UNIT THREE COMPREHENSIVE REVIEW QUESTIONS (Chapters 11, 12, 13, 14, 15) 1. Descr
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UNIT THREE COMPREHENSIVE REVIEW QUESTIONS (Chapters 11, 12, 13, 14, 15) 1. Describe the phlogiston theory of combustion, discuss why it might seem reasonable, and explain how it was disproven and by whonm. 2. Who first proposed the modern periodic table of the elements, what principles were used in organizing the elements on the table, and what predictions did the table m proposed? 3. How did Rutherford's "gold foil" experiment disprove the "plum pudding" model of the atom and what model of the atom did Rutherford propose in its place? 4. What is meant by the line spectrum of an element? Give an example. Use the Bohr model of the atom to explain the fact that an element exhibits a line spectrum. 5. "The periodic table represents both a summary of many observations and a reminder of theories regarding the structure of the atom." Explain this statement. Use several elements as examples. 6. The element potassium reacts very violently and explosively when placed in water. A beginning chemistry student sees that a salt substitute contains potassium chloride, and warns you that it would be very dangerous to use in water Explain why the salt substitute would not be explosive in water. 7. Why are the gases hydrogen and chlorine diatomic, while helium and neon are gases? ples of three fossil fuels and the fuel values typical for each and explain the trend in he concept of fuel value. Why is it ad antagecous for a good transportation fuel to fuel values. 9. Explain t have a high fuel value?Explanation / Answer
1.A fire-like element called phlogiston is contained within combustible bodies and released during combustion.
Phlogiston, in early chemical theory, hypothetical principle of fire, of which every combustible substance was in part composed. In this view, the phenomena of burning, now called oxidation, was caused by the liberation of phlogiston, with the dephlogisticated substance left as an ash or residue.
Phlogiston theory states that phlogisticatedsubstances are substances that contain phlogiston and dephlogisticate when burned. Dephlogisticating is when the substance releases the phlogiston inside it and that phlogiston is absorbed by the air. Growing plants then absorb this phlogiston, which is why air does not spontaneously combust and also why plant matter burns as well as it does.
The phlogiston theory was discredited by Antoine Lavoisierbetween 1770 and 1790. He studied the gain or loss of weight when tin, lead, phosphorus, and sulfurunderwent reactions of oxidation or reduction (deoxidation); and he showed that the newly discovered element oxygen was always involved. Although a number of chemists—notably Joseph Priestley, one of the discoverers of oxygen—tried to retain some form of the phlogiston theory, by 1800 practically every chemist recognized the correctness of Lavoisier’s oxygen theory.
2 . Modern periodic table was proposedby Henry Moseley in 1913.
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The modern periodic table
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Dmitri Mendeleev’s early periodic table was further refined in the early 20th century in light of the discovery of protons, neutrons and electrons. This allowed elements to be placed in appropriate groups according to atomic numbers instead of atomic masses, which produced the periodic table we use today.
The development of the modern periodic table
Dmitri Mendeleev put the elements in order of their relative atomic mass, and this gave him some problems.
For example, iodine has a lower relative atomic mass than tellurium, so it should come before tellurium in Mendeleev's table.
In order to get iodine in the same group as other elements with similar properties - such as fluorine, chlorine and bromine - he had to put it after tellurium, which broke his own rules.
However, the discovery of protons, neutrons and electrons in the early 20th century allowed Mendeleev’s table to be refined into the modern periodic table. It involved an important modification – the use of atomic number to order the elements. An element’s atomic number (also called proton number) is the number of protons in its atoms.
Using atomic number instead of atomic mass as the organising principle was first proposed by the British chemist Henry Moseley in 1913. It explained why Mendeleev needed to change the order of some of the elements in his table.
For example, tellurium has a higher atomic mass than iodine, but iodine has a higher atomic number than tellurium. So, even though he didn't know why, Mendeleev was (as it turned out) right to place iodine after tellurium.
Predictions made earlier where like this :
The elements, if arranged according to their atomic mass, exhibit an apparent periodicity of properties.
Elements which are similar as regards to their chemical properties have atomic weights which are either of nearly the same value (e.g., Pt, Ir, Os) or which increase regularly (e.g., K, Rb, Cs).
The arrangement of the elements, or of groups of elements in the order of their atomic masses, corresponds to their so-called valencies, as well as, to some extent, to their distinctive chemical properties; as is apparent among other series in that of Li, Be, B, C, N, O, and F.
The elements which are the most widely diffused have small atomic weights.
The magnitude of the atomic weight determines the character of the element, just as the magnitude of the molecule determines the character of a compound body.
We must expect the discovery of many yet unknown elements – for example, elements analogous to aluminium and silicon – whose atomic weight would be between 65 and 75.
The atomic weight of an element may sometimes be amended by a knowledge of those of its contiguous elements. Thus the atomic weight of tellurium must lie between 123 and 126, and cannot be 128.
Certain characteristic properties of elements can be foretold from their atomic masses.
3.
Thomson's plum pudding model viewed the atom as a massive blob of positive charge dotted with negative charges.
A plum pudding was a Christmas cake studded with raisins ("plums"). So think of the model as a spherical Christmas cake.
When Rutherford shot ? particles through gold foil, he found that most of the particles went through. Some scattered in various directions, and a few were even deflected back towards the source.
He argued that the plum pudding model was incorrect. The symmetrical distribution of charge would allow all the ? particles to pass through with no deflection.
Rutherford proposed that the atom is mostly empty space. The electrons revolve in circular orbits about a massive positive charge at the centre.
His model explained why most of the ? particles passed straight through the foil. The small positive nucleus would deflect the few particles that came close.
The nuclear model replaced the plum pudding model. The atom now consisted of a positive nucleus with negative electrons in circular orbits around it .
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