Atoms, Molecules and Ions

Chapter 2

Foundations of Atomic Theory• Law of conservation of mass: Antoine Lavoisier

– Mass is neither created nor destroyed. The total mass of a compound must be the same as the total mass of individual elements.

• Law of definite composition: Joseph Proust– a chemical compound contains the same elements in

exactly the same proportions by mass regardless of the size of the sample or source of the compound

HgO Hg + O433.2 g 401.2g + 32g

Sugar: 42.1 % Carbon 51.4 % Oxygen

6.5 % HydrogenWhether you have a teaspoon or a truckload!

8 X2Y16 X 8 Y+

• Law of multiple proportions: John Dalton– Applies to different compounds made from the

same elements– The mass ratio for one of the elements that

combines with a fixed mass of the other element can be expressed as a whole number ratio.

H2O H2O2

Water Peroxide 2g H 2g H 16g O 1:2 Ratio 32g O

2

Dalton’s Atomic Theory (1808)1. Elements are composed of extremely small

particles called atoms. All atoms of a given element are identical, having the same size, mass and chemical properties. The atoms of one element are different from the atoms of all other elements.

2. Compounds are composed of atoms of more than one element. The relative number of atoms of each element in a given compound is always the same.

3. Chemical reactions only involve the rearrangement of atoms. Atoms are not created or destroyed in chemical reactions.

J.J. Thomson, measured mass/charge of e-

(1906 Nobel Prize in Physics)

JJ Thomson’s Cathode Ray Tube

Negatively Charged Electrode

Positively Charged Electrode

Cathode Ray Tube• Scientists studied the flow of electric current in a

glass vacuum tube with electrodes at each end.

• When connected to electric current the remaining gas glowed forming a BEAM OF LIGHT.

• The beam always originated at the NEGATIVE electrode and toward the POSITIVE electrode.

• The electrode is named by what type of particle it attracts– Cathode: Negative (-)– Anode: Positive (+)

• 1897 JJ Thomson used magnets to deflect the beam proving that particles had a NEGATIVE CHARGE.

• Now with the knowledge of electrons, and knowing the atom is neutral, there must be a particle that is positive to balance the negative charge.

• A small paddle wheel was placed inside and it rolled toward the anode, providing evidence that some PARTICLE MUST BE STRIKING THE WHEEL to make it move.

JJ THOMSON DISCOVERED A NEGATIVE PARTICLE

CALLED THE

ELECTRON!

CRT Video

Plum Pudding Model

Plum Pudding OR Chocolate Chip Cookie

Robert A. Millikan

• Performed the Oil Drop Experiment

• Determined the exact charge of an electron

e- charge = -1.60 x 10-19 C

Thomson’s charge/mass of e- = -1.76 x 108 C/g

e- mass = 9.10 x 10-28 g

Measured Charge of e-

(1923 Nobel Prize in Physics)

Oil Drop Experiment Video

Radioactivity• Becquerel

– discovered RADIATION• uranium would expose photographic plates in the dark

– The properties of an element changed as it gave off radiation

• Curie– Discovered radium and polonium

The radioactive emissions of alpha, beta and gamma rays were identified.

(Uranium compound)

Ernest Rutherford’s Gold Foil Experiment

• Set up Gold Foil with a detection sheet around it.

• Set up radioactive source emitting alpha particles.

• ALPHA PARTICLES shot at gold foil.

• MOST particles went through the gold foil

• SOME particles BOUNCED back

Rutherford’s Gold Foil Experiment

particle velocity ~ 1.4 x 107 m/s(~5% speed of light)

1. atoms positive charge is concentrated in the nucleus2. proton (p) has opposite (+) charge of electron (-)3. mass of p is 1840 x mass of e- (1.67 x 10-24 g)

Gold Foil Conclusions

1. The atom is made up of mostly EMPTY SPACE

2. The center of the atom contains a POSITIVE CHARGE

3. Rutherford called this positive bundle of matter the NUCLEUS

Gold Foil Experiment Video

Rutherford’s Model of the Atom

atomic radius ~ 100 pm = 1 x 10-10 m

nuclear radius ~ 5 x 10-3 pm = 5 x 10-15 m

Goldstein and Wien - 1886

• Cathode Ray Tube with perforated cathode• Discovered collection of positively charged

particles

**DISCOVERED THE PROTON!!

Chadwick’s Experiment - 1932

• Found that alpha particles shot at beryllium made a beam form

• The beam had the same mass of a proton but was electrically neutral

**DISCOVERED THE NEUTRON!!

Chadwick’s Experiment (1932)

H atoms - 1 p; He atoms - 2 p

mass He/mass H should = 2

measured mass He/mass H = 4

+ 9Be 1n + 12C + energy

neutron (n) is neutral (charge = 0)

n mass ~ p mass = 1.67 x 10-24 g

Niels Bohr – 1913

• Developed a new diagram of the atom• Electrons can only be at certain energies• Electrons must gain a specific amount of

energy to move to a higher level, called a quantum

**DISCOVERED ENERGY LEVELS!!

Bohr’s Model of the Atom

Subatomic Particles (Table 2.1)

Particle Mass

(g) Charge

(Coulombs) Charge (units)

Electron (e-) 9.1 x 10-28 -1.6 x 10-19 -1

Proton (p+) 1.67 x 10-24 +1.6 x 10-19 +1

Neutron (n) 1.67 x 10-24 0 0

mass p = mass n = 1840 x mass e-

Atomic number (Z) = number of protons in nucleus

Mass number (A) = number of protons + number of neutrons

= atomic number (Z) + number of neutrons

Isotopes are atoms of the same element (X) with different numbers of neutrons in their nuclei

XAZ

H11 H (D)2

1 H (T)31

U23592 U238

92

Mass Number

Atomic NumberElement Symbol

How many protons, neutrons, and electrons are in C146 ?

How many protons, neutrons, and electrons are in C116 ?

6 protons, 8 (14 - 6) neutrons, 6 electrons

6 protons, 5 (11 - 6) neutrons, 6 electrons

Do You Understand Isotopes?

Period

Group

A molecule is an aggregate of two or more atoms in a definite arrangement held together by chemical bonds

H2 H2O NH3 CH4

A diatomic molecule contains only two atoms

H2, N2, O2, Br2, HCl, CO

A polyatomic molecule contains more than two atoms

O3, H2O, NH3, CH4

An ion is an atom, or group of atoms, that has a net positive or negative charge.

cation – ion with a positive chargeIf a neutral atom loses one or more electronsit becomes a cation.

anion – ion with a negative chargeIf a neutral atom gains one or more electronsit becomes an anion.

Na 11 protons11 electrons Na+ 11 protons

10 electrons

Cl 17 protons17 electrons Cl-

17 protons18 electrons

A monatomic ion contains only one atom

A polyatomic ion contains more than one atom

Na+, Cl-, Ca2+, O2-, Al3+, N3-

OH-, CN-, NH4+, NO3

-

13 protons, 10 (13 – 3) electrons

34 protons, 36 (34 + 2) electrons

Do You Understand Ions?

How many protons and electrons are in Al2713 ?3+

How many protons and electrons are in Se7834

2- ?

Atomic Mass Unit

• One atom is the standard – Carbon• Mass of other elements are based off of the

standard• Carbon: 6 p and 6 n = 12 amu

• 1/12 mass of Carbon atom• Periodic table lists weighted average atomic

masses of elements (like a GPA calculation)

Relative Atomic Mass

Calculation AVERAGE Atomic Mass Steps:

1. Percent to decimal

2. Multiply by mass

3. Add it up!

75% 133Cs20% 132Cs5 % 134Cs