Chapter 4.2 Structure of Atoms
Parts of an Atom
Nucleus – center of atom, contains positive charge Protons, and Nuetrons have have no
charge.
Negative charged Electrons are orbiting the nucleus in an electron cloud.
Atoms have a neutral charge overall, the protons positive charge cancels out the electron’s
negative charge. Make some very reactive. Also the opposite charges (electric force) holds
the atom together.
Atoms want to be stable. Either react and combine with other elements/molecules, or
become an ….
Ions- atoms that have gained an electron, become anions. Atoms that have lost an electron
become a cation. Ions are more stable than neutral atoms.
Key Terms to Know
Atomic number – each element has the same number of Protons; Elements have specific
Atomic numbers
Average Atomic Mass – the average mass of an element, represents one mole of a that
substance
Mass Number – the average atomic mass rounded to a whole number; represents the
number of neutrons and protons in the nucleus of an atom
Isotopes – Elements that have a different number of neutrons than the average.
Abundance of isotopes causes the average atomic mass to stray more from the mass
number.
Contrast Carbon vs. Boron and Hydrogen vs. Copper
Molar Math
Unified Atomic Mass – unit of mass that represents 1/12 of the mass of a carbon 12 atom.
(normal carbon)
Mole – SI base unit used to measure small particles; atoms, ions, or molecules
Avogadro’s Number – one mole of a substance equals 6.0223 x 1023 particles
Average atomic mass – one mole of substance equals the average atomic mass in grams
Atom
Mass to Moles
Moles to Mass
diatomic molecules
molecules
4.1 The Development of Atomic History
4th Century Greek Philosopher, Democritus, suggested Universe was made of invisible units
called atoms – “that which cannot be divided.” No evidence, so not many believed him.
Atomic Theory and Models
(grew as a series of models that developed from experimental evidence. As more evidence
was collected, the theory and models were revised)
1808- Dalton –
Smooth Balls
1897- Thomson –
Cookie Dough
1911- Rutherford –
The Peach
John Dalton Atomic Theory
J.J. Thomson and Smaller
Parts
Earnest Rutherford and the
Nucleus
*All elements are composed
of atoms that cannot be
divided
*Atoms of same element are
exactly alike and have the
same mass, different
elements have atoms of
different masses.
*Atoms of one element can’t
be changed into an atom of
a different element.
*Atoms cannot be created or
destroyed in any chemical
experiment, only
rearranged.
*Compounds of atoms are
combined in specific ratios.
Law of Definite Proportions
1897- experimenting with
Cathode Ray Tubes, found
atoms contain negatively
charged particles, then
reasoned due to the neutral
charge of atoms, there must
be a positive charge particle
to balance the charge.
1911- One of Thomson’s
students. Found evidence,
“Gold Foil experiment” to
counter Thomson’s model.
“negative charged particles
scattered throughout a ball
of positive charges”
negative charges particles
later became known as
electrons
*Inferred that the positive
charged particles must be
clustered in a tiny region in
its center, called the nucleus.
*Atom was mostly empty
space with electron moving
around the nucleus in that
space.
Modern Atomic Model
Bohr’s Model
A Cloud of Electrons
1913 - student of both
Thomson and Rutherford
revised model again.
1920’s – electrons do not
orbit in planet like orbits,
instead they can be found
anywhere in a cloudlike
region around the nucleus
called the energy level. The
inner energy level can only
hold 2 electrons, and the
larger outer energy levels
can hold, 8, 16, 32
respectively.
*electrons only have set
energy values, leading to
orbit the nucleus in specific
orbital.
“Onion Model”
J.J. Thomson’s experiment
1932- James Chadwick
discovered the neutron and
completed the Modern
Atomic Model.
*Neutron had no electrical
charge,
Rutherford’s – “Gold Foil” Experiment
Rutherfords Model – The Stadium Analogy
4.3 Electron Configuration
In the Bohr’s modern atomic model, electrons can only be found in certain energy levels, not
between. Electrons must gain energy to move to a higher level and lose energy moving to a
lower level.
A new model, suggested that electrons act more like waves on a string, than like particles.
Thus their exact location cannot be predicted precisely. Scientists can predict the electrons
orbital, the region where there is the greatest probability finding an electron.
4 types of Orbital, each can hold a specific number of electrons.
Orbital
Axis orientation –each axis
holds 2 electrons
s
1
p
3
d
5
f
7
Total electrons
2
6
10
14
#refer to periodic table handout and ptable.com
Electron Transitions
We see because light is “reflected” back to our eyes, but how does this occur?
1. Photons are bundles of electromagnetic energy, each vibrating at a certain
frequency. Higher the frequency, higher the energy.
2. If this frequency is in visible spectrum, the atoms absorb this energy, causing the
electrons to get “excited” and jump from the “ground state” to new energy levels
within the atom now called an “excited state.”
3. Then they immediately return to “ground state,” releasing the energy (photon) they
absorbed at a similar frequency.
4. Our eyes are able to pick up this photons vibration, (frequency) and our brain
interprets it as color!! We see due the hyper active electrons, every think about it?