Atomic Theory

cathode rays

Historical Perspective

Ancient Greece:

Democritus (c. 460 - 370 BC): matter is composedof tiny indivisible particles - atoms; besides atoms,there is “empty space”

Aristotle (384 - 322 BC): matter is composed of four elemental substances: fire, air, water, and earth.

~ 2000 years

Mass Laws and Composition

17th - 18th century

the law of mass conservation: the total mass of starting materials and products in any chemical reaction remains unchanged.

the law of constant composition: independently of its source, any chemical compound is composed of the same elements in the same proportions.

Lavoisier (1743-94)

Dalton’s Atomic Theory19th century

1. All matter consists of atoms.

2. In a chemical reactions, atoms cannot becreates or destroyed. They separate and recombine toform new substances.

3. Atoms of an element are identical in mass andother properties and are different from other elements.

4. Compounds result from a combination ofatoms in specific ratios.

Discovery of the electron

Electricity in vacuum, J.J. Thompson’s experiments:

Discovery of the electron

… electron weights 1/1000 of the lightest atom and is negativelycharged

Chocolate chip cookie model

Rutherford’s experiments

Alpha-particles: heavy, positively charged

Rutherford’s experiments

1 in ~ 20,000alpha-particlesis reflected!

Nuclear Structure of the Atom

Structure of the Atom Today

1. All matter is composed of atoms.

2. All atoms are made of a tiny (<1% atomic volume), yetheavy (>99.9% total atomic weight) positively chargednucleus surrounded by a cloud of negatively chargedelectrons (<0.1% of atomic weight).

3. The nuclei are composed of positively charged protonsand neutral neutrons. They determine the atomic mass.

Structure of the Atom TodayProperties of subatomic particles

Name (Symbol)Charge

(relative)Mass

(relative) Location in atom

proton (p+) +1 1.007 ≈ 1.0 Nucleus

neutron (n+) 0 1.009 ≈ 1.0 Nucleus

electron (e-) -1 0.001 ≈ 0.0 Around nucleus

* Relative to the mass of a hydrogen atom: the lightest element

*

Structure of the Atom TodayProperties of subatomic particles

Name (Symbol)Charge

(relative)Mass

(relative) Location in atom

proton (p+) +1 1.007 ≈ 1.0 Nucleus

neutron (n+) 0 1.009 ≈ 1.0 Nucleus

electron (e-) -1 0.001 ≈ 0.0 Around nucleus

* 1/12 the mass of a carbon atom Relative to the mass of a hydrogen atom: the lightest element

*

1. Atomic nucleus of each element is characterized bystrictly defined number of protons. (if it has adifferent number of protons, it’s a totally differentanimal).

2. The number of neutrons in the atomic nucleus of thesame element may vary.

3. Hence Isotopes: Atoms of the same element with adifferent mass.

Structure of the Atom TodayContinued

Atomic Number, Atomic Mass,Atomic Symbol

A = Z + NN = A - Z

N = number of neutrons

Cl35

17

mass number (A)

atomic number (Z)

atomic symbol

(Chlorine-35)

Atomic Number, Atomic Mass,Atomic Symbol

H10 neutrons

H2 H31 neutron 2 neutrons

HYDROGEN

Atomic Number, Atomic Mass,Atomic Symbol

H10 neutrons

H2 H31 neutron 2 neutrons

HYDROGEN

Fe54

28 neutrons

Fe56 Fe57

30 neutrons 31 neutrons

ISOTOPES OF IRON26

If There Are Isotopes, What’s ThatAtomic Weight in the Periodic Table?

atomic weight

average weight of all naturally occurring isotopes

each isotope contributes according to its abundance(fraction)

If There Are Isotopes, What’s ThatAtomic Weight in the Periodic Table?

Isotope Atomic MassNatural

Abundance (%)Contribution to Average

Atomic Mass54Fe 53.940 5.8 53.940×0.058=3.12956Fe 55.935 91.8 55.935×0.918=51.34857Fe 56.935 2.1 56.935×0.021=1.19658Fe 57.933 0.3 57.933×0.003=0.174

atomic weight

TOTAL 55.847

Mass Spectroscopy

The best method to determine the weight of particles(atoms, molecules, ions) with high accuracy

Is based on a deflection of moving charged particles ina magnetic field

[Determines a mass/charge ratio (m/z)]

Compounds: Introduction to Bonding

Most elements exist as compounds in combination withother elements

Chemical bonds are formed by an exchange of electrons

Two types of bonds:

• Ionic: as a result of transferring electrons• Covalent: as a result of sharing electrons

Ionic Compounds

Ions, charged particles, are formed when an atom (or agroup of atoms) gains or loses electrons.

Metals lose electrons → cations (+)

Non-metals gain electrons → anions (-)

All elements want to be like noble gases

Ionic Compounds: Example

Sodium Chloride, NaCl

All elements want to be like noble gases