Lecture 6-7 Atoms, Elements, Isotopes, the Mol Energy Crisis Chem 021

Lecture 6-7 – Atoms, elements, isotopes, the mol Energy Crisis – Chem 021

This lecture introduces the building blocks of matter. Whether air, gasoline, coal or nuclear fuel, all consist of atoms. There are only a slightly over 110 different atoms in the world not counting isotopes. (More if counting isotopes.) A few common atoms are N, O (the major components of air), C, H (major components of natural gas and oil), Si and O (as in sand). Material consisting of only one kind of atom is called an element. Different atoms combine to form molecules.

Distribution of elements in %, by weigh not atom %: in the earth’s crust In the human body O 50 O 65 Si 26 C 18 Al 8 H 10 Fe 5 Ca 3 The approximate abundances of some other important elements relative to Si in the earth’s crust: P 10-5 C 10-6 N 10-7 H 10-5 U 10-8 Au 10-10 He 10-6, but much more abundant in the solar system than Si.

Atoms either form molecules, such as is the case for the most abundant components of air: N2 (78%), O2 (21%). The rest of air consists of other elements and small molecules including water (H2O) and CO2 [reminder about the meaning of subscripts in chemical formulas]. All matter (gas, liquid or solid) consists of molecules which in turn consist of atoms. Examples, such as table salt (NaCl), iron ore (mostly iron oxides, Fe2O3 or Fe3O4), glasses, sand (mostly SiO2). Silicates comprise 90% of the earth crust), e.g. cement (Aluminum-silicates plus CaO). Plastics are large molecules, called polymers, (mostly C, H, some O, N, S), etc. Complexity of studying chemistry: the valency of some elements in not entirely fixed, e.g. Fe as in the above example.

All atoms of the same kind are identical providing a lot of exact knowledge. (E.g. there are 2 electrons, 2 protons and 2 neutrons in 4He, which completely determine its properties.) All 4He atoms are identical, but are different from all the others. From a chemical point of view, the isotope number (number of neutrons) is not important, only the atomic number (A, number of protons = number of electrons) matters.

The exact identity of atomic particles, such as electrons, nuclei, atoms, and molecules, is one of the basic points of quantum mechanics, a field of science that studies the law of the very small. The identity stated on an example: all He atoms are identical, though different from all C atoms, which in turn are all identical, they are not distinguishable.

1 [Comments on the key difference between the laws of the very small and that of larger, everyday objects. Former are not distinguishable, latter are.]

Atoms consist of Z number of electrons and a nucleus. The electrons determine the properties of the atom, and so the atomic symbol reflects Z: e.g. H for Z=1, He, for Z=2,a d C for Z=6. The nucleus consists of Z number of protons and N neutrons. The charge of the electron is – qe (negative), that charge of the proton is qe (positive). [The numerical value, which as very small, is given in the book.] The two kinds of charges exactly compensate each other, and so atoms are neutral. The sum of Z and N is often called the mass number (A), as mentioned on p. 174 of the textbook.

A=Z+N.

Notation for differentiating isotopes can be based on either A or N or both.

A A Z ElementN or, omitting the redundant N=A-Z, we have: Z Element (Even Z is redundant, since the element determines Z)

Practice: How many electrons (Z), protons (Z) and neutrons are in 235U (this is the isotope that can be used as nuclear fuel, to be discussed later) and in 238U (which is the more abundant isotope)?

Z=92 from the PT for Uranium, so there are 92 electrons and protons in both 235U and in 238U. There are A-Z neutrons, 235-92=143 in 235U and 238-92=146 in 238U.

Electrons ‘revolve’ or ‘orbit’ around the very small nucleus. Without going into the details of electronic structure, electrons occur in well-organized shells. The outermost shell determines the chemical properties of the atoms. The consequence of this causal (not casual) relationship is that elements in the same column of the periodic table have similar (some cases very similar, other cases somewhat similar) properties. E.g. all elements in column 1 (modern, IUPAC, notation) have one electron in the outermost shell giving rise to their property that they only form one bond. This column-by-column chemical similarity leads to the names of the elements in columns, e.g. Col. 1: alkali metals, col. 17: halogens, or col 18: noble gases, which form weak bonds or no bonds with other atoms.

Most elements are metallic, except for H (except under enormous pressures), and the elements between the C – Rn line and above. (11 elements plus 6 noble gases). C has the richest, most varied chemistry, sometimes called the element of life. It is the main component of fossil fuels, mostly originating from dead but earlier live matter, since almost all fossil fuels are left over material that some time ago formed living matter. For a recent discovery on methane of non-biological origin, see: http://www.nytimes.com/2004/09/14/science/14meth.html

A few examples of elements and their properties.

2 The mol. is a certain, very large number of atoms of the same kind or the same number of molecules of the same kind. This number NA (Avogadro’s number), rounded to 4 digits, is 6.022 x 1023. Sometimes we will round this to 6 x 1023. Knowing this number, we can do simple calculations in order to determine the quantity of matter in a process, or heat generated in a process involving atoms or molecules, because many data in tables and other sources refer to quantities of mol when providing information. This is arbitrary, but has been universally adopted in science. (This is why NA has been determined up to 9-10 figures, the very small differences do not concern us, but my be of importance in very accurate experiments.)

The reason that the number chosen is so large (6*1023= billion * billion * 600,000) is that the numbers in tables refer not to the tiny values corresponding to atomic/molecular events (e.g. the heat generated by burning on C atom) but rather to sizes of everyday objects.

E.g. how much does one mol of C atoms weigh (e.g in the form of diamond or graphite, the two elemental forms of carbon). We need the value of the atomic mass unit, amu = 1.66E-24 grams, as expected, a very small number compared to the mass of ordinary things.

NAvogadro * ACarbon [in amu] * g/amu = 12 grams = 0.42 oz.

12.011 1.66E- 6.02E+23 amu 24g/amu 12.01g 0.42 oz

Note that the mol amount of atoms in grams is the same number as the Atomic number. This is in fact the basis of the arbitrary choice of the size of the mol.

Other examples:

NAvogadro ACarbon * amu = 12 grams 1.66E- Fe 6.02E+23 55.85 24 55.85 1.66E- U 6.02E+23 238.03 24 238.03 1.66E- He 6.02E+23 4 24 4.00

We need the concept of the mol to be able to answer questions like these: How much more heat is generated by a certain amount of fueld (e.g. 1 lb of coal) compared to some other fuel. We will do these calculations in the next couple of classes.

Examples and some practical chemistry information on a few elements and a few of their compounds.

Compounds have properties that bear no direct relationship to the constituent elements.

3 Hydrogen: One bond, forms water with oxygen. H2 is a gas under ‘normal’ pressure and temperature. [‘normal’ has an exact definition, we don’t need it here] Oxygen: two bonds, form H2O, CO2, and other oxides, needed for most combustion reactions when heat energy is generated from burning fuel, such as coal, oil products, and natural gas. O2 is a gas under ‘normal’ pressure and temperature Carbon: Four bonds. A few examples: CO2, needed by plants for photosynthesis. They use the energy of the Sun’s EM radiation energy (light) to break up the bonds in CO2 and make the carbon available for use in the cells. In turn, we use the fossil fuels to burn and generate energy and produce CO2. CO2 is a gas under ‘normal’ pressure and temperature. A few calculations:

CO: carbon monoxide. (Violates the rules of valence: 2 ‘unsatisfied’ bonds on C in this molecule), generated by ‘incomplete’ burning of C containing fuels, it is a poison.

Hydrocarbons: Molecules consisting of only C and H: CnHm. Examples of hydrocarbons:

CH4: methane. Highly symmetrical molecule, can be represented by a tetrahedron, one of the Platonic solids. [ http://www.friesian.com/polyhedr.htm ] How many oxygen molecules are needed to burn one molecule of methane?

CH4 + xO2  2H2O + CO2 + heat For more reading about methane, see http://scifun.chem.wisc.edu/chemweek/methane/methane.html (not required material).

Ethane is the next of the saturated hydrocarbons, so called alkanes. Discuss the first 4 members, with their names. General formula is CnH2n+2. See Table 2.4 for their names and which ones occur in natural gas (n=1-4) and gasoline (n=5,8).

A calculation with octane, a major component of gasoline, C8H18.

First the oxidation reaction:

C8H18 + xO2  9H2O + 8CO2 + heat

From this, x=8 + 9/2 = 12.5. In such cases, it is customary to recalculate the equation so that only whole molecules appear on both sides. Taking 2 octane and 2*12.5 = 25 oxygen molecules will do the trick.

2C8H18 + 25O2  18H2O + 16CO2 + heat [book give the following value for gasoline, which is similar to that of octane: 1.32 *108 J/gallon of gasoline]

[test: conservation of matter requires that there are the same number of atoms of the same kind on both sides, e.g. 16 C atoms.]

4 l gallon of octane produces how many lb’s of CO2? l gallon = 3.78 L(iter)

The density of gasoline is less than that of water, which is 1kg/L=1g/mL. (A drop of gasoline will not mix with water and stay on the top, just like a drop of oil in a bowl of soup.) The actual number is about 0.7 g/mL. This amounts to

d(ensity) = m(ass)/V(olume)  m=d*V, m=0.7kg/L * 3.78L = 2.646 kg

How many mols are in this amount of octane?

The molecular weight of octane is 8*12.011 + 18*1.001 = 114.106

1 mol of octane would be 114.106 g. How many mols are in 2.646 kg = 2,646 g?

2,646 / 114.106 = 23.19 mols.

Every mol of octane produces 8 mols of CO2. We burn 23.19 mols, so the final number of mols of CO2 is

8* 23.19 =185.5 mols of CO2.

How many grams is this?

1 mol weighs 12.011 + 2* 16 = 44.011

Thus our 185.5 mols amount to 44.011*185.5= 8,165 g = 8.165 kg. [Is there a mistake, there is

more CO2 than octane even though the H atoms go into the new water molecules?]

How much heat is generated by burning l gallon of octane?

Assuming that the energy content of octane and gasoline is similar, the value is about 1.32 *108 J. Small deviations occur due to the precise composition of the gasoline and its ‘octane number’. [The latter is related to the engine ‘knock’ and a specific test running motors with and without load that determines this number. The details of how the molecule is burning up matters in this test.]

Alcohols contain –OH groups, one of the –H is replaced by an –OH in a hydrocarbon. E.g. Ethane  ethanol (also called ethyl alcohol):

C2H5OH (Notation retains some structural information by keeping the OH group separately, the chemical formula is not a simply C2H6O).

For an important mammalian energy source, glucose, the heat of combustion is:

C6H12O6 + 6O2  6H2O + 6CO2 + 2,800 kJ [2,800 kJ=2.80MJ (for one mol)]

Fuel values of some common fuels [in kJ/g] ______Wood 18

5 Coals 31-34 Texas Crude 45 Gasoline 48 Natural gas 49 Hydrogen 142 .

Note that 1g of hydrogen produces a lot more energy than 1g of gasoline, but the number of mols in 1 g of hydrogen is much larger, due to its very small molecular mass. (What is the molecular mass of hydrogen?)

Skills: Use of the Periodic Table (PT)

Starting with the symbol of the element, be able to find its location (row and column) in the PT. How many electron and protons does the atom of the element have? What is the name of the element? Which other elements are likely to have similar chemical properties? What other properties can you obtain from the PT? What are these concepts: atomic number, atomic weight, Memorize the atomic symbols of the following elements important for this course: C, H, O, N, S, Si, Se, He, D, T, F, Na, Fe, U, Pt, Cs, Th, P, Pb.

______Interesting web sites about the PT: My top choice is: http://pubs.acs.org/cen/80th/elements.html which provides well written short articles about each element in the PT. You need a GU ID number and use a computer on campus or the library proxy server. (For info on the library proxy server, see the library web site or see a reference librarian or a science librarian).

Other sites: http://www-tech.mit.edu/Chemicool/ (In addition to physical and chemical data, you may find prices for most elements.) http://www.chemistry.org/portal/a/c/s/1/acsdisplay.html?DOC=sitetools %5Cperiodic_table.html# (This site requires ‘Macromedia Shockwave ®’ but does not offer much more.)

I have used tables and data from “Chemistry, the Central Science” by T.L. Brown et al. 9th edition, 2003.