Introductory Chemistry

Quiz: Atoms and subatomic particles.
Answer key.

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1. Proton (charge +1 per particle) and electron (charge -1 per particle). This gives an electrically neutral atom.

2. Electrons

3. Protons and neutrons

#1-3 all describe basic features of atoms, at the level you learn them in introductory chemistry. Note that only #1 has a logical explanation, at our level.

4. First, let's figure out how many there are of each kind of particle. Since it is beryllium, Z = 4, it must have 4 protons. Since it is a neutral atom, the number of electrons must also be 4, for electrical neutrality. Since the mass number is given as 6, and the number of protons is 4, there must be 2 neutrons. Now, make a sketch, showing a "tiny" nucleus with 4 protons and 2 neutrons; put 4 electrons in a large "cloud" around the nucleus.

Don't expect to actually find a 6Be atom. Nearly all Be atoms in nature are 9Be. (The atomic weight shown on the PT is near 9.) 6Be atoms have a half life of about 10-20 seconds.

It is important here to make the distinction between the mass number of a specific atom (or isotope) and the atomic weight (mass) of the element. This particular Be atom has a mass number = 6, meaning it has a total of 6 protons and neutrons. The element beryllium has an atomic weight of 9.012, reflecting the precise weight of Be atoms, and the distribution of the isotopes in nature.

5. a. True. The atomic mass is based on the average of the atoms in nature. If "all" are mass 9, then clearly the average is mass 9.

b. False. Imagine that half of all Be atoms were mass 8 and half were mass 10. The average would be mass 9 -- even though there are no atoms of that mass.

A real example of this occurs with bromine. Natural bromine consists of (approximately) 50% Br-79 and 50% Br-81. As a result, the atomic weight of Br is 80. However, there is no Br-80 in nature.

Note, then, that you cannot predict what the major isotope of an element is by looking at its atomic weight -- unless you know there is only one major isotope. As a practical matter, the number of major isotopes is small (often 1) for lighter elements, and larger for heavier elements. Is it a "problem" that you cannot predict isotopes? Not really. In ordinary chemistry, you usually don't care about the isotopes. And if you do care, you look them up. Tables of isotopes are in many handbooks, perhaps in your textbook -- and available on the web. One such web source is listed on my page Internet resources: Introductory Chemistry, section on "Nuclei; Isotopes; Atomic weights".

6. a. cobalt, Co. 29 protons means cobalt. The symbol must be written as shown, with a capital C and a small o. Anything else is incorrect, and if what you write is ambiguous, I would not give you the benefit of the doubt. CO is the formula for carbon monoxide, a poisonous gas.

b. Don't know. Why? (If you think you do know, let's talk about it.)

7. a. It has the same number of protons. C has 6 protons. Period.

b. It has more neutrons than average. Average C has mass 12.01 (from the periodic table). This atom has mass 14. Since the only thing that affects the mass -- given that it is C -- is the number of neutrons, this must have more neutrons.

The two questions above both emphasize some very important basic points about atoms. First, the number of protons identifies the chemical identity of the element. Z = P, where Z is the atomic number, and P is the number of protons. (Of course, for a neutral atom, Z = P = E.) Second, the number of neutrons can vary -- and is not predictable by any laws of chemistry. However, the number of neutrons does determine the mass: A = P + N, where A is the mass number of a specific atom and N is the number of neutrons.

8. The first issue is to make sure you find these two elements on the PT. Their names are similar -- probably the two most easily mixed up names, at least for major elements. Magnesium (Mg) is in group 2 (2A), Z = 12; manganese (Mn) is in group 7 (7B); Z = 25. Now, having found them... Main group elements, in the A groups (those near the sides of the PT), tend to behave predictably, whereas the transition metals (B groups), typically have more complex behavior. This is an oversimplification, and you will explore the complexities as you go on in chemistry. But for now it is a useful start. So... magnesium is a main group element, and more likely to be predictable.

For those who want a bit more beyond that explanation... magnesium is in group 2A, so its common ion should be (and is) 2+. Therefore, we expect magnesium chloride to be MgCl2. Manganese forms both MnCl2 and MnCl3 -- neither of which is predictable based on anything we have said so far (except that charges 2+ and 3+ are common for transition metals).


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Last update: August 14, 2011