Unit+3+-+Atomic+Theory

Praise Harry Potter! The one and TRUE savior of the human race! In the name of Dumbledore, Potter, and Rubeus Hagrid, Amen.

1) Methane is a compound in which is 75% carbon, by mass, the balance being hydrogen. If a sample of methane weighs 24 g, what mass of hydrogen does it contain?

A) 2.0 g B) 4.0 g C) 6.0 g D) 8.0 g E) 18.0 g

C- 6 grams. Methane is CH4. We know that 75% of 24 grams is Carbon, so if we take 75% of 24, we get 18 g. Because H is the only other element in methane, we can subtract the mass of the methane to get the mass of the Hydrogen. 24 - 18 = 6 grams.

2) Which of the following ideas was not part of Dalton's atomic theory postulates?

A) Matter consists of atoms which are indestructible. B) Atoms of an element are identical in all respects. C) Compounds consist of atoms of different elements combined in specific ratios. D) Atoms of one element cannot be converted into those of another element. E) Atoms combine to form small groups called molecules.

E- Dalton did not say anything about groups of atoms called molecules.

3) J.J. Thomson A) discovered the nucleus. B) measured the charge of the electron. C) discovered X-Rays. D) measured the charge/mass ratio of the electron. E) invented the cathode ray tube.

E- J.J. invented the cathode ray tube thing.

4) How many protons (p), neutrons (n) and electrons (e) are there, respectively, in an atom of the chlorine-39 isotope?

A) 17 p, 17 n, 22 e B) 39 p, 38 n, 39 e C) 22 p, 17 n, 17 e D) 17 p, 39 n, 17 e E) 17 p, 22 n, 17 e

E- First of all, there are both 17protons and 17electrons in the element chlorine, you can find this out just by looking at the atomic number, which is of course 17. Then, since the 39 represents the number of protons and neutrons in the nucleus you just subtract 17(number of protons) from 39(number of protons and electrons) to get 22(number of neutrons.)

5) Isotopes of an element differ in their A) chemical symbol B) atomic number C) mass number D) chemical reactivity E) number of protons

C- Finally an answer other than E, anyway, the mass number indicates both the number of protons and neutrons in the nucleus and because different isotopes have different amounts of neutrons the mass number will change.

6) Magnesium consists of three isotopes with the following abundances and atomic masses: 24Mg 78.70% 23.985 amu 25Mg 10.13% 24.986 26Mg 11.17% 25.983

Based on these data the atomic weight of magnesium is A) 24.99 B) 24.53 C) 24.31 D) 23.99 E) none of the above

C- Figuring this out is actually much easier than it may seem, all you have to do is take the amount of amu's and multiply that by the percentage of that isotope in the element. Then add them together.

Ex. 23.985 x .7870 = 18.88, now just do this same thing for the other two isotopes and add all 3 together you should get 24.31.

7) Rutherford's experiment with alpha particle scattering by gold foil established that

A) electrons have a negative charge. B) protons are 1840 times heavier than electrons. C) protons are not evenly distributed throughout an atom. D) atoms are made of protons, neutrons and electrons. E) the nucleus contains protons and neutrons.

C: When Rutherford did the Gold Foil Experiment, what he did was shoot a whole bunch of Alpha particles towards a thin piece of gold foil. He found that most of the particles passed through the gold foil in a straight line, some passed through the gold foil but changed their direction slightly and a small number reflected back. So, Rutherford concluded that the atom must be mainly empty space and that the positive charge was not spread out but it was located in the center.

8) The number of neutrons in an atom of Iodine A) 53 B) 74 C) 126 D) 127 E) 180

B- 74. The atomic mass is roughly 127. So, take the number of protons (the same as its atomic number, 53) and subtract that from the Atomic mass, 127, to get 74 neutrons.

9) The element antimony has an atomic weight of 121.757 amu and only two naturally occurring isotopes. One isotope has an abundance of 57.3% and an isotopic mass of 120.904 amu. What is the mass of the other isotope? A) 52.479 amu B) 121.757 amu C) 122.393 amu D) 122.610 amu E) 122.902 amu

E: (Weight of Isotope x Abundance of Isotope) + (Weight of Isotope 2 x Abundance of Isotope 2) = Weight of Element.

Now just plug things in and solve the equation:

(.573 * 120.904) + (.427x) = 121.757 and 122.901658 is your answer.

10) What value or values of ml are allowable for an orbital with l = 2? A) 0 B) 2 C) -1 D) none of the above E) all of the above

E: All of the above.

Rules:
 * The three quantum numbers (//n//, //l//, and //m//) that describe an orbital are integers: 0, 1, 2, 3, and so on.
 * The principal quantum number (//n//) cannot be zero. The allowed values of //n// are therefore 1, 2, 3, 4, and so on.
 * The angular quantum number (//l//) can be any integer between 0 and //n// - 1. If //n// = 3, for example, //l// can be either 0, 1, or 2.
 * The magnetic quantum number (//m//) can be any integer between -//l// and +//l//. If //l// = 2, //m// can be either -2, -1, 0, +1, or +2.

11) According to the quantum-mechanical model, how many orbitals in a given atom have n = 3? A) 4 B) 7 C) 9 D) 10 E) 18

E- If n = 3 then that means we're going to the 3rd energy level and just counting across the periodic table. Just don't forget to include the d orbital because it's not lined up w/ the s and p orbitals.

12) According to the quantum mechanical treatment of the hydrogen atom, which set of quantum numbers is not allowed? A) n = 3, l = 2, ml = 0 B) n = 3, l = 0, ml = 0 C) n = 3, l = 1, ml = 1 D) n = 3, l = 1, ml = -1 E) n = 3, l = 1, ml = 2

E - according to the l this element would be in the p orbital, but if ml is 2 then the element has to be in the d or f orbitals.

13) Consider this set of quantum numbers: n = 3, l = 2, ml = -1, ms = +1⁄2 The maximum number of electrons in an atom which can share the above set of quantum numbers is A) 1 B) 14 C) 3 D) 10 E) none of the above

A - only one electron can have an atomic number. Be careful if there's a question like this on the final, make sure that the numbers all work and it's even possible at all.

14) An atom in its ground state contains 30 electrons. How many of these are in orbitals with l = 2? A) 2 B) 4 C) 6 D) 8 E) 10

E - Okay, we know that l is two, so this means that we're in the d orbital so you just start counting at Sc and count over to Zn since Zn has 30 electrons.

15) What are the possible values for the angular momentum quantum number (l)? A) integers from -l to 0 to +l B) 1, 2, 3, etc. C) 2, 4, 6, etc. D) +1⁄2, -1⁄2 E) integers from 0 to 3

E - Read my thing below about quantum numbers.

16) The electronic configuration of the element whose atomic number is 26 is: A) 1s2 2s2 2p6 3s2 3p6 4s0 3d8 B) 1s2 2s2 2p6 3s2 3p6 3d6 4s2 C) 1s2 2s2 2p6 3s2 3p6 4s2 3d6 D) 1s2 2s2 2p6 3s2 3p6 4s2 3d4 4p2 E) none of the above

C - Just do that stupid counting thing, just remember that the d orbital starts at energ level 3.

17) The set of quantum numbers that correctly describes an electron in a 3p orbital is A) n = 3; l = 0; ml = 0; ms = 0 B) n = 3; l = 2; ml = -2, -1, 0, 1, or 2; ms = +1⁄2 or -1⁄2 C) n = 3; l = 1; ml = -1, 0, or 1; ms = +1⁄2 or -1⁄2 D) n = 4; l = 0; ml = -1 ,0, or 1; ms = +1⁄2 or -1⁄2 E) none of the above

18) An atom in its ground state contains 18 electrons. How many of these are in orbitals with ml = 0? A) 2 B) 4 C) 6 D) 8 E) 10

A - This is really easy and requires no work, ml = 0 is just naming an electron pair, a pair = 2.

19) The configuration for the six outer electrons in ground state oxygen atoms is A) 2s3 2p3 B) 2p6 C) 2s2 2px2 2py2 D) 2s2 2px2 2py1 2pz1 E) 2s4 2p2

D - Because Mrs. Wilson said so.

20) Which of the following is the electron configuration for chromium, element 24? A) 1s2 2s2 2p6 3s2 3p6 4s2 B) 1s2 2s2 2p6 3s2 3p6 4s2 3d4 C) 1s2 2s2 2p6 3s2 3p6 3d6 D) 1s2 2s2 2p6 3s2 3p6 4s1 3d5 E) 1s2 2s2 2p6 3s2 3p6 4s2 3d1 3d1

B- Just count on the periodic table.

Here's what Mr. Guch has to say about some of this stuff:

Finding Protons, Neutrons, and Electrons

First, let's examine one of the entries in the periodic table (this should show up at home, but not at school, because the site that hosts the image is blocked at school for some unknown reason):

external image subatomicparticles.gif

That’s helium, and the atomic number is found at the top. The atomic number = the number of protons that the element has, and as long as the atom is neutral, the number of electrons. Now, the reason that different atoms of the element have the same number of protons is because it determines the element at hand.

The atomic mass is different for each element and for different isotopes of each element. It can be found by adding the number of protons and neutrons together, which is handy because with a PT, you can find the mass of an element and the number of protons, so you could use this formula to find neutrons [Atomic Mass - # of protons = # of neutrons.] Why? Because protons and neutrons weigh about the same thing, 1 amu. (Electrons are ignored because their mass is tiny.)

To summarize: Atomic number = number of protons and electrons. Atomic symbol allows us to find the atomic number because you can just look it up on the periodic table. Atomic mass = number of protons + number of neutrons.

Finding the number of valence electrons of an element

First, let’s review a few terms. The Octet rule says that all elements want to have the same electron configurations as the nearest noble gas to them. Noble gases are much more stable, and therefore, elements ant to be stable too. Elements become like the nearest noble gas by gaining, sharing, or loosing electrons.

Valence electrons are the number of s- and p- electrons in the outmost energy level. To find the number of Valence electrons, just count backwards from the element you’re looking at to the last noble gas. For some elements, though, you must skip over the d- and f- parts of the periodic table.

Using these two terms, we can determine the charge that an element will have when it forms ionic compounds. To find the charge, count backwards/forwards from the element that you're interested in to the nearest noble gas. If counting backwards takes you to the nearest noble gas (as in the case of lithium and magnesium), the charge is equal to +[however many elements you need to count across]. Lithium is +1, for instance, because you count back 1 to get to the nearest noble gas, hydrogen. If counting forwards takes you to the nearest noble gas the charge is equal to -[the number of elements you need to count across]. Oxygen is -2 because you count forward two elements to get to the nearest noble gas.

The Following is directly taken from www.chemfiesta.com. Thank you to Mr. Guch.

The number of valence electrons allows us to determine how many electrons an element has that can do covalent bonding, and for this reason is extremely important for nonmetals. For example, when oxygen bonds with two hydrogen atoms, we know that our resulting Lewis structure will need to show eight electrons, as oxygen has six valence electrons (determined by counting backwards to He) and the two hydrogens each have one valence electron.

The Atom in History

Most people think that the atom is a very boring thing to study. However, that can’t possibly be true – after all, lots of people have studied it for thousands of years. It must be exciting! Let’s take a look at what we’ll discuss here:


 * The Greeks talk and talk and talk about atoms.
 * Some new ideas crop up from somewhere or other.
 * Dalton actually says something smart about atoms.
 * Thomsen plays with electricity.
 * Rutherford plays with gold.
 * Rappin’ Neils Bohr and the planetary model

Let’s get started!

The Greeks and their imaginary atom:

Even though the Greeks had very little in the way of high technology, they still felt that they could use the power of their brains to figure out what matter was made up of at the smallest levels. As a result, lots of them talked about it a lot.

Democritus was one of these guys. He came up with a model of the atom that said:


 * Atoms are solid and indestructible.
 * Different atoms have different shapes and sizes – this is why different materials have different properties.

Because Aristotle disagreed with him and everybody thought that Aristotle was a big hotshot, practically nobody paid attention to Democritus. The moral of the story: Don’t mess with Aristotle. Or something like that.

Some ideas that nobody had ever thought of before:

For a really long time, nobody really thought that Aristotle was wrong. Eventually, however, with the advance of science, people started to rethink their devotion to the dead Greek guy. Here are some of the discoveries that changed this:


 * Law of conservation of mass: The amount of stuff you form in a reaction is equal to the amount of stuff you started with.
 * Law of definite composition: Every chemical compound has one and only one chemical formula. For example, no matter what process you use to make water, the formula will always be H2O.
 * Law of multiple proportions: If two elements can combine to form more than one chemical compound, the ratio of the mass of one element that combines with a fixed mass of the other element will be a whole number ratio for the compounds. Since this doesn’t make any sense, let’s use the example of two compounds where hydrogen reacts with oxygen: H2O and H2O2 (hydrogen peroxide). In the first compound, the amount of oxygen that’s needed to combine with 2 grams of hydrogen is 16 grams. In the second compound, the amount of oxygen that’s needed to combine with 2 grams of hydrogen is 32 grams. Since the ratio of 32/16 works out to a 2:1 ratio, it follows this law. Seems like a lot of words for a simple idea, huh?

Dalton and his not entirely imaginary atom:

In the 1800’s, some English guy named John Dalton came up with his own idea of what atoms were like. His theory included the following ideas:


 * Everything is made of atoms (which is true!)
 * All atoms of an element are identical in every way (which is false, because of the existence of isotopes).
 * Atoms of different elements are different (which is true).
 * Atoms can’t be broken (which is true for chemical reactions, but not for nuclear ones).
 * Atoms combine in whole number ratios to form compounds (i.e. you can’t have half an atom in a compound – this isn’t really surprising, given his idea that you can’t break an atom) – this is true.
 * In chemical reactions, atoms are rearranged (this is true).

Overall, people seemed pretty happy with Dalton’s laws. That is, until his idea of what atoms are like was disproved by…

Thomsen and his cathode ray tube: Since every chemistry textbook in the world shows a picture of the cathode ray tube experiment, I’m not going to reproduce it – I suggest you turn to it, though, since it might help with my explanation.

Anyway, one day Thomsen was goofing around the lab with these cathode ray tubes he found somewhere. What he found was that when he connected these big long hollow tubes to batteries, a beam of light would go from one end to another. Since he had a lot of time on his hands, he decided to figure out what the deal was with the light. After all, if there was nothing in the tube to start with, where’d the light come from? He figured, it must come from the electrodes – since the electrodes were made of atoms, the atoms must somehow be coming apart.

Among other things, Thomsen got a magnet and held it near the beam. When he did this, he found that the beam would bend toward the positive side of the magnet and away from the negative side. From this, he figured that the beam must contain very small particles from the atom and that they must have negative charge.

This led directly to his “plum pudding” model of the atom, named after a dessert that nobody can eat without throwing up. Think of a chocolate chip cookie, instead. His idea was that the dough in the chocolate chip cookie made up most of the atom and that it had positive charge. The chips represented the little tiny bits of negative charge that made up the light he was messing around with – unlike the dough, they could leave the atom if you gave them a shove (with a battery, for example).

For this discovery, Thomsen is forever known. This, despite the fact that his model was almost instantly disproved.

Rutherford and his gold foil:

Rutherford was a scientist who liked to play with radioactive stuff. His favorite radioactive thing to play with was alpha particles, which are helium nuclei (they have a mass of 4 amu and a charge of +2). One day, he decided to shoot a bunch of alpha particles at a really thin piece of gold foil.

When he did this (again, you can find MUCH better pictures of this in your textbook than I can make), he found that most of the particles went right through the foil, while some of them either passed through or bounced off at irregular angles. Why is this?

His idea was to come up with a model of the atom in which most of the atom is empty space with electrons floating around in it. The protons, however, are all concentrated in the middle of the atom (called the nucleus) – according to this model, the positively charged alpha particles would go straight through the atom most of the time and only be deflected on the rare occasions when they passed very close to the tiny nucleus. For this discovery, we will always know Rutherford as the man with the gold foil. And the bad model of the atom, because we now know that this model isn’t right, either.

Random interlude: Chadwick and the discovery of the neutron

In 1932, James Chadwick discovered the neutron (which has no charge at all) by doing some really complicated experiments that I don’t understand even a little bit. Don’t worry, though – your teacher probably doesn’t understand it either (unless they’re a nuclear scientist or something), so if you just remember that Chadwick discovered it, you’re probably fine.

Neils Bohr and the planetary model:

As tends to be the case with models of the atom, nobody really bought into Rutherford’s model for very long because it turned out to be wrong. Neils Bohr was one of the guys that didn’t buy it, due to the discovery that when you add energy to an atom, it gives off light that has only a few very particular colors. Since Rutherford’s model didn’t explain how this could be, Neils (as his buddies called him) came up with a different model (which, as is the case with many things, is much better drawn in your textbook).

His idea was that electrons traveled only in certain circular paths around the nucleus, much as the planets circle the sun. When energy is added to the electrons, the electrons jump from their normal orbit (called the “ground state” orbital) to a higher energy orbital farther from the nucleus (called the “excited state” orbital). Since the world likes to exist at low energy more than at high energy, the electrons eventually return to their ground state orbitals. When this happens, the energy that they absorbed is given off as light. Since the color of light is very closely related to its energy, you only see very particular colors of light being given off by the very particular energy differences between the ground state and the excited state.

His idea further went on to say that the energies of the orbitals were different for every atom. As a result, the colors of light given off by every element is unique, which allows us to identify them via the magic of spectroscopy (specifically, this phenomenon is called atomic emission spectroscopy).

Like all models of the atom, this was overturned in about 15 minutes by a bunch of guys who invented something called quantum mechanics. However, don’t feel sorry for the planetary model of the atom – it still has a healthy and thriving life in elementary school textbooks (which for some reason refuse to acknowledge the existence of quantum mechanics).


 * Quantum Numbers**

As we all learned at some point this year quantum numbers are four digit numbers that identify elements, because apparently just saying the element's name isn't good enough. So, here's how you do it. As I already said there are for different numbers in a quantum number, unfortunately chemists can't be satisfied with just numbers and have to represent the numbers with letters. n is the first letter, l is the second number, ml is the third number, and ms is the fourth number.
 * n -** this just lets us know what energy level the element is in from 1 all the way up to 7.
 * l -** this tells us what orbital the element is in, s,p,d, or f. Each of these letters has a number that goes with it s = 0, p = 1, d = 2, and f = 3.
 * ml -** this is the number that's the most confusing, this is the number that deals with the separate electron pairs in the atom. This has to do with the up and down arrows that we did. Each orbital has a different number of electron pairs s has 1, p has 3, d has 5, and f has 7. when you fill in the electrons whichever pair the last electron falls is what ml will equal. The middle electron pair gets the number zero and the other pairs get either positive or negative integers depending on were they are at compared to the middle pair. Like a number line.
 * ms -** this number is really easy to find. if the last electron is an up arrow the number is 1/2 if the last electron is a down arrow the number is -1/2.