Atomic Structure

Chapter 4

Dalton’s Atomic Theory 4.1

John Dalton – 18th century• All matter is composed of atoms• All atoms of a given element are identical• Atoms cannot be created, divided or

destroyed• Different atoms combine to form

compounds• In chemical reactions atoms are

rearranged.

Subatomic Particles 4-2

Discovering the Electron-

• Thomson identified the electron

through use of a cathode ray tube.

• Electrons carry a negative charge

• Electrons have an extremely small mass

• Thomson created the plum pudding model (chocolate chip cookie dough model)

Electrons --- charge = -1 e-

Plum Pudding Model

Negatively charged electrons were distributed throughout a uniform positive charge

The Nuclear Atom 4.2

Rutherford’s Gold Foil Experiment

• Discovered the nucleus

• Atoms consist of mostly empty space through which the electrons move

• There is a tiny dense region (nucleus) which contains all of the atom’s positive charge and almost all its mass

Gold Foil Experiment

Plum Pudding Model

Nuclear Model

Because some of the alpha particles bounced back – they must have hit something with a positive charge

What’s in the nucleus

Protons – subatomic particle with a charge equal and opposite to an electron

Proton --- charge = +1 p+

Neutrons – subatomic particle with a mass almost equal to that of a proton but it has no charge – neutral

Neutron --- charge = 0 n0

How atoms differ 4.3

Atomic Number:

• The number of protons in an atom is the elements atomic number

• The number of protons in an atom identifies it as an atom of a particular element

• Atoms are neutral so # protons = # electrons

Example: Helium - He

Atomic # = 2 so it has 2 p+ and 2 e-

How atoms differ 4.3

Isotopes and Mass Number

• Not all atoms of a particular element are identical. They all have the same atomic number (protons), but they can have a different number of neutrons

• Isotopes - same # of protons

different # of neutrons

How atoms differ 4.3

Example:

• 3 types of potassium - all have 19 protons (that’s what makes them K)

- 20 neutrons; 21 neutrons; 22 neutrons

• Differ in mass, but they have the same chemical properties

Mass # = protons + neutrons

How atoms differ 4.3

Example cont.

3 isotopes of Potassium

(same protons, different neutrons)

Potassium – 39 (19 + 20)

Potassium – 40 (19 + 21)

Potassium – 41 (19 + 22)

Mass of Individual Atoms 4.3

Atomic Mass Units (AMU) Table 4-2

1 amu is almost equal to the mass of one

proton or one neutron

The atomic mass of an element is the average mass of the isotopes of that element

Mass of Individual Atoms 4.3

Example: Chlorine - two isotopes

Chlorine – 35 and Chlorine – 37How many protons and neutrons does each have?

Isotopes Mass % abundance

Chlorine - 35 35.00 amu 75.0%

Chlorine - 37 37.00 amu 25.0%

Atomic Mass = 35.00 x 0.75 = 26.25

37.00 x 0.25 = 9.25

26.25 + 9.25 = 35.5 amu

Mass of Individual Atoms 4.3

Practice Problem:

Isotopes Mass % abundance

Boron – 10 10.013 amu 19.8%

Boron – 11 11.009 amu 80.2%

10.013 x 0.198 = 1.982

11.009 x 0.802 = 8.829

1.982 + 8.829 = 10.811 amu

Mass of Individual Atoms 4.3

Isotopes Mass % abundance

X – 6 6.015 amu 7.50 %

X - 7 7.016 amu 92.5 %

You Try:

6.015 x 0.0750 = 0.451 0.451

7.016 x 0.925 = 6.489 + 6.489 = 6.940 amu

What element has a average atomic mass of 6.94 amu?

Lithium (Li)

Radioactive Decay 4.4

Nuclear Reactions – a rxn that involves a change in the atoms nucleus

• change in the # of protons

• atoms of one element turn into atoms of another element

Radioactive Decay 4.4

Radioactivity – the process by which some substances spontaneously emit radiation

• They do this because they are unstable – they have a unstable ratio of protons to neutrons in their nucleus.

• They lose energy and undergo radioactive decay to become stable, non-radioactive atoms

Radioactive Decay 4.43 types of radiation: Alpha, Beta, & Gamma

Alpha Radiation-

Alpha Particle - 2 p+ and 2 n0

- has a charge of 2+

- equivalent to a Helium -4 nucleus

- symbol = α or 42 He

Ex:

Radioactive Decay 4.4

Beta Radiation-

Beta Particle – fast moving electrons

- negative charge ( -1)

- symbol = 0-1β

Ex:

Radioactive Decay 4.4Gamma Radiation

Gamma Rays - 00γ

- high energy

- have no mass, no charge

- usually accompany alpha or beta

- don’t make a new atom by themselves

Ex: