Chapter 4Atomic Theory

Matter

• All matter is made of atoms

o Alone as elements• Au, Na, O, He

o In combination of elements as compounds

• H2O, NaCl, LiO2

• Democritus (460-370 B.C.) proposed & believed thato Matter was not infinitely

divisibleo Made up of tiny particles

called atomos or “indivisible”

o Atoms could not be created, destroyed, or further divided

o He did not do any experiments that backed up his theory.

Early Theories of Matter

John Dalton, 19th Century

• School teacher

• Dalton revised Democritus's ideas based upon the results of scientific research he conducted

• Dalton’s atomic theoryo Not totally correct

Dalton’s First Atomic Theory:

o Elements are made up of small indivisible particles called atoms

o Atoms of the same element are identical, different elements are different- (Same size, mass and chemical properties)

o Atoms are not created or destroyed in a chemical reaction

o A compound always has the same relative numbers and kinds of atoms

What is an atom?• An atom is the smallest particle of an element

that retains the properties of the element

Atomic Models

• Thomson: Plum Pudding Model

• Based off experiments using cathode rays

Atomic Models• Based off of his Gold

foil experiments• Rutherford:

Electron Cloud Model

Rutherford: The Nuclear Atom

• His model consisted of the following ideas:

o an atom consists mostly of empty space through which electrons move

o electrons are held within the atom by their attraction to the positively charged nucleus

o small, dense, positive charged nucleus

Bohr Model

• Based on Rutherford’s model

• “Planetary” model

• Adds idea of “quantized” energy levels

Quantum Model• Schrodinger (1920’s) came up with the modern

atomic model: proposing that electrons travelled in wave-like patterns

Inside an atom: subatomic particles

• These particles have mass and charge

• Nucleus holds protons & neutronso Protons: + chargeo Neutrons: Ø charge

• Outside of nucleus is the electron cloudo Electron: - charge

Parts of the AtomName Symbol Charge Mass Location

proton p +1 1 amu nucleus

neutron n 0 1 amu nucleus

electron e- -1 ~ 0 amu outside nucleus

Atomic Mass Unit (amu)

• Small mass #’s are not easy to work with, so the atomic mass unit (amu) was developed

• The mass of 1 amu is nearly equal to the mass of one proton or neutron

Reading the Periodic Table

Atomic Number• The number of protons determine the

element (ALWAYS!!!)

• Number of protons are unique to each element

• Examples:o Carbon (C) has 6 protons

• Atomic number is 6

o Copper has 29 protons• Atomic number is 29

Atomic Number

• In uncharged atoms, atomic number is also the number of electronsoWhy? o If an atom is charged, then it is an ion

• Uncharged atom:

Atomic number = # of protons = # of electrons

Charge = # protons - # electrons

Mass Number

• To find the Mass number # protons + # neutrons = mass number

• To find # neutrons mass number – proton (or atomic number) =

neutrons

• Mass numbers are always WHOLE #’s!!

Symbols for Atoms

X= symbol of elementA= mass numberZ= number of protons

X or X

- Can also be written “element - A”- i.e. carbon - 12

AZ

A

Mg-25 Zn

atomic #

proton

neutron

electron

mass #

12

12

13

12

25

30

30

40

30

70

7030

Turn to your partner and summarize…

• How does the number of protons affect the identity of the atom?

• How do the number of protons and electrons relate to each other in a neutral atom?

• What is an ion?

Isotopes and Mass Number

C C carbon-12 carbon-13

Isotopes are atoms with the same number of protons but different number of neutrons

13

6126

Isotopes and Mass Number

• Example: 3 types of Potassium

All 3 types contain 19 protons and __ electrons

# of Protons # of Neutrons Mass Number

19 20 19 21 19 22

19

39

40

41

What’s the difference between mass

number and average atomic

mass(weight)?

C carbon-12

But if you look on the periodic table, the number states 12.01…

Atomic Mass (atomic weight) – a weighted average of the masses of all of the isotopes of that element. It is not the same as the mass number.

12 6

What’s the difference between mass

number and average atomic mass?

• Mass number- specifically about one isotope; the number of protons + neutrons

• Average atomic mass- includes the masses of all the different isotopes for that element

Mass of Individual Atoms

• Average atomic mass:o The weighted average mass of the isotopes of

an element

o Example: Chlorine• Mixture of 75% chlorine-35 and 25%

chlorine-37

Atomic mass = (0.75)*35.0 + (0.25)*37.0 = 35.5 amu

Try this one• 3 isotopes of neon:

o Ne-20 (90.92%)o Ne-21 (0.25%)o Ne-22 (8.83%)

What is the average atomic mass of Ne?

(20)*(0.9092) + (21)*(0.0025) + (22)*(0.0883) =

20.18 amu

Radioactivity – when the nucleus of an atom is unstable causing it to decompose into another nucleus

There are three types of radioactive decay:

1.Alpha Decay

2.Beta Decay

3.Gamma Decay

Shielding: Alphas, Betas, Gammas and Neutrons

Alpha Decay• An alpha particle (α ) is produced • An alpha particle is just a helium nucleus,

He+242

Beta Decay• An e- is kicked out of the nucleus (a neutron

breaks up into a proton and e-), e 0-1

Gamma Decay• Only ENERGY is released from the nucleus• the nucleus itself does not change, but almost

always accompanies alpha and beta decay

•Alpha Decay230

90Th 42He + 226

88Ra

22288Ra

•Beta Decay234

90Th 0-1e + 234

91Pa

13153I

•Gamma Decay238

92U 42He + 234

90Th + energy

42He + 218

86Rn

0-1e + 131

54Xe

What are the products of Po-218 after it undergoes alpha decay followed by a beta decay, followed by beta decay followed by alpha decay?

21884Po 4

2He + 21482Pb

21482Pb 0

-1e + 21483Bi

21483Bi 0

-1e + 21484Po

21484Po 4

2He + 21082Pb

Where Does the Radiation Come From?• The radiation you receive can be either Natural or Man-

made

Turn to your partner and summarize…

• List the 3 types of radioactive decay and the particle that is released in each

• Which type of radioactive decay is most penetrating?

Half-Life (t1/2) - time required for one half of the original sample of nuclei to decay.

•The half-life of Ra-223 is 12 days. If you start with 100.0 grams of Ra-223, how much will be left after 36 days?

100.0 g 50.00 g 25.00 g 12.50 g

•The half life of Ra-225 is 15 minutes. If you have 10.0 grams now, how much did you start with 60 minutes ago?

•10.0 g 20.0 g 40.0 g 80.0 g 160.g

Fission – splitting a nucleus into two or more smaller nuclei

1 n + U Kr + Ba + 3 n + energy

• This is what takes place in a nuclear reactor or an atomic bomb.

235 92

9236

141 56

F ission• chain reaction - self-propagating reaction

Fusion – combining of two nuclei to form one nucleus of larger mass

H + H He + 1n + energy

• This is how all of the elements were created in nature and occurs naturally in stars.

21

31

42