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Honors Chemistry Unit 2 Study Guide
The Atom
Symbolic representation for an element:
A
E where E represents any element,
-Z represents its’ Atomic Number = # of protons(and electrons in a
neutral atom), and
-A represents its’ Mass Number = The atomic mass rounded to a
whole number.
-The number of neutrons in the atom is calculated by subtracting
A – Z.
z
The structure of the atom:
Nucleus = A very small, dense, central region of the atom that contains the two
subatomic particles, the protons and neutrons.
- Another name for the protons and neutrons collectively is nucleons.
Proton = A positively charged particle with an atomic mass of 1.0 amu.
Neutron = An uncharged or neutral particle with an atomic mass of 1.0 amu.
Electron = A negatively charged subatomic particle that moves outside of the
nucleus and has little to no mass.
Particle
Symbols
1
p, 1H
proton
1
neutron
Relative
electric
charge
+1
Mass #
1
Relative
mass
(amu)
1.007276
Actual Mass
(kg)
1.673 x 10
-27
0
1
1.008665
1.675 x 10
-27
-1
0
0.0005486
9.109 x 10
-31
1
1
n
0
electron
-
e-, 0e, 0,
-1
-1
The numerical value of the charge of an electron is -1.602 x 10-19 Coulombs.
Electrons occupy regions of probability known as clouds or orbitals.
An atoms electron cloud is very large compared with the size of the nucleus.
-
Electrons possess energy; this is how they repel the pull inward from the
oppositely charged nucleus.
Neils Bohr came to this conclusion after studying the bright line emission
spectrum of hydrogen gas.
Waves
– They are repetitive in nature.
- Waves can be :
Diffracted/Refracted = Bent.
Absorbed = Taken in by what they strike.
Reflected = Sent back from what they strike.
show Interference = Superimposed to form a resulting wave of
greater or lesser amplitude or size.
- Wavelength () = The distance between corresponding points on
adjacent waves.
- Frequency (or f) = Number of waves that pass a given point in
a specific time (one wave /second = Hertz, Hz).
-  and  are related mathematically by the following equation:
c= 
Where c is the speed of light and equal to 3.00 x 108 m/s
- c is constant for all electromagnetic radiation and so  and  are
inversely proportional to one another ie. as increases, 
decreases.
Electromagnetic Radiation = A form of energy that show wavelike behavior when
travelling through space.
- This energy takes many forms.
- Electromagnetic Spectrum = All the forms of electromagnetic
radiation.
- In order of increasing wavelength they are: gamma rays, X-rays,
Ultraviolet rays(UV), visible light (this is the ROYGBIV rainbow
in reverse), Infrared rays, Microwaves, Radiowaves.
-
Bright Line–Emission Spectra = Characteristic pattern of lines viewed
through a prism and formed when an excited atom returns to ground state.
Electrons can be excited by passing an electric current through a vacuum
tube containing only the gas in question.
Ground state = Lowest energy state for the electrons in an atom.
Excited state= State in which an atom has higher potential energy than it
has in the ground state.
Bohr compared white light with that of hydrogen gas.
The H spectrum had 4 distinct lines. He believed that these lines were
formed as a result of H’s one electron gaining energy and jumping to outer
-
energy levels. Upon falling back to a lower energy level, light energy would
be emitted that corresponds to the energy difference between the levels.
Since electrons possess specific amounts of energy, certain colors of
specific frequencies are seen according to Planck’s equation (see below).
The electron is believed to behave as both a wave and a particle.
This is only true for very small particles.
See the history behind this theory below.
Historical Background:
Max Planck helped develop the Quantum Theory of the atom. He explained that
energy is transferred in small discrete packets known as quanta and not just as a
continuous wave. This was referred to as the particle theory of light. Planck
studied the emission of light from hot objects and found that they do not emit
energy continuously (wave-like), rather only when they have a specific amount of
energy. Planck proposed the following relationship between a quantum of energy
and the frequency of the radiation:
E = h
Where E is the energy of the quantum of radiation,  is the frequency of the
emitted radiation and h is Planck’s constant, which is equal to 6.626 x 10-34J.s
So, different from the past, light was now seen as both a wave and a particle and
these two are not mutually exclusive. Bohr later transferred this idea to explain
the electrons emission spectrum.
Albert Einstein explained the photoelectric effect using Planck’s idea about quanta
and called the packets of light photons.
Photoelectric effect = Emission of electrons from a metal when light is shone on it.
Photon = A discrete packet of light energy.
- Einstein reasoned that photons contained specific amounts of energy and that
light is absorbed by matter only in whole numbers of photons. In order for an
electron to be ejected from a metal surface, the electron must be struck by a
single photon possessing at least the minimum energy required to knock the
electron loose. Energy is related to frequency by Planck’s equation. Electrons in
different metals are bound more or less tightly and so require different minimum
frequencies to exhibit the photoelectric effect. (Mass and energy are equivalent
and related: E = mc2. Where E is the energy of the quantum of radiation, m is mass
and c is the speed of light, which is equal to 3.00 x 108 m/s)
- Erwin Shrodinger developed the equations that gave us the quantum numbers that
describe the 3D position of the electron in the atom.
- The solutions to his equation are known as wave functions.
- He established the field of wave mechanics that was the basis for the
development of the quantum model of the atom.
- This current model of the atom is also referred to as the Quantum Mechanical
Model/Charge Cloud Model/Orbital Model.
Law of Conservation of Mass = Mass is neither created nor destroyed during
ordinary chemical reactions nor physical changes. (Lavoisier)
Law of Definite Proportions/Composition = Chemical compounds always contain the
same elements in exactly the same proportions by mass regardless of the amount or
source of the sample. (Proust)
EX. NaCl always contain 39.34% by mass of Na and 60.66% by mass of Cl.
Law of Multiple Proportions = If two or more different compounds are formed from
the same 2 elements, then the ratio of the masses of the 2nd element combined
with a certain mass of the 1st element is always a ratio of small whole numbers.
(Dalton)
EX. CO and CO2 each contain C and O. The ratio of O:C in CO2 is 32:12 = 2.66.
The ratio of the mass of O:C in CO is 16:12 = 1.33. 2.66:1.33 = 2:1.
John Dalton studied the weather. He looked at mixtures of gases and measured
the mass of atoms. He explained the three laws stated above with his Atomic
Theory.
1. All matter is composed of small particles called atoms.
2. Atoms of the same element are all identical; atoms of different elements
are different.
3. Atoms can’t be subdivided, created nor destroyed.
4. Atoms of different elements combine in simple whole number ratios to form
compounds.
5. In a chemical reaction, atoms are combined, separated or rearranged.
His work resulted in the first quantitative evidence for discrete particles or atoms.
He created a model of the atom resembling a billiard ball, which is solid and
circular. Today we know that #2 of Dalton’s Atomic Theory is not true due to the
existence of isotopes.
Isotope/nuclide = Atoms of the same element that differ in their number of
neutrons and hence their mass.
Also, #3 is not true due to the fact that atoms are made of subatomic particles
such as protons, etc.. and nuclear chemistry reactions do subdivide the atom.
Isotope examples:
Ex. protium H-1
deuterium H-2
tritium H-3
(In this form of notation, the number following the hyphen is the mass
number, A.)
21
22
Ex. 20Ne
90.92%
Ne
0.26%
Ne
8.82%
10
10
10
To calculate the atomic mass of Ne, we take a weighted average of these
masses.
20 x .9092 =18.18 amu
21 x .0026 = 0.055 amu
+ 22 x .0882 = 1.94 amu
20. 18 amu
Atomic Weight/Mass = Weighted average value of the masses of the
isotopes in a naturally occurring sample of the element. (Isotope ratios are
constant throughout nature).
- Atomic mass is not the actual mass but Carbon was
arbitrarily chosen as a standard and all other element masses
were calculated relative to carbon by looking at the mass %
ratio in binary compounds (2 element compounds).
- The unit used to express atomic mass is not a real unit since
the masses are calculated and not measured.
- The unit is an amu = atomic mass unit.
Compound=Made from the atoms of 2 or more substances that are
chemically bonded. Ex. glucose(C6H12O6).
There are 2 classifications of compounds:
Formula Unit = A compound formed by an ionic bond (atoms become ions then
stick together electrostatically due to their opposite charge) between a
metal and a nonmetal.
Molecule = A compound formed by the covalent bond (sharing of electrons)
between two nonmetals.
For compounds:
Formula Mass = Sum of the atomic masses of all the atoms represented by
the formula of a compound.
Ex. formula mass of H2 is 2.02amu and is calculated as follows:
H
2 X 1.008 amu = 2.016 amu
Molecular Mass = Sum of the atomic masses of all of the atoms represented
by the formula of a molecule.
Ex. molecular mass of H2O is 18.015 amu
Gram atomic mass/gram-atom = The mass of an element in grams numerically
equal to its atomic mass.
Ex. The gram atomic mass of H is 1.01 g
- There are 2 ways to say this:
1. The gram atomic mass of H is 1.01 g
2. One gram atom of H is 1.01 g
For compounds:
Gram formula mass = The mass of compound in grams numerically equal to its
formula mass.
Gram molecular mass = The mass of molecule in grams numerically equal to
its molecular mass.