matter is
made of tiny indivisible particles – atomos.


_________- the smallest particle of an element
that retains the properties of the element.
B. ____________________- (late 1700’s)

1. Law of Conservation of Matter
Early Ideas Cont:
C. __________- (1799) – Law of Constant
Composition

Atoms form cmpds in specific ratios
determined by mass.
Early Ideas Cont:
D. _________________ (1803) English

1. Atomic Theory of Matter
a.) Each element is composed of atoms
(Democritus)
 b.) All atoms of an element are the same;
atoms of different elements are different
 c.) Atoms are not created or destroyed in rxns.
(Lavoisier)
 d.) Cmpds have the same relative # and kinds
of atoms. (Proust)

Early Ideas Cont:
E. __________________ (late 1700’s)

a) electric charges: like repel; opposites
attract
F. ____________(1839) - atoms
contain particles with electric charges
G. ____________________ (1870’s) –

a. Crookes’ Tube exp.
Early Ideas Cont:
H. _______(1896) – Cathode ray tubes
I. _____________ (1896) – radioactivity
J. ________________ (1909) – Oil
Drop Exp.
K. ___________________ (1909) –
Gold Foil Exp. (Alpha scattering exp.)
II. Early Experiments
A. ____________________ – key experiment

1. Results – two kinds of charge: + & - like
charges repel; opp. attract
B. Crookes’ Tube Experiment


1. Partially evacuated tube filled w/a low pressure
gas.
2. 2 electrodes inside – anode (+) cathode (-)
Early Exp. Cont:

3. When voltage was applied, a green glow
originated in the tube.


a) produced a shadow at the end opposite the cathode;
 it had to originate at the cathode CATHODE RAY; the
tube was then called a CATHODE RAY TUBE.
4. Was it light or charged particles??



a) Beam was deflected by a magnet,  it had to be a
stream of charged particles.
www.chem.uidaho.edu/~honors/crookes.html
http//library.thinkquest.org/28582/history/dalton.htm
Early Exp. Cont:
C. Thomson’s Exp.

1. Build a CRT with a fluorescent screen at
end opposite the cathode.


Glowed where particles struck.
2. Added a magnet and a pair of charged
plates.

a) the beam was 1st deflected by the magnet,
but was attracted to the + charged plate;
therefore the particles had to have a – charge.
Early Exp. Cont:

3. Changing the electrodes and the gases
used always yielded the same results; 
the particles were identical and were found
in all atoms. These particles were called
_______________________.
Early Exp. Cont:
D. The PROTON
1. If atoms had – particles, they also had to
have + particles, because atoms were
electrically neutral.
 2. Discovered through exp. w/CRTs.
 3. 1885- Thomson used H gas in a CRT.
When voltage was applied, the ray moved
to the cathode (-); therefore it had to be
positive.

Early Exp. Cont:
4. The amount of deflection varied with the
different gases.
 5. _____________________- electrons
were randomly distributed throughout a +
charged “gel”.

Early Exp. Cont:
E. Becquerel’s discovery- __________
1. Placed a piece of uranium on a
photographic plate and placed it in a dark
drawer.
 2. Later found the plate had been exposed
by some unknown type of radiation
originating from the U.

Early Exp. Cont:
F. Ernest Rutherford- radioactivity

1. Through exp. determined the 3 types of
radiation
G. Ernest Rutherford (1909)
1. Gold Foil Exp. (alpha-scattering Exp.)
a) Aimed a beam of high speed α particles (+)
at a very thin piece of gold foil.
 b) Almost all particles passed through

Early Exp. Cont:
c) Some particles had slight deflections
 d) Very few particles reversed direction and
came straight back.
 e) Results: All + charge is concentrated at the
center of the atom – __________________.


1. Credited with the
___________________________ of the ATOM.
III. Atomic Structure
A. Nucleus
1. + Charged
 2. Composed of p+ (+) and n (no chg)

B. Electron Cloud
1. e- travel around nucleus in discrete
energy levels
 2. In a neutral atom, the # e- = the # p +

IV. Atomic Numbers
A. Discovered by Henry Moseley

1. Atoms of @ element have a unique +
chg. on the nucleus
B. The identity of the element depends
on the # p+ in nucleus
ATOMIC NUMBER
V. Ions
A. Ions- an electrically charged atom
B. Formed when atoms gain or lose e-;
or share eC. ATOMS NEVER LOSE PROTONS
D. If gain e- = - chg; If lose e- = + chg
Isotopes
________________ – atoms of the
same element with the same number of
protons but different numbers of
neutrons
Ex: H-1, H-2, H-2

C-12, C-13, C-14

Atomic Mass
Atoms are too small to see individually
They are also too small to mass with
normal everyday lab balances
So, how is the mass of a SINGLE atom
measured????
Atomic Mass
Atomic Mass Unit – the mass of a single
proton or neutron
Protons and neutrons have essentially
the same mass – they make up 99% of
the mass of the atom
Therefore, if we add those numbers up
we get the mass of a single atom
Atomic Mass
Scientists chose to define an atomic
mass unit in terms of an arbitrary
standard – the C-12 atom
1 atom of C-12 is exactly 12 amu
So, why are the atomic masses listed on
the periodic table not whole numbers?
Atomic Mass
_____________________ – the
fractional occurrence in nature of a
specific isotope of an element
_________________________ – the
weighted average of all the isotopes of
an element.
Atomic Mass
Example: