AP Chemistry Part 1: Structure of Matter FRQ Test # 1

Q1. Explain each of the following in terms of atomic and molecular structures and/or forces.

(a) The first ionization energy for magnesium is greater than the first ionization energy for calcium.

Answer explanation

Ionization energy is the energy required to remove an electron from an atom. The outermost electron in Ca is at the 4s energy level. The outermost electron in Mg is at the 3s level. The outermost electron in Ca is at a higher energy level and is more shielded from the nucleus, making it easier to remove.

Q1. Explain each of the following in terms of atomic and molecular structures and/or forces.

(b) The first and second ionization energies for calcium are comparable, but the third ionization energy is much greater.

Answer explanation

Calcium has two electrons in its outer shell. The second ionization energy will be larger than the first but still comparable because both electrons are being removed from the same energy level. The third electron is much more difficult to remove because it is being removed from a lower energy level, so it will have a much higher ionization energy than the other two.

Q1. Explain each of the following in terms of atomic and molecular structures and/or forces.

(c) The difference between the first ionization energies of sodium and magnesium are comparable but the different between the second ionization energies for the said elements is much greater.

Answer explanation

Magnesium has two electrons in its outer shell. The second ionization energy will be larger than the first but still comparable because both electrons are being removed from the same energy level. The third electron is much more difficult to remove because it is being removed from a lower energy level, so it will have a much higher ionization energy than the other two. While, as for sodium, it has one electron in its outer shell so the second ionization energy will be much higher due to the same reason.

Q1. Explain each of the following in terms of atomic and molecular structures and/or forces.

(d) The first ionization energy for aluminium is lower than the first ionization energy for magnesium.

Answer explanation

The valence electron to be removed from magnesium is located in the completed 3s subshell, while the electron to be removed from aluminum is the lone electron in the 3p subshell. It is easier to remove the electron from the higher-energy 3p subshell than from the lower energy (completed) 3s subshell, so the first ionization energy is lower for aluminum.

Q2. The mass spectra for the hypochlorite ion, ClO- is given.

Oxygen has only one stable isotope, which has a mass of 16 amu.

(a) How many neutrons does the most common isotope of chlorine have? (Work it out from the diagram given)

Answer explanation

The most common mass of a ClO- ion is 51 amu. 51 amu-16 amu = 35 amu, which must be the mass of the most common isotope of chlorine. As mass number is equal to protons + neutrons, and chlorine has 17 protons (its atomic number), 35 - 17 = 18 neutrons.

Q2. The mass spectra for the hypochlorite ion, ClO- is given.

Oxygen has only one stable isotope, which has a mass of 16 amu.

(b) Using the mass spectra, calculate the average mass of a hypochlorite ion.

Answer explanation

51(0.75) + 53(0.25) = 51.5 amu

Q2. The mass spectra for the hypochlorite ion, ClO- is given.

Oxygen has only one stable isotope, which has a mass of 16 amu.

(c) Does the negative charge on the ion affect the value of masses in the mass spectra? Justify your answer.

Answer explanation

No. The only subatomic particles that contribute to the mass of any atom are neutrons and protons. Changing the number of electrons does not change the mass significantly.

Q2. The mass spectra for the hypochlorite ion, ClO- is given.

Oxygen has only one stable isotope, which has a mass of 16 amu.

(d) The negative charge in the ion is located around the oxygen atom. Speculate as to the electronegativity differences between both elements.

Answer explanation

An oxygen atom is smaller than a chlorine atom, and as such is more electronegative. The electrons in the bond are thus more attracted to oxygen than chlorine.