• 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’ meant.

• 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 laboratory.”

Standard PS-2

• The student will demonstrate an understanding of the structure and properties of atoms.

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 “electrons”

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 levels”

• 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 accurate

Standard PS-2

• PS-2.2: Illustrate the fact that the atoms of elements exist as stable and unstable isotopes.

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

neutrons• Number of Protons + Number of Neutrons

= “Mass Number”

• Mass number will be different for each isotope

Standard PS-2

• Symbolizing Mass Numbers to represent isotopes:– Mass numbers NOT found on periodic table,

usually– 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(not needed, if periodic table around)

Subscript, left

C126

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 &

neutrons)– Unstable nucleus wants to become stable, so

the nucleus emits particles…Radioactive Decay

• “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 environment.– 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 isotope)

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-2http://ed.sc.gov/agency/offices/assessment/Programs/endofcourse/periodtable022304.pdf

• Note – Period numbers– Group numbers– Lanthanides/Actinides– 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• Lanthanides• Actinides

Standard PS-2

• Valence electrons (outermost number of electrons)– Group number Valance electrons

1 1

2 2

13 3

14 4

15 516 6

17 7

18 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