A 2. Electrons have a negative charge (-1) but protons have a positive charge (+1). Electrons are much, much smaller than protons. Protons are at the center of the atom, in the nucleus. Electrons are not in the nucleus, they orbit it. Back to the question

A 3. The term "nucleon" refers to protons and neutrons. Nucleons are in the atom’s nucleus. It is the total nucleon count which tells us the atom’s atomic mass. And remember, isotopes of an element have the same number of protons, but different numbers of neutrons and, thus, have different numbers of nucleons. NOTE: Don’t get neutrons and nucleons mixed up. Neutrons are nucleons, but so are protons! Remember: "nucleon" is the name for both the proton and the neutron because they are in the nucleus. Neutrons (are neutral and) are in the nucleus, so they are a kind of nucleon. But protons are nucleons too! As you read and study, pay attention to this easily missed difference. Otherwise, you’ll get confused. Back to the question

A 4. The number of protons determines the chemical behavior of an atom. Because most atoms have equal numbers of protons and electrons, someone might argue that it is the number of electrons which determine behavior. Don’t be mislead. Atoms which do not have the same number of protons as electrons (ions) will behave according to the number of protons. Those ions will have the properties determined by their protons, but they will have a strange electron shell. Back to the question

A 5. An ion is an atom with a (net) charge. This is caused by it having more or fewer protons than electrons. If you add up all the protons and subtract all the electrons (because protons are +1 and electrons -1) you find the atom’s (net) charge. If it is anything but zero, it’s an ion! Back to the question

A 6. Oxygen is listed as having an atomic mass of 15.994 because it represents a MIX of oxygen isotopes. Most oxygen atoms in the universe have 8 protons and 8 neutrons, so an atomic mass of 16. But there must be some oxygen atoms in the universe with an atomic mass of less than 16 Daltons (probably 15 Daltons). Back to the question

A 7. Oxygen-15 (15O) must have 8 protons, in order for it to be oxygen. (I reminded you that oxygen has 8 protons, in Answer 6.) If it had 7 protons it would not be oxygen, but something else (it would be nitrogen, but you may not have remembered that). Therefore, this isotope of oxygen must have only 7 neutrons. 7 neutrons + 8 protons = 15 nucleons. Back to the question

A 8. An alpha particle is a helium nucleus. It has no electrons, so it has a charge of +2 (from the two protons, which make it helium). Back to the question

A 9. A beta particle is an electron, so it has a charge of -1. Back to the question

A 10. A gamma ray is a powerful beam of light (too powerful to be seen but very capable of causing damage). It doesn’t have a charge at all! Back to the question

A 11. Transmutation is the transformation of one atom or element into another. This requires a change in the number of protons. Both alpha and beta particles are involved in transmutation.. When an atom spits out an alpha particle, it loses two protons (and one or two neutrons) so it must become another element. Alpha decay lowers the atom's atomic number by two and lowers its atomic mass (nucleon count) by three or four (usually four). When an atom spits out an electron it does so because it has created a new proton from a neutron. (It’s not magic but it sure is close!). It spits out the electron (beta particle) to conserve charge. Beta decay increases the atom's atomic number by one, but it still has the same number of nucleons so its atomic mass is not changed. (We don't count the wee difference in mass between the proton and neutron.) Oh, by the way, atoms often loose their electrons without involving beta decay. They may pick up a lot of energy (heat) from their environment and simply toss away some of their electrons. This is ionization, not beta decay. Beta decay is a "nuclear transformation event". It starts in the nucleus. Ionization is an "orbiting electron event" and it has to do with the electrons already around the atom (and has nothing to do with transmutation or the nucleus). Gamma rays are sometimes produced during transmutations of elements we haven’t yet discussed, but they are not (directly) involved in transmutation. Back to the question

A 12. Yes and no. Alpha decay causes the atom to change into a different element and to lose some mass (usually 4 Daltons). Beta decay doesn’t change the atomic mass because a neutron is transformed into a proton and they both have the same mass (or close enough). To put it another way, alpha decay causes the atom to loose nucleons and thus change its mass, but beta decay causes no change in the nucleon count, just their distribution. (Neutrons becoming protons is just swapping among the nucleons.) Back to the question

A 13. A Dalton is a unit of atomic mass (not weight). Hydrogen is the fundamental unit and has a mass of one Dalton. A better way to look at it is to say that each nucleon has a mass of one Dalton. We calculate the atomic mass by adding up the number of nucleons and express that number as Daltons. Back to the question

A 14. Hydrogen has three "common" isotopes. Normal hydrogen (hydrogen one, 1H) is just a proton. Deuterium (hydrogen two, 2H) is a proton and a neutron. Tritium (hydrogen three, 3H) is a proton and two neutrons, but it's radioactive and beta decays into helium three (3He). Back to the question

A 15. Atomic number is the number of PROTONS in the atom. It defines the element and determines its chemical properties and behavior. Each element has one (and only one) atomic number. Atomic mass is the number of NUCLEONS in the atom. Each nucleon has a mass of one Dalton. Each element can have a variety of isotopes, each differing by the numbers of neutrons and causing it to have a different atomic mass. Relative atomic mass is the AVERAGED mass of all of an element’s isotopes. The relative proportions of an element’s isotopes are calculated along with the atomic mass of each isotope. The relative atomic mass is a convenient way to represent the mass and abundance of all that element’s isotopes, all in a single number. Back to the question

A 16. This question asks you to calculate the amount of tritium needed to leave 1 kilogram of tritium available (for the bomb) in 36 years. Tritium has a half-life of 12 years, so in 36 years it will have gone through 3 half-lives. Therefore, 8 kilograms of tritium must be packed into the bomb so there will be 1 kilogram undecayed in 36 years. NOTE: The answer is not 3 kilos. Some folks get confused about the half-life math. They think you can multiply the number of half-lives by the amount needed at that time, in order to figure out what you need to start with. This is wrong because the math is not used correctly. (Students with advanced math education will recognize that half-lives are an EXPONENTIAL function, not a LINEAR one. And they will use exponetials to arrive at the same answer more quickly than the method described here.) Working backward through time: In 36 years from now we need 1 kilogram, so 12 years before (24 years from now) we need to have 2 kilograms of tritium. In 24 years we need 2 kilograms of tritium, so 12 years before that time (12 years from now) we need to have 4 kilograms of tritium in the bomb. In 12 years we need 4 kilograms of tritium, so 12 years before that time (now) we need to have 8 kilograms of tritium in the bomb. Just to be sure, check this answer by working the problem out in the other direction: 8 kilograms of tritium today will be 4 kilograms in 12 years, then 2 kilograms in 24 years and finally only 1 kilogram in 36 years. After that length of time, the bomb would not have enough tritium to allow it to explode. Back to the question

A 17. For an atom to change from an atomic mass of 224 to 220, it must lose 4 nucleons. Alpha particles (helium nuclei) are 4 Daltons in mass, thus alpha decay is the process which will cause an element to lower its atomic mass by 4 Daltons. The particle emitted is an alpha particle (a slow moving helium nuclei). You can easily protect yourself from alpha particles with a thin piece of paper, or just a bit of distance. Put about a meter of air between you and the radioisotope and you will be safe from its alpha particles. The reason the radioisotope changes to a new element is because two protons are lost. Elements are defined by the number of protons and the element is transmutated by the alpha decay into a different element. Back to the question

A 18. There are lots of ways to do the math. Arthur’s way goes like this: 74 billion disintegrations per second (74 bdps) equals 2 Curies. (Because 1 Curie is 37 bdps, and 74 divided by 37 is 2). Specific activity is expressed as "Curies per gram", so the specific activity of the coin is 2/5 (or 0.4) Curies per gram. (Because 2 Curies is divided by 5 grams to give 2/5 = 0.4). I agree with Arthur, but like to write it all out in a long equation (because it helps me keep track of the units). I do it this way: The Coin 74 bdps ---------------- 5 grams times x The Conversion 1 Curie --------------- 37 bdps equals = The Answer (almost) 2 Curies ---------- 5 grams or = The Answer (finally) 0.4 Specific activity (Curies/gram) Back to the question

A 19. Nitrogen was created inside a star billions of years ago. After the star had used up all its hydrogen (creating helium), it underwent a contraction, which started a new type of nuclear fusion. By smashing a beryllium atom (with 4 protons) into a lithium atom (with 3 protons) a star made nitrogen. There may have been other combinations of atoms fused together that can make nitrogen, but the end result is a nucleus with 7 protons. That's nitrogen. Back to the question

A 20. Gold, with an atomic number of 79, has 79 protons. This is far too many for mere nuclear fusion inside a star. This requires a huge explosion of the star, a nova, to smash heavy iron nuclei together. It would take three iron nuclei and a hydrogen to give you enough protons to make gold. Or maybe four iron nuclei fused, creating an atom with 104 protons which could then undergo various types of nuclear decay, spitting out 25 protons (and a bunch of neutrons) to bring it down to the number 79. There are lots of different combinations of fusion and decay which could make gold. Perhaps you've thought of others. It doesn't matter exactly which combination you used. Just so long as the math worked out. Back to the question

A 21. NOTHING! There really is no difference between the atoms in your body and those in a distant star. Except, of course, for the distance between them. Back to the question

A 22. The inner most shell is the K-shell and it holds only 2 electrons. The next shell is the L-shell which can hold no more than 8 electrons. This is followed by the M-shell which can hold up to 18 electrons. Electrons are "assigned" to the inner shells first and one works outward, successively filling shells, until all the electrons are used up. Note: for large atoms (which have lots of electrons) this method of "filling shells from the bottom up" doesn't always work. Shells define the size of an atom by showing us where the outermost shell lays. That's the largest cloud and therefore represents the outer boundary of an atom. Back to the question

A 23. The nitrogen atom has 7 electrons (because it isn't an ion). You must distribute them. First among the (principle) shells and then among the orbitals (subshells). First the shells. The K-shell is the closest shell and it will hold only 2 electrons. The L-shell is next and it can hold up to 8 electrons, so there's plenty of room for the remaining 5 electrons. Now do the orbitals (subshells). All shells, including the K-shell, have an s orbital. In fact, the K-shell ONLY has an s orbital. So the two electrons in the K-shell are in an s orbital. This is a sphere. An inner sphere. The next shell, the L-shell, has an s orbital (obviously) and p orbitals. The orbitals of this outer shell will give the atom its shape. The remaining 5 electrons must be assigned to them. This makes things more complex. The first 2 electrons go into the L-shell's s orbital because s orbitals are of lower energy than p orbitals so electrons fill them first. The remaining 3 electrons go into the (higher energy) p orbitals. But which ones? There are three p orbitals to choose from; x, y and z. If you put one electron into each orbital you did it right! You remembered Hund's rule and used it correctly. What about spins? There are two spin rules to recall. Hund's rule tells you that when orbitals are "half full" (have only one electron in each) they should all be parallel. That means they should have the same spins. Therefore, all three electrons in nitrogen's p orbitals are either all "up" or all "down". The other spin rule comes from Pauli. He said that when two electrons share an orbital, they must have opposite spins, one "up" and the other "down". In nitrogen, the K-shell s orbital and L-shell s orbital are full, so both will have pairs of electrons with opposite spins. A summary of the nitrogen atom's electronic structure would look like this: K-shell has 2 electrons (s2) [Not drawn, but it would look like a smaller version of the L-shell's s orbital] L-shell has 5 electrons (s2, x-p1, y-p1, z-p1) The L-shell gives nitrogen its shape. It is hard to draw (clearly), but you can imagine it as a sphere (the s orbital) with three thin lobes (containing half-full p orbitals) sticking out in all three orientations. Each lobe is at "right angles" (squared) to the others. Note: just because an orbital has only one electron, it does not mean it forms a one-sided p orbital. Any electron within an orbital moves freely around it trying to fill both lobes at once. Nitrogen's shape is determined by overlapping ALL the orbitals in the L-shell. That gives you this weird cloud. Notice it is made of a full s orbital, and three half-full (less dense) p orbitals. Back to the question

A 24. Boron has 5 protons so it must have 5 electrons. Two of them fill the K-shell and the other three electrons go into the L-shell. The two electrons in boron's K-shell are in an s orbital (the only kind of orbital the K-shell can have) and they have opposite spins. They have nothing to do with the atom's size or shape. It is the outer shell that tells us that. Two of the three electrons in boron's L-shell are in the s orbital. Those two electrons also have opposite spins. The remaining electron can go into the x-p, y-p or z-p orbital. It doesn't matter which. So, boron's K-shell has 2 electrons (s2) [Not drawn] and its L-shell has only 3 electrons (s2, x-p1). Here I've shown that lone electron in the x-p orbital, but it could just as well have been in the y-p or z-p orbital. When the orbitals of the L-shell are overlapped on the nucleus we see boron's final shape. It has a thick sphere from the full s orbital and a single pair of lobes which are occupied by a lone electron. If this atom had one more electron (like carbon), it would have placed it in another orbital to give two pairs of half-full lobes at right angles to each other. (And those two electrons would have had parallel spins.) Back to the question

A 25. If you guessed that M-shells have d orbitals, you are right! (Don't be disappointed if you missed that one. This is new stuff). The M-shell has an s orbital (because all shells have an s orbital) which hold two electrons. It also has the normal types of p orbitals (x-p, y-p and z-p) which can hold another six electrons. That leaves 10 electrons to go somewhere. Perhaps you remembered that there are five different types of d orbitals. We didn't talk about them, but you may remember that ANY orbital can hold up to two electrons. That means an entire set of d orbitals can hold ten electrons. Therefore a full M-shell would have 2 electrons in the single s orbital, 6 electrons in three p orbitals, and 10 electrons in the five d orbitals. That gives you a total of 18 electrons that can find a home in the M-shell. Don't feel bad if you missed that. But do try to understand my explanation. A funny thing about these larger shells is that they can hold more electrons, but they don't like to! They don't like to use their d and f orbitals! For example, calcium (Ca) has 20 electrons (because it has an atomic number of 20). If you were assigning calcium's electrons to shells and orbitals you would expect the electrons to be placed like this... K-shell gets 2 electrons in its s orbital (and it does), L-shell gets 8 electrons in its s and three p orbitals (and it does). M-shell gets the remaining 10 electrons in its orbitals, BUT IT DOESN'T! There's plenty of room for those ten electrons in calcium's M-shell. Calcium puts two electrons into its M-shell s orbital (like it should) and six electrons into its M-shell p orbitals (filling all three types). You would expect the remaining two electrons to go into d orbitals (whatever they are shaped like). Instead, calcium puts them into the NEXT shell, the N-shell! Isn't that odd? So the electronic configuration of calcium is ... K-shell has 2 electrons in its s orbital. L-shell has 2 electrons in its s orbital and 6 in its three p orbitals. M-shell has 2 electrons in its s orbital and 6 in its three p orbitals. N-shell has 2 electrons in its s orbital. Weird! I'm pointing this out because many students stumble across it. In the large atoms, electrons are often assigned into the next shell's s orbital before the d orbital have been touched. And the even larger atoms which have f orbitals to work with don't use them until they have filled both the s AND p orbitals of the next larger shell. If you think this has to do with some part of quantum mechanics we haven't discussed, you'd be right! Back to the question