MCAT Physical : Periodic Trends

Study concepts, example questions & explanations for MCAT Physical

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Example Questions

Example Question #21 : Periodic Trends

Electronegativity is an important concept in physical chemistry, and often used to help quantify the dipole moment of polar compounds. Polar compounds are different from those compounds that are purely nonpolar or purely ionic. An example can be seen by contrasting sodium chloride, NaCl, with an organic molecule, R-C-OH. The former is purely ionic, and the latter is polar covalent.  

When comparing more than one polar covalent molecule, we use the dipole moment value to help us determine relative strength of polarity. Dipole moment, however, is dependent on the electronegativity of the atoms making up the bond. Electronegativity is a property inherent to the atom in question, whereas dipole moment is a property of the bond between them.

For example, oxygen has an electronegativity of 3.44, and hydrogen of 2.20. In other words, oxygen more strongly attracts electrons when in a bond with hydrogen. This leads to the O-H bond having a dipole moment.

When all the dipole moments of polar bonds in a molecule are summed, the molecular dipole moment results, as per the following equation.

Dipole moment = charge * separation distance

Electronegativity is closely associated with the principle of ionization energy. Which of the following defines ionization energy?

Possible Answers:

Exactly the amount of energy needed to add an electron to a species at STP

The amount of energy needed to remove an electron from a species in its gaseous state

The amount of energy needed to either remove or add an electron to a species in its gaseous state

Exactly the amount of energy needed to remove an electron from a species at STP

The amount of energy needed to add an electron to a species in its gaseous state

Correct answer:

The amount of energy needed to remove an electron from a species in its gaseous state

Explanation:

Even though adding an electron would generate an ion (anion), ionization energy is defined as the energy needed to remove an electron.

Example Question #21 : Periodic Trends

Electronegativity is an important concept in physical chemistry, and often used to help quantify the dipole moment of polar compounds. Polar compounds are different from those compounds that are purely nonpolar or purely ionic. An example can be seen by contrasting sodium chloride, NaCl, with an organic molecule, R-C-OH. The former is purely ionic, and the latter is polar covalent.  

When comparing more than one polar covalent molecule, we use the dipole moment value to help us determine relative strength of polarity. Dipole moment, however, is dependent on the electronegativity of the atoms making up the bond. Electronegativity is a property inherent to the atom in question, whereas dipole moment is a property of the bond between them.

For example, oxygen has an electronegativity of 3.44, and hydrogen of 2.20. In other words, oxygen more strongly attracts electrons when in a bond with hydrogen. This leads to the O-H bond having a dipole moment.

When all the dipole moments of polar bonds in a molecule are summed, the molecular dipole moment results, as per the following equation.

Dipole moment = charge * separation distance

Electronegativity is closely associated with the principle of ionization energy. Which of the following is true of second, third, and successive ionization energies?

Possible Answers:

They generally decrease in magnitude, with large jumps when you move from a noble gas configuration to a non-noble gas configuration

They generally increase in magnitude linearly with the magnitude of resulting ionic charge

They generally increase in magnitude, with large jumps when you move from a noble gas configuration to a non-noble gas configuration

They generally decrease in magnitude linearly with the magnitude of resulting ionic charge

They can increase or decrease in magnitude depending on the species

Correct answer:

They generally increase in magnitude, with large jumps when you move from a noble gas configuration to a non-noble gas configuration

Explanation:

As you ionize atoms, you generate charged ionic species. These charges will resist further ionization (cations will more strongly attract the electrons you are trying to pull away). Noble gas configurations are particularly stable, so you would expect a large increase in needed energy to ionize away from this state.

Example Question #21 : Periodic Trends

Which of the given atoms has the lowest ionization energy?

Possible Answers:

Rn

Cs

Pt

W

Correct answer:

Cs

Explanation:

Cesium, tungsten, platinum, and radon are all in the same row (period) of the periodic table.

The ionization energy of an atom, defined as the energy required to remove an electron from an atom’s orbit, increases as one move from left to right across the periodic table. This is primarily because the atoms decrease in atomic size, meaning that the outermost electrons that would be removed to create an ion are closer to the nucleus and require more work to remove. Cesium has the largest atomic radius and attains an octet if an electron is removed, making it more stable as an ion, meaning that it will be fairly easy to remove an electron from cesium compared to the other options.

Example Question #22 : Periodic Trends

Which of the given atoms has the greatest ionization energy?

Possible Answers:

I

Cl

Br

F

Correct answer:

F

Explanation:

Fluorine, chlorine, bromine, and iodine are all halogens in the same group of the periodic table.

The ionization energy of an atom, defined as the energy required to remove an electron from an atom’s orbit, decreases as one moves down a group of the periodic table. This is primarily because the atoms increase in size as one moves down the group, thus decreasing the force between the nucleus and an electron in an outermost orbit. It requires less energy to remove an electron that is farther from the protons, than to remove one that is held closer to the nucleus.

Example Question #23 : Periodic Trends

Which element has the highest first ionization energy?

Possible Answers:

Nickel

Lithium

Nitrogen

Carbon

Chlorine

Correct answer:

Chlorine

Explanation:

Ionization energy describes the energy required to remove a single electron from an atom. The periodic table trend for ionization energy is to increase from left to right or a period, and decrease from top to bottom of a group. Typically, elements that have seven valence electrons, one electron away from having a complete valence shell, have very high ionization energies. These elements are found adjacent to noble gases on the periodic table: the halogens. Chlorine would thus have a very high ionization energy.

Alkali metals have the lowest ionization energies, as removing a single electron will give them a full octet configuration.

Example Question #22 : Periodic Trends

Which answer choice has the greatest first ionization energy?

Possible Answers:

Copper

Krypton

Oxygen

Bromine

Lithium

Correct answer:

Krypton

Explanation:

Noble gasses, like krypton, have a full octet in their valence shells. Their stability makes it very hard to alter their valence configuration. First ionization energy increases from left to right across the periodic table.

Bromine, which is right next to krypton, also has a high first ionization energy because it would much rather capture an electron than release one. As you move down the table, the energy for that first ionization gets lower, but as you move across from left to right it gets higher. Transition elements don't follow the trend exactly, but the energy required for them is not higher than halogens or noble gasses.

Alkali metals, like lithium, have the lowest first ionization energies because they obtain a full octet by releasing an electron.

Example Question #21 : Periodic Trends

Using your knowledge of periodic trends, determine which atom or ion has the largest first ionization energy.

Possible Answers:

Correct answer:

Explanation:

Ionization energy refers to the amount of energy necessary to remove a single electron from an atom or ion in the gas state, resulting in an increase in charge.

To find our answer, we need to know that ionization energy increases as we move left to right across a period and decreases as we travel down a group. Alkali metals, on the left side of the table, gain stability with the removal of a single electron, while halogens lose stability. Atoms or ions with a electron configurations similar to those of the noble gases are very stable. Due to this stability, larger amounts of energy are required to alter the electron configuration.

When a halogen gains an electron, it achieves a stable valence octet; thus, the fluoride and bromide ions are incredibly stable and will have very high first ionization energies. Bromine is lower on the periodic table than fluorine, so its electrons are separated from its positively-charged nucleus by more energy shells. This effect, called shielding, leads to a smaller amount of energy required to remove one electron from bromine. , therefore, is our solution.

Example Question #21 : Atoms, Elements, And The Periodic Table

Which element would you expect to have the greatest second ionization energy?

Possible Answers:

Potassium

Magnesium

Calcium

Titanium

Strontium

Correct answer:

Potassium

Explanation:

Ionization energy is the energy required by an element to remove a valence electron and gain a positive charge. Because alkali metals and alkaline earth metals have one and two valence electrons, respectively, their first ionization energies are both relatively low; shedding a valence electron and absorbing a positive charge actually stabilizes these elements.

Alkaline earth metals can easily release their second valence electron, and have relatively low second ionization energies. Removing this second electron gives these elements a noble gas electronic configuration. This is why the ions of alkaline earth metals are most commonly Mg2+ and Ca2+. In comparison, alkali metals cannot easily shed a second valence electron because it would require removing an electron from an already-filled valence shell. Because of this, alkali metals have extremely HIGH second ionization energies in comparison to alkaline earth metals.

Of the answer choices, potassium would have the highest second ionization energy because it is an alkali metal, rather than an alkaline earth metal.

Example Question #24 : Periodic Trends

Which of the given atoms has the least metallic character?

Possible Answers:

Ru

Xe

Rb

Sn

Correct answer:

Xe

Explanation:

Rubidium, ruthenium, tin, and xenon are all in the same row (period) of the periodic table.

Metallic character decreases as one moves from left to right across a period. This is generally due to a reduction in the number of electrons in the outermost shell being from the s orbital. Increased numbers of protons reduce the atomic radius, making it harder for outer shells to have electron motility. Metals will generally have larger radii, allowing for greater electron freedom, ionization, and conductivity. Remember that metallic characteristics include malleability, ductility, luster, and shininess.

Example Question #22 : Periodic Trends

Which of the given atoms has the greatest metallic character?

Possible Answers:

Li

Fr

K

Rb

Correct answer:

Fr

Explanation:

Lithium, potassium, rubidium, and francium are all alkali metals in the same group of the periodic table.

Metallic character increases as one moves down each group the periodic table. This is generally due to an increase in the distance between the electrons and the nuclear protons, making it easier for an electron to be lost to a bond or to move freely during electric conduction. Remember that metallic characteristics include malleability, ductility, luster, and shininess.

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