MCAT Physical : Periodic Table Groupings

Study concepts, example questions & explanations for MCAT Physical

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Example Questions

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Example Question #1 : Elements And The Periodic Table

Which of the following is true about transition metals?

I. Transition metals form acidic oxides

II. A transition metal can have multiple oxidation states

III. Transition metals can be reduced, but can never be oxidized

Possible Answers:

II only

I and II

I only

I, II, and III

Correct answer:

II only

Explanation:

Transition metals are classified as metals; therefore, the oxides they form are called metal oxides. Metal oxides are basic compounds, whereas non-metal oxides are acidic compounds. Metal oxides, such as  and , give rise to high pH values, and non-metal oxides, such as  and , give rise to low pH values.

An oxidation state is defined as the degree of oxidation a compound can achieve. It is often calculated by observing the gain and loss of electrons in an atom. For example, an atom that loses two electrons will have an oxidation number of , whereas an atom that gains two electrons will have an oxidation number of . Some transition metals have multiple oxidation states because they can lose varying amounts of electrons. This occurs because the energy difference between the outermost  orbital and the outermost  orbital is small; therefore, the energy required to remove the electron from the subsequent  orbital is comparable to removing electrons from the  orbital.

For example, iron () can have an oxidation state of  or . Iron’s electron configuration is . When it loses two electrons, iron will have an empty  orbital. The electron configuration of  will be . The amount of energy difference between the  orbital and the  orbital is very small; therefore, it is easy for iron to lose another electron from the  orbital and become . On the other hand, it is very hard to remove electrons from a filled  orbital. This explains why non-metals, such as the oxygen and halogen groups, and metals, such as alkali and alkaline earth metals, have only one oxidation state.

Reduction involves gaining electrons and oxidation involves losing electrons. This means that reduction will decrease the oxidation number and oxidation will increase the oxidation number. It is generally favorable for a transition metal to lose electrons and become oxidized, though reduction can be achieved by adding enough energy to the system (such as in a electrolytic cell).

Example Question #51 : Atoms, Elements, And The Periodic Table

Which of the following is not true of halogens?

Possible Answers:

Reactivity is somewhat higher for elements farther up in the halogen group, due to decreasing atomic radius

They have low electron affinity, and generally are not very reactive

Out of seven total valence electrons, five are from the p subshell

Their most common oxidation state is -1

They want to gain one electron to achieve a full valence shell

Correct answer:

They have low electron affinity, and generally are not very reactive

Explanation:

Elements are generally more reactive the closer they are to the stable noble gas configuration (eight valence electrons). Halogens have seven valence electrons, so only need one more to achieve a full valence shell, thus they are said to have high "electron affinity" since they react easily to gain the final valence electron. All the other options are correct statements about halogens.

Example Question #21 : Periodic Table Groupings

Which of the following is true about insulators?

Possible Answers:

An insulator is a material in which charges do not move freely

Copper is an example of a good insulator

An insulator is a material in which charges can move freely

Sound moves fastest through an insulating material

Correct answer:

An insulator is a material in which charges do not move freely

Explanation:

Good insulators are usually non-metals, in which the electrons are not able to freely move. Insulators, unlike conductors, do not carry current. This is because charges cannot move freely in an insulating material, making this choice the correct answer.

Sound would travel slowest through an insulating material, as there is less ability to compress and propagate the sound wave. Copper is an example of a good conductor, and is a poor insulator.

Example Question #3 : Elements And The Periodic Table

A researcher has substance X and substance Y. After performing several tests, he finds out that substance X is a metal oxide and substance Y is a non-metal oxide. is  What will the researcher observe when he measures the pH of each oxide in aqueous solutions of equal concentrations?

Possible Answers:

The pH will be 7 because metal and non-metal oxides are products of a neutralization reaction

Substance X will have a lower pH because metals produce acidic oxides

Both oxides will have a pH greater than 7

Substance Y will have a lower pH because non-metals produce acidic oxides

Correct answer:

Substance Y will have a lower pH because non-metals produce acidic oxides

Explanation:

One of the key distinguishing characteristics of metals and non-metals is the acidity of their oxides. Metal oxides are basic oxides, whereas non-metal oxides are acidic oxides. Remember that acids have low pH (high hydrogen ion concentration), whereas bases have high pH (low hydrogen ion concentration); therefore, non-metal oxides will have the lower pH.

A neutralization reaction is a special type of reaction that occurs between an acid and a base. This type of reaction will produce a pH of 7 if the reacting species are a strong acid and a strong base. Metal and non-metal oxides do not involve acid-base reactions; therefore, neutralization reaction is irrelevant to this question.

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