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: All matter is made up of atoms Atoms are neither created nor destroyed in chemical reactions (Law of conservation of mass) Atoms of different elements combine in whole-number ratios Each element is made of a different kind of atom 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 clouds Regions 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

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 amu Each neutron has a mass of 1 amu Electrons 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? NO Isotopes – atoms that have the same number of protons but different number of neutrons Each element has a limited number of isotopes that occur naturally

Mass number Protons + neutrons = mass number Used 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 Example Thermal 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 Properties Substance 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 Does not react with vinegar Rubbinmg alcohol Clear liquid Red food coloring Red color Reacts with bleach and loses color Iron malleable Reacts with oxygen Tin