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Introduction

Our understanding of the bonding between atoms in molecules tells us a great deal about the structures and properties of molecules. We can use the valence shell model of atoms and the Lewis model of bonding to explain and even predict what types of molecules will be stable, like CH 4 , and which are not expected to exist, like CH 5 . This is very powerful and quite beautiful. It means that the enormous number of known substances generally fit a fairly simple set of rules that describes their molecules. Without these rules, we would be lost trying to understand the properties of the more than 50 million different substances that chemists have identified. With these rules, though, we cannot only understand the properties of each molecule but even predict the properties of other molecules even if they have never been created before.

Chemistry, though, is about changes in matter. How do substances react with one another? If a molecule is stable under ordinary conditions, why does it become unstable when the temperature is raised or other molecules are added to the flask? We are interested in chemical reactions, but we are also interested in some physical changes which take place in matter. What happens when a substance changes from solid to liquid or liquid to gas? Why do some substances do this so readily?

Answering these questions often requires us to understand how the properties of individual molecules create the properties we see in large quantities of a substance. This seems like a very challenging problem. It is hard enough to think of the structure of one molecule and try to visualize how its structure creates properties such as dipole moments or intermolecular forces. It seems much harder to imagine thinking of a mole of these molecules all interacting with each other at the same time.

We need a way to relate macroscopic properties to molecular properties. As simple examples, let’s compare the substances water, carbon dioxide, and nitrogen. Each of these is composed of molecules with just a few atoms, and all of the atoms have rather small masses, so the molecules all have low molecular weights. These three molecules have very similar molecular properties; however, the physical properties of these three substances are very different. Carbon dioxide and nitrogen are gases at room temperature, but water is a liquid up to 100 °C. We can only get nitrogen to condense by cooling it to -196 °C. This means that the boiling temperatures of water and nitrogen are different by almost 300 °C. Water is a liquid over a rather large temperature range, freezing at 0 °C and boiling at 100 °C. Nitrogen is very different – it is only a liquid between -210 °C and -196 °C. Carbon dioxide is even more interesting. At normal atmospheric pressure, carbon dioxide gas cannot be condensed into a liquid at all, no matter how cold we make it. If we keep cooling carbon dioxide gas to -60 °C, it converts directly to solid “dry ice.” We cannot melt dry ice either. Warming dry ice above -60 °C does not produce any liquid, but instead the solid “sublimes,” meaning that it converts directly into gas.

Questions & Answers

A golfer on a fairway is 70 m away from the green, which sits below the level of the fairway by 20 m. If the golfer hits the ball at an angle of 40° with an initial speed of 20 m/s, how close to the green does she come?
Aislinn Reply
cm
tijani
what is titration
John Reply
what is physics
Siyaka Reply
A mouse of mass 200 g falls 100 m down a vertical mine shaft and lands at the bottom with a speed of 8.0 m/s. During its fall, how much work is done on the mouse by air resistance
Jude Reply
Can you compute that for me. Ty
Jude
what is the dimension formula of energy?
David Reply
what is viscosity?
David
what is inorganic
emma Reply
what is chemistry
Youesf Reply
what is inorganic
emma
Chemistry is a branch of science that deals with the study of matter,it composition,it structure and the changes it undergoes
Adjei
please, I'm a physics student and I need help in physics
Adjanou
chemistry could also be understood like the sexual attraction/repulsion of the male and female elements. the reaction varies depending on the energy differences of each given gender. + masculine -female.
Pedro
A ball is thrown straight up.it passes a 2.0m high window 7.50 m off the ground on it path up and takes 1.30 s to go past the window.what was the ball initial velocity
Krampah Reply
2. A sled plus passenger with total mass 50 kg is pulled 20 m across the snow (0.20) at constant velocity by a force directed 25° above the horizontal. Calculate (a) the work of the applied force, (b) the work of friction, and (c) the total work.
Sahid Reply
you have been hired as an espert witness in a court case involving an automobile accident. the accident involved car A of mass 1500kg which crashed into stationary car B of mass 1100kg. the driver of car A applied his brakes 15 m before he skidded and crashed into car B. after the collision, car A s
Samuel Reply
can someone explain to me, an ignorant high school student, why the trend of the graph doesn't follow the fact that the higher frequency a sound wave is, the more power it is, hence, making me think the phons output would follow this general trend?
Joseph Reply
Nevermind i just realied that the graph is the phons output for a person with normal hearing and not just the phons output of the sound waves power, I should read the entire thing next time
Joseph
Follow up question, does anyone know where I can find a graph that accuretly depicts the actual relative "power" output of sound over its frequency instead of just humans hearing
Joseph
"Generation of electrical energy from sound energy | IEEE Conference Publication | IEEE Xplore" ***ieeexplore.ieee.org/document/7150687?reload=true
Ryan
what's motion
Maurice Reply
what are the types of wave
Maurice
answer
Magreth
progressive wave
Magreth
hello friend how are you
Muhammad Reply
fine, how about you?
Mohammed
hi
Mujahid
A string is 3.00 m long with a mass of 5.00 g. The string is held taut with a tension of 500.00 N applied to the string. A pulse is sent down the string. How long does it take the pulse to travel the 3.00 m of the string?
yasuo Reply
Who can show me the full solution in this problem?
Reofrir Reply
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Source:  OpenStax, Concept development studies in chemistry 2013. OpenStax CNX. Oct 07, 2013 Download for free at http://legacy.cnx.org/content/col11579/1.1
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