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THE ATOM
Ch 3
ARISTOTLE
• Aristotle: 384 - 322 B.C.
• Greek philosopher
• Described composition and behavior of matter in terms
of 4 qualities: hot, cold, moist, and dry
• Said any substance could be changed into another by
changing one of those qualities
• Meaning, we could change lead to gold
• Alchemy: a field of study concerned primarily with
finding potions that would produce gold
DEMOCRITUS
• Democritus: 460 - 370 B.C.
• Greek philosopher
• “matter is composed of a finite number of incredibly
small but discrete units we call atoms.”
• Coined the term atom from the Greek phrase atomos,
meaning “not cut” or “that which is indivisible”
• The texture, mass, and color of a material are a function
of the texture, mass, and color of its atoms.
LAVOISIER
• Antoine Lavoisier (1743 - 1794)
• An element is any material made of only one component
• Identified a compound as any material composed of
two or more elements
• Lavoisier is considered the “father of modern
chemistry” and ideas are in line with our present
understanding
• His definitions required experimentation—Greek
based their ideas on logic and reason.
LAVOISIER
• Hypothesized that mass is always conserved during a
chemical reaction
• Law of Conservation of Mass: amount of mass present
after the reaction is the same as amount present
before the reaction
• If I burn a piece of wood, does the wood weigh the
same after as it does before? Why?
• Lavoisier accounted for gases! When the wood
burns, gaseous carbon dioxide and water vapor
are released!
DALTON
• John Dalton: 1766 – 1844
• self-educated English schoolteacher
1. Each element consists of indivisible, minute
particles called atoms
2. Atoms can be neither created nor destroyed
in chemical reactions
3. All atoms of a given element are identical
4. Atoms chemically combine in definite
whole-number ratios to form compounds
5. Atoms of different elements have different
masses
MENDELEEV
• Dmitri Mendeleev: 1834-1907
• Russian chemistry professor
• Produced a chart summarizing the properties of
known elements
• Arranged the elements based on their properties
• Sometimes there were blanks that could not be filled
by any known elements; he left these open and others
eventually discovered the missing elements
THOMSON
• Thomson: 1856-1940
• Used cathode ray tube
experiments to discover
negative part of atom, but was
unable to calculate the charge
or mass of the particle
• We know this particle today as
the electron
• The electron determines many
of a material’s properties,
including reactivity
MILLIKAN
• Robert Millikan: 1868-1953
• Calculated the numerical
value of a single electric
charge
• He calculated the mass of a
cathode ray particle to be
considerably less than that of
the smallest known atom,
hydrogen
RUTHERFORD
• Ernest Rutherford: 1871-1937
• It was reasoned that if atoms had
negative particles, there must also
be a balancing positive force
• Used the gold foil experiment to
show that atoms are mostly empty
space with the majority of mass
concentrated in a central core
called the atomic nucleus
ATOMIC NUCLEUS
• Atomic Nucleus: tiny central core
of the atom where most of the mass
is concentrated (99.99% of mass)
• Think of a marble inside a football
stadium
• Determined that the nucleus was
the balancing positive force in an
atom and it was made up of
subatomic particles
SUBATOMIC PARTICLES
• Atoms are made up of three subatomic particles
• Neutron: neutral (no charge) particle in the nucleus
• Proton: positively charged particle in the nucleus
• Electron: negatively charged particle orbiting the
nucleus in electron cloud
• In a neutral atom, the number of protons is equal to the
number of electrons, meaning the atom is electrically
balanced!
SUBATOMIC PARTICLES
• Subatomic particles are extremely small;
therefore scientists cannot measure them
in grams
• Instead, they are measured in atomic mass
units, or amu
• 1 amu = 1.61 x 10-24 g
• 1 g is about the mass of a paperclip
ATOMIC NUMBER
• Atomic Number (Z): number of protons in a nucleus
• Used to identify the element, like a fingerprint, because it
NEVER CHANGES
• The modern periodic table lists elements in order of
increasing atomic number
ATOMIC MASS
• Atomic Mass: total mass of an atom (sum of
all components – e-, p+, and n0)
• Electrons are so much less massive than
protons and neutrons, their mass is
negligible
• Mass Number (A): total number of neutrons
and protons in an atom
• subtract atomic number from mass number to
find number of neutrons
ELECTRONS
• The trends found in the
periodic table are a result of
electron arrangement,
specifically, the number of
valence electrons
• Valence Electron: electrons that
fill the outermost energy level
IONS
• The group number of an element will tell you the
number of valence electrons it has
• Group 1: 1 valence electron
• Group 2: 2 valence e- ’s
• Skip 3-12
• Group 13: 3 valence e- ’s
• Groups 14-18: 4, 5, 6, 7, and 8 valence e- ’s respectively.
• The period number will tell you how many energy
levels have electrons
18
IONS
• Ions: charged particles (atoms
that are not neutral)
• Formed when an atom gives up
or gains electrons
• Cation: positive ion, has given
away electrons
• Anion: negative ion, has
gained electrons
IONS
• All atoms want to have 8
valence electrons in the outer
shell – making their outer shell
full and the atom stable
• Elements really close to 8
tend to be the most reactive.
• Elements already “full” are
considered inert, they don’t
react because they don’t
need to gain or lose
electrons
ISOTOPES
• Isotopes: atoms of the same
element that contain a different
number of neutrons
• Carbon-12: 6 neutrons
• Carbon-13: 7 neutrons
• Carbon-14: 8 neutrons
ISOTOPES
• Find the number of neutrons by subtracting the
atomic number (Z) from the mass number (A)
• Sometimes Z is not written because the atomic
number is always the same for a specific element