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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.
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
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