Chemistry Unit 1

History of the Atom - 

John Dalton (1766 - 1844) - 

• 1808

• Dalton’s Postulates

  • All matter is composed of atoms

  • All atoms of a given element are identical

  • Atoms cannot be divided, created, or destroyed

  • Atoms can combine in whole number ratios to form chemical compounds

  • In a chemical reaction, atoms are combined, separated, or rearranged

• Wrong on one point - the atom is made of even smaller particles

JJ Thomson (1856 - 1940) - 

• Proved that the atom was made of smaller particles using a cathode ray tube and two charged metal plates

• All atoms contained a negatively charged component - electrons

• Positively-charged cloud with small negatively-charged particles inside it (plum pudding or chocolate chip cookie)

Ernest Rutherford (1871 - 1937) - 

• Proved that the atom was mostly composed of empty space, with a small, dense nucleus at the center

• Fired a stream of alpha particles at a thin sheet of gold foil, expecting all to pass through, however, some didn’t

• Proved existence of a small, dense nucleus

• Later discovered nucleus contained protons

Niels Bohr (1885 - 1962) - 

• 1913

• Theorized that electrons moved around the nucleus in fixed orbits, like the Solar System

• Bohr’s model shows electrons in “shells”, also called energy levels

• Each atom has a specific number of these shells, and electrons with more energy are in shells further away from the nucleus

James Chadwick (1891 - 1974) - 

• Atoms seemed to have too much mass for the number of protons and electrons they contained

• Neutrons were difficult to detect due to having no charge

• Determined the neutron was real by observing the behavior of certain types of radiation

• Neutron had almost the exact same mass as a proton

Werner Heisenberg (1901 - 1976) - 

• Disagreed with Bohr’s model

• Imagined using a gamma ray microscope to observe an electron

• Electrons would be “kicked” away every time a gamma ray hit it

• The more precisely one tried to measure the position of an electron, the more its momentum would change

• “Heisenberg’s Uncertainty Principle”

  • The position and momentum of an electron cannot both be known 

Erwin Schrodinger (1887 - 1961) - 

• Described the electron not as a particle, but as a wave

• Developed and published probability functions that can be used to predict the location of electrons in an atom

The Periodic Table

Dimitri Mendeleev

• Created the first periodic table

• Organized by atomic mass and grouped elements with similar properties

• Predicted a few undiscovered elements and left space for them

• Ga, Sc, and Ge

• #101 was named after him

• Some elements are grouped together

• No atomic # yet

Henry Moseley

• Organized by increasing atomic number, instead of mass, but still grouped elements by properties

• Essentially the modern periodic table

• There is no atomic number or average mass shown

• A period on the periodic table refers to the horizontal rows

• They are numbered 1 through 7

• This tells us how many energy levels an atom has

• A group or family on the periodic table refers to the vertical columns

• They are numbered 1-18 or may use a combination of numbers and letters (ie 6A)

• These groups have several characteristics in common due to having the same number of valence electrons

• Valence electrons

  • Electrons in the highest energy level of an atom

  • Ex:) 1A has 1, 2A has 2

Metals, Nonmetals and Metalloids

• Properties of Metals

  • Most elements are metals

  • Solids at room temperature, except Hg (liquid)

  • Malleable - can be bent without breaking

  • Ductile - can be stretched into wire

  • Lustrous - shiny

  • Good conductors of heat - transmit heat well

  • Good conductors of electricity - transmit electricity well

  • High melting points (melting point - temperature at which a substanfce changes from solid to liquid)

• Properties of Nonmetals

  • Solids, liquid, or gases at room temperature (depends on the element)

  • Solids are brittle (break easily)

  • Dull-looking and sometimes colored

  • Poor conductors of heat (insulators) - do not transmit heat

  • Poor conductors of electricity - do not transmit electricity

• Properties of Metalloids

  • Have some properties of metals and some of nonmetals

  • Solid and brittle

  • Have luster

  • Semiconductors - conduct electricity under certain conditions

• Alkali Metals

  • Group 1A (except H)

  • Most reactive metals

  • Metallic properties

  • React explosively with water

  • Never alone in nature

• Alkaline Earth Metals

  • Group 2A

  • Very reactive (less than alkali)

  • Metallic properties

  • Never alone in nature

• Transition Metals

  • Groups 3B to 2B

  • All are solids at room temperature (except Mg)

  • Metallic properties

  • Form cations with various charges

• Boron Group

  • Group 3A

  • Properties vary due to containing metals, nonmetals, and metalloids

• Carbon Group

  • Group 4A

  • Properties vary due to containing metas, nonmetals, and metalloids

• Nitrogen Group

  • Group 5A

  • Properties vary due to containing metals, nonmetals, and metalloids

• Oxygen Group

  • Group 6A

  • Often found in minerals with other elements

• Halogens

  • Group 7A

  • Most reactive nonmetals

  • Means  “salt former” because compounds with halogens form salts

  • Highly toxic

  • Exist in all three states of matter (F and Cl are gases, Br is liquid, I and At are solid)

• Noble Gases

  • Unreactive 

  • Do not form compounds easily

  • Usually alone in nature

  • Often called “inert gases”

  • Group 8A

• Lanthanides

  • Found in period 6 (between barium and hafnium)

  • Highly reactive

  • Named for the element lanthanum

• Actinides

  • Found in period 7 (between radium and rutherfordium)

  • Highly reactive

  • Named for the element actinum

  • Elements after uranium are synthetic (created by humans)

• Group A elements are called Representative Elements, or Main Group Elements

• Group B elements are called Transition Metals

• Lanthanide and Actinide series are called Inner Transition Metals or Rare Earth Metals