Unit IIAtomic Structure

Obj. 1…Dalton’s Atomic Theory• Four postulates (1808)...

2. Atoms of the same element are identical...each element is unique

* proton, neutron and

electron

isotopes exist

*

• Much of this theory is still accepted, with 2 exceptions

3. Atoms of different elements can physically mix orchemically combine (compounds).

1. All elements are composed of tiny, indivisible particlescalled ‘atoms’.

4. Chemical rxns. occur when atoms areseparated, joined, or rearranged.

Obj. 2…Atomic Scientists• J. J. Thomson discovered electrons (e-) in

1897.• He passed an electric current through a

glasstube filled with gas.

• He discovered that a beam of negative chargestraveled from the cathode (-) to the anode (+).

Obj. 2 cont...• E. Goldstein discovered positive particles in

atomsin 1886.

• He observed rays traveling in the opposite direction ofcathode rays.

• These particles were termed ‘ protons’ (p+) by ErnestRutherford in 1920.

• His discovery was based on the fact thatdifferent atoms’ atomic mass and atomic #(# of protons) were not adding up.

• James Chaddwick discovered neutrons (n0)in 1932.

Obj. 2 cont...

• all e- carry exactly one unit of negative charge.

• mass of an e- is 1/1840 the mass of a hydrogen atom (p+).

• Robert Millikan furthered Thomson’s work by describingthe electron more in depth.

Obj. 3 and 4…The Bohr-Rutherford Model

1. atoms are mostly space. (football arena)

• Ernest Rutherford conducted the famous ‘gold foilexperiment’ (1911) which concluded that:

2. atoms have a solid nucleus at thecenter which contains most of the mass.

• This overturned the accepted ‘plum pudding model’ of thetime.

Obj. 3 and 4 cont...• The Gold Foil Experiment...

• Stats...• 98% of particles went straight through (expected)

• 2% of particles deflected at large angles• 0.01% of particles deflected straight back (canon

balls & tissue!)

(BBs and cream cheese!)

Obj. 3 and 4 cont...

• The progression of the atom...

• Why don’t the (-) e- fall into the (+) nucleus?

Plum Pudding Rutherford Rutherford-

BohrQuantum

• After Rutherford’s findings, Niels Bohr further explainedthe atom by concentrating on the e- (1913).

• Bohr proposed that the e- travel on concentricorbits around the nucleus.

• Each orbit has a fixed energy (energy level) and e- do notlose energy.

Obj. 5…Subatomic Particles

PROPERTIES OF SUBATOMIC PARTICLES

PARTICLE SYMBOL ELECTRICAL CHARGE

RELATIVE MASS

ACTUAL MASS

(g)

Electron

Proton

Neutron

e-

p+

n0

-1

+1

0

1/1840

1

1

9.11 x 10-

28

1.67 x 10-

24

1.67 x 10-24

Obj. 6…Characteristics of Elements

K19

Potassium

39.098

atomic #

element symbol

element name

atomic mass

Obj. 6 cont...• Atomic

#: # of p+ in an atom identifies

element # p+ = # e- in neutral atom

(change atomic # = change of element).

** (+) charge = less e- than p+

** (-) charge = more e- than p+

• Atomic mass:

mass of the nucleus

p+ + n0

units are a.m.u. (atomic mass unit)

(a.k.a. mass #)

Obj. 7…Atomic Symbols• There are two ways to represent elements:

• Symbol Form:

# X #

atomic mass

(# of p+)atomic #

element symbol

# Xatomic

mass element symbol

OR

• Shorthand Form: name of element followed by atomic mass. • Ex...

Carbon - 14

Aluminum - 27 Nitrogen - 14

• You can find mass #, atomic #, # of n0, and # of e- witheither notation!

Obj. 8…Atomic Calculations• All mass of the atom is in the nucleus. - only p+ and n0 are in the nucleus. - if you know the mass of any atom, you can find the # of n0.

• Ex...

31P 15

+1

electrical charge on atom

element = _______________

atomic mass = ___________

atomic # = ______________ # of p+ = _______________

# of n0 = _______________

# of e- = _______________

Phosphorus

31 a.m.u.s 15 15 16 14

Manganese - 55 symbol = ______________ atomic mass = __________ atomic # = ____________ # of p+ = ______________ # of n0 = ______________

# of e- = ______________

Mn 55 a.m.u.s 25 25 30 25

- if you know the # of n0 and the # of p+, you can find themass.

Obj. 9…Isotopes

- different # of n0 !!! • Three isotopes of Carbon:

Carbon - 14

Carbon - 12 Carbon - 13

(6p+, 6n0) (6p+, 7n0) (6p+, 8n0)

element of life extremely rare radioactive…carbon dating • Note: atomic # will NEVER change in

isotopes…only mass and # of n0 do!• Which of the following are isotopes of the same

element?

22X 12

+3

22X 10

25X 10

-1 Neon - 20

Neon - 22

Fluorine - 20

• Isotopes: atoms of the same element with different atomicmasses.

Obj. 10…Average Mass of Isotopes• Isotopes are naturally occurring.

• The mass # of an element (periodic table) is the weightedavg. of all isotopes that exist in nature.

- abundance of isotope is just as important as mass! • Ex...

Natural copper (Cu) consists of 2 isotopes ... Copper - 63 (mass = 62 .930 g/mole)

Copper - 65 (mass = 64 .930 g/mole) 69%

31% • To calculate avg.

mass... mass x abundance for each isotopeStep 1 :Step 2 :add the two values from step 1 together

62 .93 x .69 =

64 .93 x .31 =

43.42

20.13

43.4220.13+

63.55 g/mole

Obj. 10 cont...

• Ex...

Three isotopes of Oxygen:

Oxygen - 18

Oxygen - 16 Oxygen - 17

• The avg. mass (from P.T.) is closest to 16, therefore, Oxygen-16 is the isotope that is most abundant in nature.

99 . 759%0.037%

0.204%

• The average mass of an element is closest to the isotopethat is most plentiful in nature.