Atomic Theory Review

John Dalton’s atomic model, the first ever, could be described as which of the following?

• Electrons imbedded in a positively-charged goo, like blueberries in a muffin

• A solid sphere

• Electrons orbiting a central nucleus, like the planets orbiting the sun.

• None of these

Answer explanation

Dalton figured out that the only shape capable of exerting the exact same pressure no matter which way it hit a surface was a solid sphere.

Thomson discovered the electron. What did his discovery mean?

• Electrons were the first neutral particle found

• It proved the atom idea

• It indicated Dalton’s model was flawed and there were particles smaller than an atom

• The electrons had been defeated in a global war several years earlier

Answer explanation

Dalton had shown that the smallest particle of matter was a hydrogen atom, with a mass of 1 amu. Thomson has just discovered a negatively-charged particle with a mass 2000 times smaller than that!

Rutherford disproved the “plum pudding” model of the atom in what famous experiment, in which he shot alpha particles at metal sheet?

• The cathode ray tube experiment

• The heavy hydrogen experiment

• The electric coil experiment

• The gold foil experiment

Answer explanation

Rutherford used gold foil because gold is incredibly dense. He figured that if the alpha particles were going to hit an atom, it would help if the atoms were very closely packed together.

The Law of Constant Composition says that what compounds are made of does not change. What does this imply about chemistry?

• That atoms are following some set of rules that govern how they combine into compounds

• We cannot make new compounds because that would cause a change in composition

• It explained why everyday objects did not suddenly change form

• It implied an underlying structure to the way protons interacted with neutrons

Answer explanation

For atoms to keep combining in specific ratios, they had t be following some rule that dictates how they combine together in a compound (we now call this the "Octet Rule").

Tags

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How did Lavoisier prove mass was conserved in a chemical reaction?

• He didn’t, Dalton did this

• He weighed matter in sealed jars before and after burning it

• He froze samples of human tissue and then thawed them back out to show the tissue was still there

• He measured the volume of matter before and after burning it

Answer explanation

Lavoisier sealed metal oxides inside glass jars. He would then weigh them. Then he would heat the jar until the metal oxide changed back into pure metal, then weighed them again. He found that no matter how many times he did this, the two masses were always the same.

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Rutherford could never explain why the electrons did not quickly lose energy and spiral down into the nucleus. What solution did Niels Bohr propose that solved this problem?

• He proposed that the electrons were moving too quickly to lose energy

• He proposed that the electrons could only occupy specific energy levels (shells) around the nucleus, which he called quanta

• He proposed that the small mass of the electron meant that it would take billions of years to lose enough energy to fall into the nucleus

• He proposed that the opposing charges of electrons and protons would repel the electrons away from the nucleus

Answer explanation

Bohr proposed that the electron energies came in 'packets' and an electron could have multiples of these packets, but never any other amount.

Imagine you have three 10-dollar bills and someone asks you for 50 cents. You can't give it to them because your money only comes in 10-dollar increments. You can give them $10, $20, or $30, but you cannot possibly give them 50 cents. In this way, the $10 bills represent the energy packets the electron can contain. It cannot contain any less energy than that packet.

What did the gold foil experiment indicate about the atom?

• It was smaller than originally thought

• It could flow through solid matter

• There had to be a heavy mass of positive charge in the center

Answer explanation

Rutherford realized that the only way a heavy, positively-charged alpha particle could bounce off of something is if it hit an object more massive and also positively-charged.

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What does Proust’s discovery that elements always combine is set ratios imply about elements?

• Elements follow an underlying order when combining into compounds

• That elements can only combine in a single ratio

• The different elements are all made of the same atoms

• It proves that elements are made of atoms

Answer explanation

Again, because atoms always combine in the same numerical ratio when making a compound, there must be some rule that governs how they fit together.

A neutron has approximately the same mass as:

• a nucleus

• a beta particle

• an electron

• a proton

Answer explanation

Neutrons are a tiny bit more massive than protons, but they have essentially the same mass (for our purposes).

What new model did Bohr suggest for the atom that drove classical physicists nuts?

• Plum Pudding model

• Quantum Mechanical model

• Quartet model

• Quark model

Answer explanation

Bohr's model drove other physicists nuts because it not only contained the idea of the quantum jump, but the only way to prove it was with mathematics that Bohr himself had invented.