Introduction to Atoms

What are atoms?

An atom is the smallest particle into which an element can be divided and still be the same substance.

History of the Atom

History• Democritus – first person to use the word atom, comes from

the Greek word atoma, means “indivisible”• John Dalton came up with the atomic theory• JJ Thomson did experiments using a cathode ray tube, came

up with model that included positive and negative charges• Ernest Rutherford did the Gold Foil experiment, which

changed the model to a positive nucleus surrounded by electrons

• Niels Bohr determined that electrons move in specific distances from the nucleus based on how much energy they have

Atomic theory

The atomic theory states 5 things:1.All matter is made up of atoms

2.Atoms are neither created nor destroyed in chemical reactions (Law of conservation of mass)

3.Atoms of different elements combine in whole-number ratios

4.Each element is made of a different kind of atom

5.The atoms of different elements have different mass and properties

Parts of an atom

Protons – positively charged particles found in all atoms

Neutrons – particles found in all atoms that do NOT have a charge (neutral charge)

Electrons – negatively charged particles found in all atoms

Nucleus

Center of the atom that is extremely dense, positively charged region that contains BOTH protons and neutrons

Protons and neutrons = mass number

Electron cloudsRegions inside the atom, located outside surrounding the nucleus with negatively charged particles (electrons)

Bohr proposed that electrons move in paths at certain distances around the nucleus = BOHR MODEL

Next time – Atoms and the periodic table

Copy the entire figure, drawing and words from page 312, Figure 11

Atomic mass unit (amu)

The SI unit used to express the masses of the particles of an element

Each proton has a mass of 1 amuEach neutron has a mass of 1 amuElectrons are so small that they have a mass unit

of almost zero

Atomic number

How can you tell which elements these atoms represent?

Atomic number – number of protons in the nucleus of an atom

Isotopes

Are all atoms of an element the same? NOIsotopes – atoms that have the same number of

protons but different number of neutronsEach element has a limited number of isotopes

that occur naturally

Mass number

Protons + neutrons = mass numberUsed to determine if an element is an isotope

Atomic mass

Weighted average of masses of all the naturally occurring isotopes of an element

MATH BREAK – pg 316 , do in your science journal

Physical Property Color, odor, mass, volume

Physical Properties Definition ExampleThermal conductivity The ability to transfer

thermal energy from one area to another

Plastic foam is a poor conductor, so hot chocolate in a plastic foam cup will not burn your hand.

State The physical form in which a substance exists , such as a solid, liquid, or gas

Ice is water in its solid state.

Malleability The ability to be pounded into thin sheets

Aluminum can be rolled or pounded into sheets to make foil.

Ductility The ability to be drawn or pulled in a wire

Copper is often used to make wiring.

Solubility The ability to dissolve in another substance

Sugar dissolves in water.

Density Mass per unit volume Lead is used to make sinkers for fishing line because lead is more dense than water.

Physical vs. Chemical PropertiesSubstance Physical property Chemical property

Helium Less dense than air nonflammable

Wood Grainy texture flammable

Baking soda White powder Reacts with vinegar to produce bubbles

Powdered sugar White powder Does not react with vinegar

Rubbinmg alcohol Clear liquid flammable

Red food coloring Red color Reacts with bleach and loses color

Iron malleable Reacts with oxygen

Tin malleable Reacts with oxygen