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• Ira Remsen (1901):
“While reading a textbook of chemistry, I
came upon the statement ‘nitric acid acts
upon copper’, and I determined to see
what that meant.
Having located some nitric acid, I had
only to learn what the words ‘acted upon’
• Ira Remsen (1901):
In the interest of knowledge I was even
willing to sacrifice one of the few copper
cents then in my possession.
I put one of them on the table, open the
bottle labeled ‘nitric acid’ poured some of
the liquid on the copper, and prepared to
make an observation.
• Ira Remsen (1901):
But what was this wonderful thing which I
beheld? The cent was already changed,
and it was no small change either!
A greenish-blue liquid foamed and fumed
over the cent and over the table. The air
became colored dark red. How could I
stop this?
• Ira Remsen (1901):
I tried by picking the cent up and throwing
it out the window. I learned another fact:
nitric acid acts upon fingers.
The pain led to another unpremeditated
experiment. I drew my fingers across my
trousers and discovered nitric acid acts
upon trousers.
• Ira Remsen (1901):
That was the most impressive experiment
I have ever performed. I tell of it even now
with interest. It was a revelation to me.
Plainly the only way to learn about such
remarkable kinds of action is to see the
results, to experiment, to work in the
Standard PS-2
• The student will demonstrate
an understanding of the
structure and properties of
Standard PS-2
• Compare the subatomic particles(protons, neutrons,
electrons) of an atom with regard to mass, location,
and charge, and explain how these particles affect
the properties of an atom (identity, mass, volume,
and reactivity).
– Subatomic particles
• Protons
• Neutrons
• Electrons (“cloud”)
Isotopes (nuclear reactions & radioactivity)
Periodic Table trends (valence & atomic number)
Atomic number/Mass number
Formation of ions
Fission/Fusion (Mass defect, Chain reactions, critical mass)
Nuclear Applications (power, medicine, weapons)
Standard PS-2
• PS-2.1: Compare the subatomic particles
(protons, neutrons, electrons) of an atom
with regard to mass, location, and charge,
and explain how these particles affect the
properties of an atom (including identity,
mass, volume, and reactivity).
Standard PS-2
• For the chemist:
– Matter: the physical material of the universe
• All matter made of a 100 or so elements
– Atoms: smallest particle representative of the element
Standard PS-2
• Structure of the atom (Sub-atomic
particles, fundamental particles):
– Protons (positively charged particle)
– Neutrons (no charge particle)
• Collectively make up the nucleus of the atom
– Electrons (negatively charged particle)
• Exist in a “Cloud” (large region of space)
surrounding the nucleus
Standard PS-2
• Structure of the atom:
– Nucleus: small, dense (most of the weight)
• Protons & neutrons have about same mass
– Electrons: large, light (most of the volume)
• Small mass relative to protons/neutrons
Standard PS-2
• Structure of the atom:
– Protons: positively charged
• For convenience: each is “+1” charge
– Neutrons: no charge
• “0” charge
– Electrons: negatively charged
• For convenience: each is “-1” charge
Standard PS-2
• Structure of the atom:
– Nucleus: Protons + Neutrons
• “Charge” of the nucleus = number of protons (positive)
– Example: Helium nucleus
» 2 protons/2 neutrons = +2 nuclear charge
– Electrons (negative)
– Example: Helium
» 2 electrons = -2 electron charge
– Atoms are neutral (no charge)
• Protons = Electrons
– Ions (are not atoms): charge through loss/gain of
Standard PS-2
• Structure of the atom:
– Attractive force between nucleus & electrons (+/-)
• holds the atom together
– Repulsive forces between electrons (-/-)
• Stay as far apart as possible
– Repulsive forces between protons(+/+)
• Large amount of energy holding nucleus together
Standard PS-2
• Nucleus: small volume
• Electron: “cloud”, large volume
– Broken down into “regions of space” called “energy
• Electrons are moving about in these energy levels
– “Regions of space” further from the nucleus have
higher energy
• Stretch a spring
• Electron “moves” to a region of space with “higher energy”
requires energy input
• Electron “moves” to a region of space with “lower energy”,
excess energy given off
Standard PS-2
• Energy Levels (regions of space)
– Electrons repel (-/-)
– Farther regions of space are larger (also higher
energy levels) can accommodate more electrons
Level 1: 2 electrons
Level 2: 8 electrons
Level 3: 18 electrons
Level 4: 32 electrons
Standard PS-2
• Element: one type of atom
– The “type” of atoms – defined by the number
of protons
• Carbon – always has 6 protons
– Neutrons may vary
• Nitrogen – always has 7 protons
– Neutrons may vary
Standard PS-2
• Element: defined number of protons
– Vary number neutrons  isotopes
• Hydrogen  1 proton
• 1 proton + 1 neutron  isotope of hydrogen, “Deuterium”
• Fluorine (F) : 9 protons, 9 electrons (10
neutrons, only natural isotope)
– Group 17: gain 1 electron
– F- (fluoride): 9 protons, 10 electrons …….
– If we changed number of protons, element changes
• Alchemy
Standard PS-2
• Again, particle mass:
– Protons about same as neutrons
– Electrons much lighter (almost insignificant)
• Chemical Reactions:
– Changes in electrons, nucleus remains unchanged
• Formation of ionic bond/covalent bond
• Formation/Splitting of water
– Same number & type of elements before and after
– 2H2 + O2  2H2O (4 hydrogen & 2 oxygen before & after)
Standard PS-2
• Reactivity: (look at electrons)
– Electrons occupy most of the volume, but
almost none of the mass
– Higher energy electrons (furthest from
nucleus) are MOST reactive
– Number of Electrons farthest from nucleus
determine most of the reactivity
Standard PS-2
Microsoft Clip Art: “atom”….useful, but not
Standard PS-2
• PS-2.2: Illustrate the fact that the atoms of
elements exist as stable and unstable
Standard PS-2
• Nucleus: Protons and Neutrons
– Number of protons: define the element
• Usually found on periodic table
• “Atomic Number” = number of protons
• Increases across a row
– Number of neutrons: vary (isotopes)
• Certain numbers of neutrons are stable
• Generally most stable nuclei when number of
protons and neutrons are about the same
Standard PS-2
• Belt of Stability
• Outside the “Belt”, nucleus is not stable
– Unstable nucleus: “radioactive” isotope
– Not all isotopes are “radioactive”, only the
unstable ones
• Hydrogen & Deuterium are both stable isotopes 
neither is radioactive
Standard PS-3
• Isotopes
– Same number of protons
– Varying number of neutrons
– To differentiate isotopes, sum the protons and
• Number of Protons + Number of Neutrons
= “Mass Number”
• Mass number will be different for each isotope
Standard PS-2
• Symbolizing Mass Numbers to represent
– Mass numbers NOT found on periodic table,
– Mass numbers not really a “mass”
• 12C (mass number written superscript, left)
• Carbon-12 or C-12
Standard PS-2
• Symbolizing Mass Numbers:
the “atomic number” may be added 6
(not needed, if periodic table around)
Subscript, left
Standard PS-2
• Atomic Mass: is the “mass” found on the
periodic table
– Not related to mass number
– “Weighted average” of all isotopes
Standard PS-2
• Example:
• 99% of carbon atoms are C-12
» Mass C-12 is 12.000
• 1% of carbon atoms are C-13
» Mass C-13 is 13.100
Weighted average:
(0.99 * 12.000) + (0.01 * 13.100) = 12.011
Standard PS-2
• Periodic Table
– Carbon  12.011
• By the way….our system of atomic mass
is based on assigning the carbon-12
isotope to an “exact” mass of 12.000……
i.e. – it’s the atomic mass standard
Standard PS-2
• Radioactive isotopes:
– Unstable nucleus (combination of protons &
– Unstable nucleus wants to become stable, so
the nucleus emits particles…Radioactive
• “Radiation” is the emitted particles
Standard PS-2
• Types of particles:
– Alpha particles (2 protons/2neutrons)
• Same as a helium nucleus
• Alpha particle might be written as an “He-4”
– Beta particles (electron)
• Wait a minute…electrons from the nucleus?
• Think of neutron (0 charge) as a proton+electron
• Beta particle  converts a neutron to a proton and
the electron is given off
• Might be written as “e-”
Standard PS-2
– High energy gamma (light) rays
– “Rate of decay” can be short (seconds or
fractions) or long (millions/billions years)
• Characterized by the “Half-life” – time it takes to
cut the concentration in half
– 1  ½  ¼  etc
– Radioactive Rate of Decay
Standard PS-2
• Radioactive isotopes are natural in our
– Concentrated radioactive isotopes
• 20 kg U-238 (around 40 lbs)
• After 1 billion years, 17 kg (34 lbs) remain
– Other 3 kg is now Thorium-234 (another radioactive
Standard PS-2
• PS-2.3: Explain the trends of the periodic
table based on the elements’ valence
electrons and atomic numbers.
Standard PS-2
• Valance vs. Valence Electrons
– Valance is the number of bonds an element
will form
• Example:
– Carbon – valence of 4 (forms 4 bonds)
– Oxygen – valence of 2 (forms 2 bonds)
– Hydrogen – valence of 1 (forms 1 bond)
– Valence electrons are the total number of
electrons in the outermost energy level
Standard PS-2
• Periodic Table
– Group: “Column”
– Period: “Row”
– Metals  left
– Non-metals  right
– Metalloids  diagnoal
Standard PS-2
• Note
Period numbers
Group numbers
For an element:
Atomic symbol
Atomic number
Element name
Atomic mass
Standard PS-2
• Other information:
– For neutral atoms,
number protons = number electrons
– Number of “Energy levels” occupied = period number
– Groups with names:
Group 1: Alkali metals
Group 2: Alkaline Earth metals
Group 16: Oxygen group
Group 17: Halogens
Group 18: Nobel Gases (Inert gases)
Transition metals
Standard PS-2
• Valence electrons (outermost number of
– Group number
Valance electrons
8 (except He)
Standard PS-2
• For the most part, ignore transition metals
– Periods below 3, that is 4-7
– Work with what are sometimes called “Main
Group” elements
• Periodic table trends:
– Valence increases across a period
– Within a group, valence is the same
Standard PS-2
• Periodic table trends:
– Energy level same across a period
– Energy level increases down a group