Kaplan General Chemistry: Chapter 2 - The Periodic Table

The content in this chapter should be relevant to about 10% of all questions about general chemistry on the MCAT.

What are groups in the Periodic Table?

Groups are columns in the Periodic Table, and elements in the same group share similar valence electron configurations and chemical properties.

What are valence electrons?

Valence electrons are the electrons in an atom’s outermost energy shell and are involved in chemical bonding.

How many periods are there in the periodic table, and what do they represent?

There are seven periods in the periodic table, representing the principal quantum numbers n = 1 through n = 7 for the s- and p-block elements.

How does the number of protons and electrons change across a period?

In each period, each element has one more proton and one more electron than the element to its left (in their neutral states).

What do elements in the same group share?

Elements in the same group have the same electronic configuration in their valence shell and share similar chemical properties.

How are valence electrons different from other electrons?

Valence electrons are farthest from the nucleus and are less tightly held, making them more available for bonding.

What does the Roman numeral above each group in the periodic table represent?

The Roman numeral above each group represents the number of valence electrons in the neutral state of the elements in that group.

What is the difference between representative and nonrepresentative elements?

Representative elements (Groups IA to VIIIA) have valence electrons in the s or p subshells, while nonrepresentative elements (transition metals and lanthanides/actinides) have valence electrons in the s and d or f subshells.

What electron configuration does an element in Group VA have?

An element in Group VA has five valence electrons, with the configuration s²p³.

How does the periodic table organize elements into periods and groups?

The periodic table arranges elements into periods (rows) and groups (columns) based on increasing atomic number and similar electronic configurations.

How do the periodic table groups determine an element's chemical properties?

Elements in the same group have similar valence electron configurations, which leads to similar chemical properties.

What is the trend for valence electrons in the periodic table?

Groups contain elements with the same number of valence electrons, and elements in the same period increase in the number of valence electrons as you move from left to right.

Where are metals found on the periodic table?

Metals are found on the left side and in the middle of the periodic table.

What are some physical properties of metals?

Metals are lustrous (shiny) solids (except mercury, which is liquid), have high melting points and densities, and are generally malleable and ductile.

What is malleability?

Malleability is the ability of a metal to be hammered into shapes.

What is ductility?

Ductility is the ability of a metal to be pulled or drawn into wires.

What are the atomic characteristics of metals?

Metals have low effective nuclear charge, low electronegativity, large atomic radius, small ionic radius, low ionization energy, and low electron affinity.

What makes transition metals unique in terms of bonding?

Transition metals often have two or more oxidation states (different charges when bonding with other atoms) due to their loosely held valence electrons.

Where are the valence electrons of metals located?

• Active metals: s subshell

• Transition metals: s and d subshells

• Lanthanides and actinides: s and f subshells

Which transition metals are known for being nonreactive?

Transition metals

What makes alkali and alkaline earth metals metallic in nature?

Alkali and alkaline earth metals are metallic because they easily lose electrons from the s subshell of their valence shells.

Where are nonmetals primarily located on the periodic table?

Nonmetals are found predominantly on the upper right side of the periodic table.

What are the physical properties of nonmetals?

Nonmetals are generally brittle in the solid state, show little or no metallic luster, and are poor conductors of heat and electricity.

What are the atomic characteristics of nonmetals?

Nonmetals have high ionization energies, high electron affinities, high electronegativities, small atomic radii, and large ionic radii.

Why are nonmetals poor conductors of heat and electricity?

Nonmetals are poor conductors because they are unable to easily give up electrons, making it difficult for them to conduct heat and electricity.

What are some common characteristics of carbon as a nonmetal?

Carbon is brittle, nonlustrous, and a poor conductor of heat and electricity, fitting the general characteristics of nonmetals.

Where are metalloids located on the periodic table?

Metalloids form a stair-step group of elements separating the metals and nonmetals on the periodic table.

What are metalloids also known as?

Metalloids are also called semimetals because they share characteristics with both metals and nonmetals.

How do the electronegativities and ionization energies of metalloids compare to metals and nonmetals?

The electronegativities and ionization energies of metalloids lie between those of metals and nonmetals.

What are some examples of metalloids and their properties?

Examples include silicon (Si), which has a metallic luster but is brittle and a poor conductor.

How do metalloids react with different elements?

The reactivity of metalloids depends on the element they are reacting with. For example, boron (B) behaves like a nonmetal when reacting with sodium (Na) but like a metal when reacting with fluorine (F).

What is the effective nuclear charge (Zeff) and how does it change across a period?

Zeff is the net positive charge experienced by the outermost electrons due to the electrostatic pull from the nucleus. As you move from left to right across a period, Zeff increases, as more protons are added to the nucleus, pulling the valence electrons closer.

How does the effective nuclear charge (Zeff) change as you move down a group?

As you move down a group, the Zeff remains relatively constant. This is because the increased number of inner electron shells (shielding) reduces the electrostatic attraction between the valence electrons and the nucleus.

What effect does shielding have on the valence electrons as you move down a group?

As you move down a group, shielding from the inner electron shells reduces the electrostatic attraction between the valence electrons and the nucleus, causing the valence electrons to be held less tightly.

What is the octet rule, and why is it important?

The octet rule suggests that atoms are most stable with eight electrons in their valence shell, similar to the noble gases. This is a guiding principle in understanding chemical bonding, although many exceptions exist.

How do increasing principal quantum numbers affect the electrostatic attraction between valence electrons and the nucleus?

As the principal quantum number increases (moving down a group), the valence electrons are further from the nucleus, and the electrostatic attraction between the valence electrons and nucleus decreases due to the increased number of inner electron shells.

How is atomic radius defined?

Atomic radius is the distance from the center of an atom's nucleus to the outermost electron shell, defined as half the distance between two atoms of the same element in contact.

What is the difference between atomic radius and ionic radius?

Atomic radius refers to the size of a neutral atom, while ionic radius depends on the atom’s ionization—whether it gains or loses electrons to form an ion.

What happens to atomic radius as you move across a period from left to right?

Atomic radius decreases as you move across a period from left to right, because the increasing nuclear charge (Zeff) pulls electrons closer to the nucleus, making the atom smaller.

What happens to atomic radius as you move down a group in the periodic table?

Atomic radius increases as you move down a group because the valence electrons are farther from the nucleus due to the increasing number of inner electron shells (higher principal quantum number).

What is the trend in atomic radius within a group and across a period?

Atomic radius increases going down a group and decreases across a period from left to right.

Which element has the largest atomic radius, and why?

Cesium (Cs) has the largest atomic radius because it is in the farthest left and bottom position in the periodic table, where atoms have more electron shells and a larger radius.

Which element has the smallest atomic radius, and why?

Helium (He) has the smallest atomic radius because it is in the top-right corner of the periodic table, where atoms have fewer electron shells and a stronger nuclear pull on the electrons.

What happens to ionic radius when metals form cations (positive ions)?

When metals form cations, their ionic radius decreases because they lose electrons, resulting in a smaller electron cloud and a stronger attraction between the remaining electrons and the nucleus.

How does the ionic radius of nonmetals change when they form anions (negative ions)?

When nonmetals form anions, their ionic radius increases because they gain electrons, causing greater electron-electron repulsion and a larger electron cloud.

How do metalloids behave in terms of ionic radius, and what influences their behavior?

Metalloids can behave like either metals or nonmetals depending on their position relative to the metalloid line. They typically gain electrons like nonmetals or lose them like metals, influencing their ionic radius accordingly.

What is ionization energy (IE)?

Ionization energy (IE) is the energy required to remove an electron from a gaseous atom or ion. It is an endothermic process, meaning it requires the input of energy.

How does ionization energy change as you move across a period and down a group in the periodic table?

Ionization energy increases from left to right across a period and from bottom to top in a group.

What is the relationship between effective nuclear charge (Zeff) and ionization energy?

As Zeff increases, the ionization energy increases because the valence electrons are more tightly bound to the nucleus, making them harder to remove.

What is the first ionization energy?

The first ionization energy is the energy required to remove the first electron from a neutral atom, forming a univalent cation (X⁺).

How does the second ionization energy differ from the first?

The second ionization energy is the energy required to remove a second electron from a monovalent cation (X⁺), forming a divalent cation (X²⁺). It is generally larger than the first ionization energy because the electron is being removed from a positively charged species.

Why do alkali metals (Group IA) have low ionization energies?

Alkali metals have low ionization energies because they have only one valence electron, which is easy to remove, leading to a stable, filled valence shell after ionization.

Why do halogens (Group VIIA) have high ionization energies?

Halogens have high ionization energies because they are one electron short of a stable, filled valence shell. Removing an electron disrupts this stability, making it more difficult to ionize.

How do alkaline earth metals (Group IIA) differ from alkali metals in terms of ionization energy?

Alkaline earth metals have two valence electrons, so it takes more energy to remove both electrons compared to the single electron removed from alkali metals. However, their ionization energies are still relatively low.

What makes noble gases (Group VIIIA) have the highest ionization energies?

Noble gases have stable electron configurations, so they are extremely unwilling to give up an electron, resulting in the highest ionization energies.

What units are used to measure ionization energy?

Ionization energy is typically measured in electron volts (eV) or kilojoules per mole (kJ/mol). The units depend on the context, but eV is commonly used for individual atom measurements, and kJ/mol is used for moles of atoms.

What is electron affinity?

Electron affinity refers to the energy dissipated by a gaseous species when it gains an electron. It is the opposite of ionization energy, which involves the removal of an electron.

Is the process of gaining an electron exothermic or endothermic?

Gaining an electron is an exothermic process because energy is released when the atom acquires an electron. The process has a negative ΔHrxn.

Why is electron affinity reported as a positive number?

Even though electron affinity is an exothermic process (energy is released), it is reported as a positive number because it reflects the energy dissipated when the electron is gained.

How does effective nuclear charge (Zeff) affect electron affinity?

The greater the Zeff, the stronger the electrostatic pull between the nucleus and the valence shell electrons, leading to a greater release of energy when the atom gains an electron. Thus, electron affinity increases as Zeff increases across a period.

How does electron affinity change across a period (from left to right)?

Electron affinity increases from left to right across a period because the nucleus' pull on the valence electrons becomes stronger, making it easier to gain an electron and release more energy.

How does electron affinity change down a group (from top to bottom)?

Electron affinity decreases as you move down a group because the valence electrons are farther from the nucleus, reducing the electrostatic attraction and making it harder to gain an electron.

Do metals tend to have high or low electron affinity values?

Metals generally have low electron affinity values because they are more likely to lose electrons rather than gain them.

What is electronegativity?

Electronegativity is the measure of the attractive force an atom exerts on an electron in a chemical bond. It indicates how strongly an atom will attract electrons when bonded to another atom.

How are electronegativity and ionization energy related?

The higher the ionization energy, the higher the electronegativity. Conversely, the lower the ionization energy, the lower the electronegativity. Both are measures of an atom's ability to attract electrons.

Do noble gases have high or low electronegativity?

Noble gases have negligible electronegativity because, despite having high ionization energies, they do not often form bonds.

What is the range of values on the Pauling electronegativity scale?

On the Pauling scale, electronegativity values range from 0.7 for cesium (least electronegative) to 4.0 for fluorine (most electronegative).

How does electronegativity change as you move across a period?

Electronegativity increases from left to right across a period due to the increasing effective nuclear charge (Zeff), which draws electrons more strongly toward the nucleus.

How does electronegativity change as you move down a group?

Electronegativity decreases as you move down a group because the valence electrons are farther from the nucleus, resulting in a weaker pull on the bonding electrons.

Which elements tend to have higher electronegativity values, metals or nonmetals

Nonmetals tend to have higher electronegativity values compared to metals because they are more likely to attract electrons in a chemical bond.

Periodic Trends Left → Right

• Atomic radius ↓

• Ionization energy ↑

• Electron affinity ↑

• Electronegativity ↑

Periodic Trends Top → Bottom

• Atomic radius ↑

• Ionization energy ↓

• Electron affinity ↓

• Electronegativity ↓

What are the physical properties of alkali metals?

Alkali metals (Group 1) possess classic metal properties like malleability and ductility, but their densities are lower than those of other metals (e.g., lithium is less dense than water).

Why do alkali metals have the largest atomic radii in their respective periods?

Alkali metals have very low Zeff (effective nuclear charge), which results in weak attraction between the nucleus and the valence electron, causing the electron cloud to be farther from the nucleus, giving them the largest atomic radii.

How do alkali metals react with nonmetals?

Alkali metals react readily with nonmetals, especially halogens, to form univalent cations. A classic example is the reaction with chlorine to form NaCl. They also react violently with water.

What are the physical properties of alkaline earth metals?

Alkaline earth metals (Group IIA) share many properties with alkali metals, such as being malleable, ductile, and good conductors of heat and electricity, but they have slightly higher effective nuclear charges and slightly smaller atomic radii.

How many valence electrons do alkaline earth metals have, and what is their ionization behavior?

Alkaline earth metals have two valence electrons in their outer shell, both of which are easily removed to form divalent cations (2+).

Why are alkali and alkaline earth metals referred to as active metals?

Alkali and alkaline earth metals are called active metals because they are highly reactive and are not naturally found in their elemental (neutral) state.

How do alkaline earth metals compare to alkali metals in terms of atomic size and reactivity?

Alkaline earth metals have slightly smaller atomic radii and higher effective nuclear charges compared to alkali metals, but they are still highly reactive and easily form divalent cations

What is the general reactivity of chalcogens (Group VIA or Group 16)?

Chalcogens are less reactive than halogens, but they are still crucial for normal biological functions.

How many valence electrons do the chalcogens have, and what are their atomic and ionic radii characteristics?

Chalcogens have six valence electrons. They generally have small atomic radii and large ionic radii due to their proximity to metalloids.

What are some biological roles of chalcogens?

Oxygen is essential for water, carbohydrates, and biological molecules. Sulfur is vital for amino acids and vitamins, and selenium protects against oxidative stress in microorganisms.

How many valence electrons do halogens (Group VIIA or Group 17) have, and what is their main chemical characteristic?

Halogens have seven valence electrons and are highly reactive, as they tend to gain one electron to complete their octet.

What are the physical states of the halogens at standard conditions?

he halogens can exist in different physical states at standard conditions: gaseous (F₂, Cl₂), liquid (Br₂), and solid (I₂).

Which halogen has the highest electronegativity, and why are halogens so reactive?

Fluorine (F) has the highest electronegativity of all elements. Halogens are highly reactive due to their high electronegativity and electron affinity, making them especially reactive toward alkali and alkaline earth metals.

Why are halogens not typically found in their elemental state?

Halogens are not naturally found in their elemental state because they are so reactive. Instead, they are found as ions (called halides) or as diatomic molecules (e.g., F₂, Cl₂).

What is a halide?

A halide is a chemical compound formed when a halogen (Group 17 element) bonds with another element, typically forming an ionic bond where the halogen becomes a negatively charged ion (anion).

What is a diatomic molecule?

A diatomic molecule is a molecule composed of two atoms, which can be of the same element (e.g., O₂) or different elements (e.g., CO).

Which elements naturally exist as diatomic molecules at standard conditions?

The 7 diatomic elements are: H₂, N₂, O₂, F₂, Cl₂, Br₂, and I₂. These are known as the "diatomic seven."

What is a key characteristic of noble gases (Group VIIIA or Group 18)?

Noble gases are chemically inert because their valence shells are full, making them stable and minimizing their chemical reactivity.

Why do noble gases have high ionization energies?

Noble gases have high ionization energies because their valence electrons are tightly bound due to their stable, filled electron shells, making it difficult to remove an electron.

What is the electronegativity of noble gases?

Noble gases, such as He, Ne, and Ar, have little or no measurable electronegativity because they don't tend to form bonds by gaining or losing electrons.