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The Atom
I. Early ideasA. ___________ (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) English1. 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 ExperimentsA. ____________________ – 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 PROTON1. 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- radioactivity1. 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 deflectionsd) 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. Nucleus1. + Charged2. Composed of p+ (+) and n (no chg)
B. Electron Cloud1. e- travel around nucleus in discrete
energy levels2. In a neutral atom, the # e- = the # p +
IV. Atomic Numbers
A. Discovered by Henry Moseley1. 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 e-
C. 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 neutronsEx: 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.