Question

1. an isotope has an atomic number 51 and a mass number 123. how many protons, neutrons and electrons does it have if its neutral?

circle the letter of the choice that best completes the statement.

1. what is the best way to describe where electrons are in an atom?

a. electrons are in the nucleus.
b. electrons are in orbits around the nucleus like planets go around the sun.
c. electrons can either be in the nucleus or outside the nucleus.
d. electrons are in \cloud - like\ areas called orbitals outside of the nucleus.

1. the mass of an electron is

a. smaller than the mass of a proton.
b. smaller than the mass of a neutron.
c. a tiny fraction of the mass of an atom.
d. all of the above.

1. one atomic mass unit is

a. 1/12 the mass of a carbon - 12 atom.
b. 1/16 the mass of an oxygen - 16 atom.
c. exactly the mass of one proton.
d. approximately the mass of one proton plus one neutron.

1. the atomic mass of an atom is usually not a whole number because it accounts for

a. only the relative abundance of the atoms isotopes.
b. only the mass of each of the atoms isotopes.
c. the mass of the atoms electrons.
d. both the relative abundance and the mass of each of the atoms isotopes.

1. which is most like what we believe an atom to be like?

a. chocolate chip cookie dough
b. pool balls
c. the sun with planets orbiting
d. a very small nucleus with cloud - like orbitals surrounding it

Explanation:

• Question 16: Electrons are not in the nucleus (eliminating a and c). The Bohr model (electrons in orbits like planets) is an oversimplification. The modern view is that electrons are in "cloud - like" orbitals (quantum mechanical model), so d is correct.
• Question 17: Electrons have a mass of approximately \(9.1\times10^{- 31}\) kg. Protons have a mass of about \(1.67\times10^{-27}\) kg and neutrons about \(1.69\times10^{-27}\) kg. So electrons are smaller than protons and neutrons. Also, since protons and neutrons make up most of the atomic mass, electrons are a tiny fraction of the atom's mass. Thus, d (all of the above) is correct.
• Question 18: By definition, one atomic mass unit (amu) is \(1/12\) the mass of a carbon - 12 atom. So a is correct.
• Question 19: Atomic mass is a weighted average. It takes into account both the mass of each isotope and its relative abundance. So d is correct.
• Question 20: The modern atomic model has a very small nucleus (containing protons and neutrons) with electron - cloud (orbitals) surrounding it. So d is correct.

Answer:

1. d. Electrons are in "cloud - like" areas called orbitals outside of the nucleus.
2. d. all of the above.
3. a. \(1/12\) the mass of a carbon - 12 atom.
4. d. both the relative abundance and the mass of each of the atom's isotopes.
5. d. a very small nucleus with cloud - like orbitals surrounding it.