Atomic Structure

Atomic StructureAtoms, Elements &

Isotopes

Dalton’s Atomic Theory proton, neutron

and electron

isotopes exist

Four postulates (1808)...1. All elements are composed of tiny,

indivisible particles called ‘atoms’.2. Atoms of the same element are

identical... each element is unique3. Atoms of different elements can

physically mix or chemically combine (compounds).

4. Chemical reactions occur when atoms are separated, joined, or rearranged.

Much of this theory is still accepted, ….. with 2 exceptions.

Atomic Scientists• J. J. Thomson discovered electrons (e-)

in 1897.• He passed an electric current through a

glass tube filled with gas.• He discovered that a beam of negative

charges traveled from the cathode (-) to the anode (+).

• E. Goldstein discovered positive particles in atoms in 1886.

• He observed rays traveling in the opposite direction of cathode rays.

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

• James Chaddwick discovered neutrons (n0) in 1932.• His discovery was based on the fact

that different atoms’ atomic mass and atomic # (# of protons) were not adding up.

Atomic Scientists

Atomic Scientists• Robert Millikan furthered Thomson’s work

by describing the electron more in depth• all e- carry exactly one unit of negative

charge.• mass of an e- is 1/1840 the mass of a

hydrogen atom (p+).

The Bohr-Rutherford Model• Ernest Rutherford conducted the

famous ‘gold foil experiment’ (1911) which concluded that:• 1. atoms are mostly space. (football

arena)• 2. atoms have a solid nucleus at

the center which contains most of the mass

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

The Bohr-Rutherford Model• 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!)

The Bohr-Rutherford Model• After Rutherford’s findings, Niels Bohr

further explained the atom by concentrating on the e- (1913).

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

• Bohr proposed that the e- travel on concentric orbits around the nucleus.• Each orbit has a fixed energy (energy

level) and e- do not lose energy.• The progression of the atom...

Plum Pudding

Rutherford Rutherford-Bohr

Quantum

Subatomic Particles

PROPERTIES OF SUBATOMIC PARTICLES

PARTICLE SYMBOL ELECTRICAL CHARGE RELATIVE

MASSACTUAL MASS (g)

Electron e- -1 1/1840 9.11 x 10-

28

Proton p+ +1 1 1.67 x 10-

24

Neutron n0 0 1 1.67 x 10-

24

Characteristics of Elements

K19

Potassium

39.098

Atomic Number

Element Symbol

Element Name

Atomic Mass

Characteristics of Elements• Atomic Number : Number of p+ in an

atom• identifies element (change atomic #

= change of element).• # p+ = # e- in neutral atom• (+) charge = less e- than p+ • (-) charge = more e- than p+

• Atomic mass: (a.k.a. Mass Number)• mass of the nucleus

• p+ + n0

• units are a.m.u. (atomic mass unit)

Atomic Symbols• There are two ways to represent elements:

• Symbol Form: # X #

atomic massatomic # (# of p+)

element symbol

# Xatomic mass element

symbol

OR

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

• Ex... • Aluminum - 27• Nitrogen – 14• Carbon – 14

• You can find mass number, atomic number, number of n0, and number of e- with either notation!

Atomic Symbols• There are two ways to represent elements: • Symbol Form:

Shorthand Form: name of element followed by atomic mass.

Ex... Aluminum – 27Find the following elements on the Periodic Table: Beryllium, Copper, Sulfur, Neon and write them in both notations.

# X #

atomic massatomic # (# of p+)

element symbol

# Xatomic mass element

symbolOR

Atomic Symbols• Using the symbol form for neutral elements:

• Symbol Form:

# X #

atomic mass

(# of p+)atomic #

element symbol

• Ex...

3. Sulfur-

1. Beryllium-2. Copper -

• Now find for each element the:

• atomic mass• atomic number• # of p+ (same as

atomic#)• # of n0 (mass# -

atomic#)• # of e- (same as # of

protons)

4. Neon -

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

mass.• Ex... electrical charge on atom

Manganese - 55

31P 15

+1

element = Phosphorusatomic mass = __31

a.m.u.s atomic # = 15

# of p+ = 15# of N0 = 16# of e- = 14

symbol = __Mn atomic mass = __55

a.m.u.s atomic #

= 25

# of p+ = 25# of n0 = 30# of e- = 25

Isotopes

22 X 12

+3

22 X 10

25 X 10

-1

Neon - 20

Neon - 22 Fluorine - 20

• Isotopes: atoms of the same element with different atomic masses.• - different # of n0 !!!

• Three isotopes of Carbon:• Carbon - 12 (6p+, 6n0) element of life • Carbon - 13 (6p+, 7n0) extremely rare • Carbon - 14 (6p+, 8n0) 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?

Average Mass of Isotopes

• Isotopes are naturally occurring. • The mass # of an element (periodic table) is

the weighted avg. 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) @ 69% • Copper - 65 (mass = 64 .930 g/mole) @ 31%

• To calculate avg. mass... • Step 1 : mass x abundance for each isotope• Step 2 :add the two values from step 1

together• 62 .93 x .69 = 43.42 43.42• 64 .93 x .31 = 20.13 + 20.1363.55

g/mole

Average Mass of Isotopes• The average mass of an element is closest

to the isotope that is most plentiful in nature.

• Ex... Three isotopes of Oxygen:

Oxygen - 16 99 . 759%Oxygen - 17 0.037%Oxygen - 18 0.204%The avg. mass (from P.T.) is closest to 16, therefore, Oxygen-16 is the isotope that is most abundant in nature.