Chapter 2--The Chemical View of Matter

Elements, Compounds, Matter, Balance

Last revised: Monday, January 15, 2007--Paul S. Russo

Overview

After a brief introduction to chemistry’s place compared to the other sciences, we introduce atoms and molecules. We do not yet peek inside the atom (which was once supposed to be impossible) to see what makes it tick, or figure out how they combine to make molecules. We do try to distinguish between molecular forms of matter and compounds, mixtures and solutions. The various atoms, called elements when in pure form with no "different" atoms, are periodic. If you made a list of them and sorted that list according to the mass of the atom, you would find that trends repeat. For example, metal atoms that bind one fluorine atom occur at mass 7, 23, and 39 (the units of mass are funny here). These atoms, Lithium, Sodium and Potassium, are separated by 16 mass units. They lie in a single column of our vaunted periodic table. The chapter continues with an introduction to mass and phase properties of matter. It concludes with some info about measurements and how to perform calculations.

Chemistry’s Place and The Jargon of Science

• The "Central Science" because it affects so many others. The reason it's the central science is because of the size scale of chemistry. When matter is smaller than the atoms or molecules that chemists deal with, it's very hard to comprehend. This is the realm of physics. When matter is much larger than atoms or molecules, it's the realm of engineers. So it's "central" in the sense that it is "in between". The chemical regime is the region where most people can understand and manipulate matter.
• Like any science, based on observations both

        qualitative (it's cold today)

and

        quantitative (it's -20.25 ^oC at 12:45:05 pm)

• Next come hypotheses (reasonable guesses) about HOW things are: hypothesize that acceleration is force divided by mass: a = F/m. Why? The hypothesis is based on observation: small things surely accelerate faster; larger forces help! So put one in the denominator and one in the numerator.
• Next comes experimentation. What experiment would you do to test?
• Laws are made when everyone gets bored with more experiments. No one decides what a scientific law should be beforehand (unlike religion, unlike civil law). Laws are statements of experimental facts that never seem to be violated (yet).
• Theory exists to predict things based on previous laws or observations. They allow y ou to extrapolate the existing knowledge to unknown cases. Theories are ideally rooted in experimental fact, as hypotheses are rooted in simple observations.
• Simulations is a new field. You can do lots of problems by computer: docking of two molecules, diffusion problems, drying of paint, etc. Simulations are more like experiments than like theory.
• Models are various constructs that help us visualize the peculiar world of chemistry. A plastic ‘57 Chevy evokes the real car.

Kinds of Chemistry

• Analytical: very quantitative. What and how much.
• Inorganic: elements > 21. Synthesis and characterization. Theory of bonding, crystals, metals, etc.
• Macromolecular: Large molecules. Can be synthetic or physical or analytical.
• Materials. Can include macromolecules, but also ceramics, films, optical devices, plastic wire, etc.
• Organic: chemistry of C. Mostly synthesis.
• Physical: Asks why? Experiments and theory.
• Zillions of others:  pharmaceutical, geo, agro, environmental, materials....

Chemistry’s Domain is "Fairly Small Stuff"

Chemistry is everywhere, but it is not everything.

Chemists are trained to appreciate and manipulate the world from the atomic scale to the macromolecular scale.

 Nomenclature: Chemical names

It wouldn’t do to go around drawing and coloring molecules all the time--too slow. Thus, names.

• H₂O = water = two atoms of hydrogen per atom of oxygen. It DOES NOT say that these atoms are really joined in a molecule! (However, these two usually are).
• MgCl₂ = magnesium chloride, which is NOT a molecule but a compound.

Difference between molecules and ordinary compounds

...is like difference between marriage and dating. Forsaking all others, these two hydrogen atoms do take this one oxygen atom forever to make a long-term relationship (marriage = molecule). There is bigamy in molecules: two hydrogens can combine with one oxygen. This assembly never seems to take on another part of an atom. For example, there are not 2.5 hydrogen atoms combined with one oxygen. The "divorce" of molecules can be accomplished, and they can remarry. To many chemists, such reactions define the subject. But until the divorce, the molecules really are something special among compounds--they go together.

Non-molecular compounds are less particular. In MgCl₂ two chlorines want ANY one magnesium. They can easily be made to swap: just dissolve the MgCl₂ in water and dry it down again. All the partners will have changed (almost). However, the bigamy will remain: it will still be two chlorines to one magnesium.

When considering whether a substance is molecular or a compound form, ask this question:

with reasonable effort, could I obtain a "piece" of matter that contains just one atom of every type indicated by the formula?

For water, the answer is "yes". With reasonable effort (boil it!) you can obtain a "piece" (called a molecule) which has one oxygen atom and two hydrogen atoms like the formula requires. For something like MgCl₂ the answer is "no". Anything you can easily do will result in a large piece that contains a huge number of Mg atoms and twice that many Cl atoms. If you heated a chunk of MgCl₂ enough, it would not boil to give MgCl₂ molecules. With luck, it might crack (when you cool it down again, for example) into smaller pieces, but these would still contain many Mg and Cl atoms.

Elemental Forms

If all the atoms in a given sample of "material" (we have yet to define this) are "the same" then the chemical is said to be an elemental form. Later, we will see that "almost the same" is what really counts. Diamond and graphite (the "lead" in your pencil) are both elemental forms containing (when pure) no atoms except carbon atoms. It obviously matters A LOT how the atoms are arranged, since you cannot propose marriage with a pencil.

Definition of matter

• Matter: has mass and occupies space. Duh!
• Mass: how easy is it to accelerate the object. F = ma
• Acceleration: change in speed with time (dv/dt = d²x/dt² for you Calculus buffs)
• Gravity provides a convenient, relatively constant, acceleration: F = mg where g = 9.8 m-s^-2
• We could define one gram as the mass of one cubic centimeter of water. Then see how much a spring is compressed with this mass on it under Earth’s field. If spring operates linearly an object that compresses it 2x more than the cubic cc of water is 2 grams.(F = kx where x is the displacement of the spring from equilibrium; this is Hooke’s law in Physics).
• When you weigh something, you indirectly obtain its mass. The real thing being measured is a force, which has other units (kg-m-s^-2).
• Click to See the truth about grams!

Compute the force (in newtons) exerted by your own body mass (in pounds).  Follow the example below:  

Use the space above to make notes.

Space

Well, this is sort of obvious, except that most of things we think are occupied are not really. The inside of most atoms (and most everything else, since atoms cannot overlap) is really empty! Most of everything is actually nothing. Oh well.

Physical States of Matter: Physical vs. Chemical Transformations

• Big 3 are: Liquid, Solid, Gas
• Other, more nebulous states exist: glass, liquid crystal
• Ice to water to steam are physical transformations:  no real change in chemical formula.
• Water electrolysis to Hydrogen and Oxygen gas is a chemical transformation: not only does the formula change, but a molecule is broken.
• In many practical operations, both physical and chemical transformations take place. An example might be an explosion.

Classifying Matter

Atoms, Elements, Compounds and Molecules (the left side of the big, blue table)  

    Now it is time to expound on the table shown above, which is really the "collective wisdom" of people who have tried to purify, isolate different compounds over many years.  

Atoms:  the name is a lie...and a challenge!  It means "not possible to cut" but atoms can be cut open.   The ancient Greeks who gave this name just did not know how.  It was more a philosophical concept, really--they just believed that you could cut (divide) material down to its smallest "chunk" and said "chunk" was the "atom".  At this point it is not at all clear whether there would be more than one kind of fundamental chunk--and all variety in our world just the result of putting it together differently--or whether there might be different kinds of chunks.  From a chemist's viewpoint, the latter is true...but physicists are still seeking the great tiny unifying chunks of matter.  As this is a chemistry class, we recognize (and revel in!) the fact that atoms can be taken apart...but we adopt the viewpoint that the intact atom is a useful thing.  And here is what we think it looks like:  

Later, we will learn the most amazing story about why we think atoms look like this.  For now, just accept that this is the best we can do.  The three green things, each carrying positive electrical charge, are protons.  The fact that there are three of them makes this a Lithium atom.  There is one blue dot, almost as heavy as a proton, but it has no charge and is called a neutron.  I don't really know if any real Lithium atom would have only one neutron--most actually have 3.  The atom does not really care too much about how many neutrons are present--any more than we spend a lot of time thinking about how many dead people are buried underground:  like neutrons, dead people don't do much.  The swirling red cloud represents 3 electrons, each of which has a negative electronic charge.  Electrons are much lighter than neutrons or protons, so they swirl.  Electrons do the real work of chemistry.  

Atomic Symbols:  We chemists tire easily, so we have invented a shorthand way of referring to different types of elements.  Here are some examples:

Unfortunately, it is an international code, so it sometimes does not make sense.  You have to know Greek or Latin sometimes  Fe (iron, one of the "ferrous" metals), Na for Natrium (which is sodium), Cu (cuprum, the original name for copper, because a lot of it comes from Cypress).  Anyway, that's the deal--you have to know some of these symbols.  You could make flashcards to memorize them.  

Elements:  when all of your sample has only one kind of atom, you have an element.  Example:  take dirt, heat the snot out of it until the "gold stuff" comes out, try various things to make that "even golder" and when you fail, you have a "pure" thing--elemental gold.  All the atoms in pure gold have 79 protons, with varying number of neutrons.  In any given "chunk" of gold, there could be more or less atoms.  There is no set number.  

    You could also isolate pure oxygen, but it is tricky:  sometimes the pure oxygen likes to contain two atoms, and sometimes three.  It matters!  The two-atom oxygen is what we breath, while the three-atom oxygen isn't very good for us.  

              Molecular Oxygen, O₂               A Different Molecular Oxygen, O₃   

Different versions of the same elemental form are called allotropes:  normal oxygen and ozone are allotropes.  We think we know most of the allotropic forms that exist now, but sometimes we are surprised.   Within the last decade or so, a new form of carbon was found, which is leading to all sorts of exciting new materials.  For fun, type "fullerenes" into any web search engine, like Google.com.  Note that some elements exist as molecules, while others do not.  So what are molecules, anyway?

Molecules:  Exact accretions of atoms.  Those two oxygen atoms in normal oxygen above are pretty strongly wedded together.  You can divorce them, and chemists are good at this kind of thing.  Another example would be H₂O, water.  Two hydrogen atoms bond to a single oxygen atom and the result is a well-defined chunk.  Those two atoms of hydrogen are married to that particular oxygen.  Only under duress will they leave for another oxygen (or something else).  

Compounds:  Exact proportions of atoms.  An example would be AlCl₃.  This might really mean Al₁₀₀₀₁Cl₃₀₀₀₃.  Or it might mean Al₂₀₀₀₂Cl₆₀₀₀₆.  No way of really telling how many in the "chunk"; it depends on the size.  Unless the compound is also molecular, it is  comparatively easy to get these atoms to date other atoms:  just dissolve them and dry them down.  For example, if we dissolve salt, NaCl, then dry it down we will find that the sodium atoms have associated with different chlorine atoms.  

Molecules are Compounds but not vice versa:  the key thing that makes a compound is that exact proportion.  It is harder to be a molecule.  It's the same with human beings:  most agree with one guy/one girl but that "for life" thing is harder.  

Formulae:  The exact arrangement of atoms to make a molecule may matter!  And then again, sometimes it does not.  So we have different ways to write our molecules, depending on how specific we need to be.  

-----BRING MODELS-----

The dashed bonds go behind the plane of the page; the solid bonds go in front.  In class, we sometimes make the dashed ones fatter towards the central atom (the "railroad track" perspective).  The convention shown in this photo is more conventional and commonly used by chemists.  The pictures shown here were drawn by a popular package, such as ChemDraw.   If you are a serious chemist, you can buy this; it is heavily discounted for students.   You can try it, or some competing package, in the Chemistry Library.  Ask the librarian for a molecular drawing package. 

Why it matters: "the formula" is too ambiguous: many different compounds have the same number and kind of atoms.

Chemical Equations:  Chemists are good at converting one compound to another.  We have discovered a few rules about doing this:

1. The total mass is never changed.
2. Atoms are not changed or lost.  
3. How the atoms are arranged, and which partners they choose, can change a lot.  

Example 1:  H₂(g) + Cl₂(g) à 2 HCl(g)

In words:  a molecule of diatomic hydrogen reacts with a molecule of diatomic chlorine to produce two molecules of hydrogen chloride.  All of them are gases (hence the little g everywhere).  

Example 2:  AgNO₃(aq) + NaCl(aq)   à AgCl(s) + NaNO₃(aq) 

In words:  an aqueous (water is the solvent) solution containing silver nitrate reacts with another containing sodium chloride to produce a precipitate (s = solid) of silver chloride and sodium nitrate, which remains dissolved as an aqueous solution.  

Example 1 is a molecular reaction--everything starts as a molecule, and a molecule is the result.  Example 2 involves compounds that are not molecular--it is best to think of it happening on zillions of AgNO₃ , equally zillions of NaCl, etc.  

Balancing Chemical Equations: 

Note the coefficient 2 on the right side of Example 1.  This is called a stoichiometric coefficient.  To get the total number of H atoms on the right or left side, we have to multiply by the stoichiometric coefficients.  Example 1 could be written:  

1 H₂(g) + 1 Cl₂(g) à 2 HCl(g)

We just find writing all those extra "1's" tedious.  Balance is an important issue because of rule 2 above:  Atoms are not changed or lost.  To emphasize this point, consider the reaction of hydrogen with oxygen:  (Blue=hydrogen; red=oxygen)

If you only wrote H₂ + O₂ à   H₂O you would be unbalanced.  You must write 2 H₂ + O₂ à   2 H₂O instead.  

 

Example:   the decomposition of potassium chlorate to potassium chloride and oxygen:  KClO₃ ---» KCl + O₂

• This reaction is not balanced because there are 3 oxygen atoms on left and only 2 on right. 
• To balance it, see the 2 - 3 oxygen relationship.  Try putting a 2 with the 3 and vice versa:  2 KClO₃ ----» ? KCl + 3 O₂
• clearly ? = 2 to balance the Cl's: 2 KClO₃ ----» 2 KCl + 3 O₂
• It is important to check the balance!!!!  Make a check table, like this: 

Atom	On left side	On right side
K	2 x 1 = 2	2 x 1 = 2
Cl	2 x 1 = 2	2 x 1 = 2
O	2 x 3 = 6	3 x 2 = 6

 Secrets of balancing:  do a lot of 'em!  Keep trying!  It is trial and error.  

Mixtures, Solutions & Suspensions (the right side of the big, blue table)  

• Solutions consist of elements or compounds mixed together at a very fine level.  That's pretty vague, but we mean mixed to the point where you cannot see any "unmixing" when you look at it by eye.  Or even with a microscope.  Don't ask how good a microscope, OK?  There is a gray area here called "colloidal suspensions".  

• Mixtures are obviously not very well mixed.