Modern Atomic Theory

Modern Atomic Theory States:

Atoms are composed of protons, neutrons and electrons.

Particle Charge Mass Location

Proton Positive 1 amu Nucleus

Electron Negative 0 amu Electron Cloud

Neutron Neutral 1 amu Nucleus

An atom’s identity comes fromthe number of protons in the nucleus.this number is called the atomic number.

The atomic number can be found onthe periodic table. It is usually located in the upper right hand corner.

If an atom is neutral (has no charge), thenumber of electrons will be the same as the number of protons.

Positive charge = negative charge

So if an atom is neutral, youcan use the atomic number to findthe number of electrons as well.

Ex. Heliumhas an atomic number of 2. Soit has 2 protons and if neutral has2 electrons.

To find the number of neutrons,you use the mass number of the element.

The mass number can be found by roundingthe average atomic mass of an element to a whole number. Ex. Hydrogen has an average atomic mass of 1.00794 so itsmass number would be 1.

Mass number = number of protons + number of neutrons

Number of neutrons = mass number – atomic number

Example: The mass number of sodium is 23. Sodium has 11 protons… so it has 12 neutrons (23-11).

Protons Electrons Neutrons

Hydrogen

Calcium

Chlorine

1 1 0

20 20 20

17 17 18

Atoms can lose and gain electrons.

An ion is an atom that has lost orgained electrons to take on a netelectrical charge.

Charge of ion = # of protons - # electrons

Na +1

Ba +2

Cl -1O-2

Example: Mg+2 always has 12 protons but becauseit has a charge of +2 it only has 10 electrons (12-10 = 2).

Ion Protons Electrons

K+1 19 18

Sr +2 38 36

Br-1 35 36

S-2 16 18

In nature, elements can be found with differentnumbers of neutrons. An isotopeis an atom that has the same number of protonsbut has different numbers of neutrons. Because of the differing number of neutrons-isotopes differ inmass.

Example: All chlorine atoms have 17 protons-but somechlorine nuclei have 18 neutrons while others have 20neutrons.

(Hyperlink)

Iron-56

Protons Neutrons Electrons

35 Cl17

Cu+2-65

26 30 26

17 18 17

29 36 27

Calculating Average Atomic Mass

• The average atomic mass of an element takes into account all of an elements isotopes. Remember… because isotopes differ in #’s of neutrons, their masses will differ

• Avg. Atomic Mass = the sum of each elements fractional abundance multiplied by its mass

Examples:

CarbonIsotope 1Isotope 2

ChlorineIsotope 1Isotope 2

SiliconIsotope 1Isotope 2Isotope 3

Mass Abundance Avg. Atomic Mass

12 98.89%13.003 1.11%

34.969 75.53%36.966 24.47

27.977 92.21%28.976 4.70%29.974 3.09%

12.011

35.45

28.09

• Emission of one of the three types of radiation:

AlphaBetaGamma

In the nucleus, two forces exist:

• Electric repulsion between protons

• Strong nuclear force which overcomes the repulsion between protons in the nucleus and keeps it together.

• ***The presence of

neutrons adds to the

net attractive force in

the nucleus.

If the strong nuclearforce is not sufficient to overcome the repulsionbetween protons, the nucleus begins to fall apart.This is what causes an element to be radioactive.

Stable Nuclei

• Elements 1-20 are stable because they have almost equal #’s of protons and neutrons.

• Beyond 20 protons, nuclei need more neutrons than protons to stabilize.

• When the atomic # exceeds 83, no # of neutrons is sufficient to hold the nucleus together.

• All nuclei w/ atomic #’s greater than 83 are radioactive.

• Stream of high energy alpha particles

• Consists of 2 protons and 2 neutrons

• Not very penetrating-can be blocked by paper or clothing

• Stream of high speed electrons

• Electrons are produced when a neutron splits into a proton and an electron (beta particle)-proton remains in nucleus, while electron is emitted as a beta particle.

• 100 times more penetrating than alpha

• Very energetic form of light-does not consist of particles

• Penetrates deeply into solid material-stopped by heavy shielding, such as concrete or lead.

• Reactions in which changes occur in the nucleus of an atom and result in a change of composition in the nucleus.

Examples:

Half-life

• No two radioactive isotopes decay at the same rate.

• Half-life (t1/2) is the time required for half the atoms of a radioactive nuclide to decay.

• More stable nuclides decay slower and have longer half-lives.

Application- Radioactive Dating

• First determine the number of half-lives that have passed:

1/8 = (1/2) x (1/2) x (1/2)Therefore, three half-lives have passed so…3 x (1.3 x 109) = 3.9 x 109 years have passed

since the rock was formed!!!

The half-life of potassium-40 is 1.3 x 109 years. A volcanic rock contains 1/8 of the amount of potassium-40 found in newly formed rocks. When was the rock formed?

Example Problem:

1. Phosphorous-32 has a half-life of 14.3 days. How many milligrams of phosphorous-32 remain after 57.2 days if you start w/4.0 mg?

2. After 4797 years, how much of the original 0.250 g of radium-226 remains if the half-life is 1599 years?

Fusion vs. Fission

Fission

Fusion

Fusion

• Nuclear fusion occurs when low-mass nuclei combine to form a heavier, more stable nucleus.

• Fusion releases more energy per gram than nuclear fission.

• In our sun, hydrogen nuclei combine to form a helium nucleus.

• A temperature of 108 K is required to induce fusion!!!!

Fission

• Fission occurs when a very heavy nucleus splits into more stable nuclei.

• Fission can occur spontaneously or when nuclei are bombarded by particles.

• Nuclear power plants utilize fission to produce electricity.