MCAT Physical : Periodic Trends

Study concepts, example questions & explanations for MCAT Physical

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Example Questions

Example Question #1 : Periodic Trends

Ionization energy __________ and atomic radius __________ down a group of the periodic table.

Possible Answers:

decreases . . . decreases

increases . . . decreases

stays the same . . . decreases

decreases . . . increases

increases . . . increases

Correct answer:

decreases . . . increases

Explanation:

Elements within a group have the same number of valence electrons, but in increasing energy levels. Elements toward the bottom of a group have valence electrons with higher energies in larger orbitals. This results in a larger radius and a weaker attractive force between the nucleus and outer electrons. The ionization energy decreases as the electrons are more removed from the attraction of the nucleus.

When moving down a group, atomic radius increases and ionization energy decreases.

Example Question #1 : Atomic Radius

Which of the following has the largest atomic radius?

Possible Answers:

Correct answer:

Explanation:

Atomic radius increases with increasing effective nuclear charge (Z). Elements toward the right and toward the top of the periodic table have the highest Z values. Protons and electrons are added in pairs as we traverse the periodic table from left to right. A attractive force is established between the positively-charged nucleus and the negatively-charged electron cloud, which increases as the number of particles grows.

When electrons are added or taken away without the same happening to a proton, an imbalance of charge accumulates. When more electrons are present than normal, the electron cloud sags farther away from the nucleus. When fewer electrons are present than normal, the electron cloud is drawn in more tightly toward the nucleus. Atoms with extra electrons (a negative charge) will have larger nuclei than their neutral counterparts. A chloride ion will thus has a larger atomic radius than argon, a potassium ion, or a calcium ion.

Example Question #1 : Atomic Radius

Which of the following elements has the greatest atomic radius?

Possible Answers:

B

N

O

C

F

Correct answer:

B

Explanation:

Atomic radius can be determined using the periodic trends. Atomic radius increases to the left of a period and down a group of the periodic table. Electronegativity, in contrast, increases to the right of a period and up a group of the periodic table. Relating the two, we can see that the greater the atomic radius, the weaker its electronegativity because the electrons are farther away from the nucleus and are unable to feel the attractive force of the protons in the nucleus.

Example Question #11 : Periodic Trends

When looking at the periodic table of elements, what is the general trend for increasing electronegativity?

Possible Answers:

To the left in a period and down in a group

To the right in a group and up in a period

To the left in a group and down in a period

To the right in a period and up in a group

Correct answer:

To the right in a period and up in a group

Explanation:

Electronegativity increases to the right and up when looking at the periodic table of elements, such that fourine is the most electronegative element. Following this general trend can help you determine the relative electronegativity between atoms within problems. Remember that periods run horizontally on the table and groups run vertically.

Example Question #11 : Atoms, Elements, And The Periodic Table

Which of the following elements has the highest electronegativity?

Possible Answers:

Sulfur

Nitrogen

Oxygen

Magnesium

Correct answer:

Oxygen

Explanation:

Remember that electronegativity increases as you approach the top right corner of the periodic table. Since oxygen is the farthest right and the highest up on the perioidic table out of these choices, we conclude that it has the highest electronegativity.

Example Question #12 : Periodic Trends

Which of the given elements has the lowest electronegativity?

Possible Answers:

C

F

Br

Fr

Correct answer:

Fr

Explanation:

Electronegativity, defined as the tendency of an atom to attract an electron, increases from left to right across a period, and from the bottom of a group to top. The least electronegative (sometimes called most electropositive) element is francium (Fr). This is because an electron that can be attracted to francium has the lowest ratio of attractive nuclear force to repulsive force by other electrons in the atom. Essentially, the distance between the attractive nuclear protons is too great for the attractive force to overcome the repulsion of the orbiting electron cloud. Francium will not be stable if it gains an electron, and is much more stable if it loses an electron and forms an octet as a positive ion.

Example Question #13 : Periodic Trends

Which of the given elements has the greatest electronegativity?

Possible Answers:

Fr

C

Br

F

Correct answer:

F

Explanation:

Electronegativity, defined as the tendency of an atom to attract an electron, increases from left to right across a period, and the bottom of each group to the top. The most electronegative element is fluorine (F). This is because an electron that can be attracted to fluorine has the greatest ratio of attractive nuclear force to repulsive force by other electrons. Essentially, fluorine is the most stable ion with a negative-one charge. It is small, allowing the nuclear protons to maintain the attractive force on the electron, and it has an octet, giving it the absolute maximum ionic stability possible.

Example Question #14 : Periodic Trends

Which of the following elements is the most electronegative?

Possible Answers:

Helium

Chlorine

Silver

Iodine

Correct answer:

Chlorine

Explanation:

The correct answer is chlorine. The most electronegative elements are those in the upper right of the periodic table, with the exception of the noble gases. Electronegativity describes how easily an element will gain an electron. The halogens (second to last group) "want" an extra electron to complete their valence shell. Iodine and chlorine are both halogens. Chlorine, however, has a smaller atomic radius, and therefore a smaller distance between the protons and outer electrons. Chlorine thus has a stronger attraction for an additional electron due to the greater effective nuclear attraction.

Example Question #11 : Periodic Trends

Which of the following atoms has the lowest electronegativity?

Possible Answers:

Argon

Fluorine

Chlorine

Potassium

Sodium

Correct answer:

Argon

Explanation:

Electronegativity is the tendency for an atom to attract electrons.

We know that fluorine is the most electronegative atom on the periodic table, and that electronegativity increases to the right across periods and upwards within groups. The trick answer might possible be potassium, since it has a very low electronegativity and is located to the left of the table. The noble gases, however, have virtually no electronegativity. Remember, noble gases are characterized by their valence octets in the ground state. This stability generates resistance to any electron change, including both electronegativity and ionization energy.

Example Question #892 : Mcat Physical Sciences

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

A scientist is studying flourine. Which of the following is a possible electronegativity value for flourine?

Possible Answers:

3.9

Undefined

2.2

0

3.4

Correct answer:

3.9

Explanation:

Flourine must have an electronegativity value higher than oxygen. Remember your periodic trends: electronegativity increases as we move up and to the right on the periodic table. We are told in the passage that the electronegativity of oxygen is 3.44, therefore, our answer must be 3.9.

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