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Chapter 4 - ATOMIC STRUCTURE
MODELS OF ATOMS:
v Ancient Greek philosopher Democritus believed matter was
made of small particles that could not be divided called ATOMS
(from the Greek word “atomos” or uncut.)
v Ancient Greek philosopher Aristotle thought there was NO
limit to the number of times matter could be divided and that
everything was made up air, water, earth or fire.
v 1800s- John DALTON‛S theory states that all matter is
made up of individual particles called atoms, which cannot be
divided. DALTON studied gases & found that gas consists of
individual particles & that all compounds have fixed composition
(always combine in the same ratio!)
All elements are composed of atoms, pictured as solid
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spheres.
All atoms of the same element have the same mass, and
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atoms of different elements have different masses.
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Compounds contain atoms of more than one element.
In a particular compound, atoms of different elements always
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combine in the same way.
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v J.J. THOMPSON‛S theory describes the atom as a positively
charged sphere with negatively charged particles spread evenly
throughout. (plum pudding or chocolate chip ice cream model.)
He was the first to discover ELECTRONS!
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Objects with like charges repel and opposite charges attract.
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A flow of charged particles is an electric current
Thompson used a cathode ray tube where the current flowing
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through was attracted to a positively charged plate outside &
repelled by a negatively charged plate!
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He concluded these “electrons” came from inside the atoms.
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v Ernest RUTHERFORD‛s model has a positively charged
NUCLEUS at the center of the atom.
Alpha particles (Helium nuclei) are fast-moving positive
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particles emitted by uranium & other elements.
GOLD FOIL EXPERIMENT- When alpha particles were shot
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at thin gold foil, they were deflected in surprising manner (some
came straight back.)
The POSITIVE charge is concentrated in the NUCLEUS in
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the center of the atom.
The total volume of an atom is 1012 times the volume of its
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nucleus!
Atoms are neutral- having both positively and negatively
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charged particles.
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v BOHR s model is like the solar system model with fixed
electron orbits around the nucleus.
o
Each electron in an atom has a specific amount of energy.
o If an atom gains or loses energy the energy of an electron
can change.
v ELECTRON CLOUD MODEL is used to describe the probable
location of electrons in an atom (i.e. propeller of a plane.)
o The possible energies an electron can have are called energy
levels.
• The electron cloud represents all the orbitals in an atom. An
ORBITAL is a region of space around the nucleus where an
electron is likely to be found.
o An electron in an atom can move from one energy level to
another when the atom gains or loses energy.
• The level in which an electron has the least energy (lowest
energy level) has only 1 orbital.
Ø Scientists can measure the energy absorbed or released
when energy is gained or lost and electrons move to a higher or
lower energy level.
•
Higher energy levels have more than one orbital
When fireworks explode we are seeing the energy released as
light when the electrons (which moved up during the explosion)
move back down. Different elements emit different colors based
on their energy levels
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SUBATOMIC PARTICLES:
ELEMENTS IDENTIFYING NUMBERS:
PROTONSà (p+) positively charged particles in the nucleus of
an atom. Each element has a different amount of protons which
identify it! +1
ATOMIC NUMBER = number of protons in an element. It never
varies, and identifies what element it is. In neutral atoms, the
number of protons = number of electrons.
ELECTRONS à (e -) Negatively charged particles found in the
space outside the nucleus. -1 Mass is 1/1836 of a proton or
neutron.
MASS NUMBER = Sum of Protons + Neutrons. (You can
subtract the atomic number from the mass number to calculate
the number of neutrons in an atom.)
NEUTRONS à (n) neutral (no charge) particle found in the
nucleus with a mass almost the same as a proton.
4
Be
9
atomic
number
Protons = 4
Neutrons = 5 (9-4)
Electrons = 4 assuming neutral atom
mass number
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Ø ISOTOPES: Atoms of the same element (same # of protons
or same atomic number) but different MASS numbers because
they have different numbers of neutrons. Isotopes are written
with a DASH and then the different mass number:
ELECTRON CONFIGURATION = the arrangement of electrons
in the orbitals (shells) of an atom. The most stable electron
configuration is where the electrons are in orbitals with the
lowest possible energies (called Ground State.)
Be-10 is a Be isotope with a mass number of 10. Its atomic
number or number of protons NEVER changes, so that is 4.
Since it‛s an isotope with a mass number of 10, it has 6 neutrons
instead of the usual 5 neutrons (10 - 4 = 6). We can assume it is
still a neutral atom, so the electrons = protons 4.
Example: Lithium has 3 electrons - 2 in the first energy level and
1 in the second. If the atom absorbs enough energy, one of its
electrons can move to an orbital with a higher energy or an
excited state. It is less stable and will eventually lose energy
and return to ground state. When He, Ne, Ar, Kr & Xe return
from excited to ground state they emit the light seen in “neon”
lights!
Most elements‛ isotopes show no difference except water. Most
water is made with H-1 (normal and most common Hydrogen.)
“Heavy water” is made with H-2 which has slightly different
properties.
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Electron Configuration & IONS
Electrons are positioned in the atom (in “energy levels”, or
“shells”) based on the following rules:
Ø
The ENERGY LEVELS (SHELLS) fill from the inside out.
Ø The first shell (closest to the nucleus) fills first with a max
of 2.
Ø VALENCE ELECTRONS = the electrons in the outermost
energy level of an atom. (NEVER MORE THAN 8!)
Ø VALENCE ELECTRONS determine the chemical properties
and ability to form bonds with other atoms.
Ø An atom is happiest or MOST STABLE when its outer shell
is full or empty (and if it is empty it is gone so the shell inside of
it is now the outermost shell and is full!)
Ø The rest of the energy shells vary, but due to the way they
fill up their shells, the OUTERMOST shell can only have a MAX
OF 8!!
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Ø Atoms that do not have a FILLED OUTERSHELL may
undergo IONIZATION
-->the gaining or losing of electrons (e-)
Ø When an atom gains an electron or gives up an electron it
becomes CHARGED (because it‛s # of protons and electrons is
no longer the same) and we call it an ION.
Ø Since Na does NOT have a full or empty outer shell, it is
NOT “happy.”
Ø Atoms always do what is EASIER, so Na gives up the 1
electron in its 3rd shell (instead of trying to gain 7 electrons.)
For example:
Sodium (Na) has an atomic # of 11.
**(Atomic NUMBER ALWAYS = # of
PROTONS!!)
Ø Sodium now has only 10 negatively charged electrons, but
still has 11 positively charged protons.
Ø Since it has ONE MORE positively charged proton than
negatively charged electrons, it is a positive one ION --> Na+1
Which means that Na has protons and ___ electrons
Following our rules, ____ of those 11 electrons would go in the
first energy level, ____ of those left would go in the 2nd energy
level and _____ would be left to go in the 3rd level
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ION = an atom that carries an electrical charge because of the
loss or gain of electrons.
ü
ü
Positive IONS have LOST electrons
Negative IONS have GAINED electrons
What kind of ion do you think the following atoms would become
in order to have a full or empty outer shell?
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