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1.8 Valence Electrons and Ionic Compounds

8 min readdecember 22, 2022

Dalia Savy

Dalia Savy

Dalia Savy

Dalia Savy

We've discussed several times in this unit's study guides already! Here is a quick recap regarding what you should know so far:

  • are the outermost electrons in an atom.

  • are found in the s and p orbital of the outermost shell.

  • A gap in could tell us how many an element has.

  • Elements in the same group on the have the same number of .

Foundational Knowledge of Bonding

Valence Electrons

From the , you could tell how many an element has.

https://firebasestorage.googleapis.com/v0/b/fiveable-92889.appspot.com/o/images%2Fv.e-n0ilH50Y3JRF.JPG?alt=media&token=553d90a5-f882-4597-90b2-5dc771a3a6b6

Image Courtesy of ck12

The graph above skips over the transition because those are much less predictable. The AP Chemistry exam is going to primarily focus on the other elements, but it is good to be familiar with several transition (like Co, Cu, Zn).

Just by looking at the , you could tell that oxygen has six and carbon has four . Remember that the number of an atom has affects the way the element bonds with others. Therefore, elements in the same group tend to bond with similar elements and form similar compounds. Here are some examples:

  • Elements in group one can all bond with chlorine: LiCl, NaCl, KCl, RbCl.

  • Elements in group two can all bond with O: MgO, CaO, SrO, BaO.

Charges of Ions

Before getting into bonds, it is also good to memorize the charges of most elements on the when they bond with another element. This will make more sense once we look into what happens when elements form a bond.

Essentially, ions are charged atoms or molecules that have gained or lost electrons. Atoms may become ions in order to achieve a more stable . Remember, most things in chemistry try to become more stable! ⚖️

https://firebasestorage.googleapis.com/v0/b/fiveable-92889.appspot.com/o/images%2F-XjZLpkhZBYeM.jpg?alt=media&token=547b25f5-f128-43b4-9ec4-ece985a05e0a

Image Courtesy of Chemistry Land

Again, the transition aren't included here because they have various charges and very different properties. Don't worry though, the AP won't question you about them. At most, you would have to write their electron configurations.

Types of Elements

There are three different types of elements: , , and .

https://firebasestorage.googleapis.com/v0/b/fiveable-92889.appspot.com/o/images%2F-YaEAnnIYHe7H.JPG?alt=media&token=c6ff0009-3fdd-467e-9c22-59b3318b2938

Image Courtesy of ck12

are good conductors of heat and electricity🔥⚡, shiny, malleable (can bend), and ductile (can be made into a wire).

are the complete opposite: bad conductors of heat and electricity and brittle.

have properties of both and and there are only seven of them.

Electronegativity

refers to how strongly a nucleus attracts electrons of another atom. This comes into play when two atoms are sharing since the pull of the electrons depends on how electronegative the atom is!

👉 Want a quick refresh on electronegativity? Check out our study guide on "Periodic Trends."

Types of Bonds

Elements bond to achieve the lowest possible energy to reach the highest stability. There are two different types of bonds you should be familiar with: and .

Ionic Bonds

are formed by the transfer of electrons from one atom to another, usually from a metal to a nonmetal.

The atom that loses an electron will gain a positive charge and is called a (usually a metal).

The atom that gains an electron will gain a negative charge and is called an (usually a nonmetal).

Some properties of ionic compounds include very strong bonds, solubility in water, and the ability to strongly conduct heat and electricity.

Example of an Ionic Bond - NaCl

In the ionic compound NaCl, sodium (Na) loses an electron and obtains a positive charge, while chlorine (Cl) gains an electron and therefore obtains a negative charge.

https://firebasestorage.googleapis.com/v0/b/fiveable-92889.appspot.com/o/images%2F-Tc86ZSRT3dxx.jpg?alt=media&token=241f7ed9-e23f-475a-8838-bedd9e3e2ffd

The one valence electron that was in Na was transferred to the chlorine atom in order for both ions to have a full octet. The is based on the idea that atoms are most stable when they have eight in their valence shell.

Group one elements and group 17 elements (halogens) often bond this way to reach stability. They have the proper number of that enable them to both reach a full octet. In fact, when forming an ionic bond with halogens, group one elements lose an entire electron shell.

When sodium and chlorine become ions, their actually matches that of the noble gas closest to it. Here is an example:

https://firebasestorage.googleapis.com/v0/b/fiveable-92889.appspot.com/o/images%2F-9LTdbrg1X4Ir.JPG?alt=media&token=a7326149-1b7a-4bd7-83ef-e203b518d229

Covalent Bonds

are formed when atoms share electrons (usually two ). Some properties of include low melting points and weak electroconductivity abilities.

There are actually two types of : and

  • are a type of bonding where electrons are unequally shared between two different . This is due to the two atoms having significantly different electronegativities.

  • are a type of bonding where electrons are equally shared between, usually, two of the same nonmetal. This bond typically forms between two atoms that have a similar tendency to attract electrons, or rather, two atoms that have very similar electronegativities.

are strong and stable, and they are actually responsible for the properties of many common substances that we interact with on a daily basis. Some examples include water, methane, carbon dioxide, and even proteins, DNA, and carbohydrates! 🤯

Polar-Covalent Example - HF

Hydrogen and fluorine create a polar covalent bond. Fluorine attracts electrons more strongly due to its high of 4.0, resulting in an unequal distribution of electrons. Think about it this way: fluorine is super greedy to fulfill its full octet of eight , so it more strongly attracts them towards it.

https://firebasestorage.googleapis.com/v0/b/fiveable-92889.appspot.com/o/images%2F-ZxQxjkGNkVZ8.png?alt=media&token=9fc71ebf-4662-425e-bc17-9df18c1ff311

Image Courtesy of Study

Note the difference in representation between a covalent bond and an ionic bond. The dash here represents two shared electrons. Rather than a transfer of electrons, hydrogen is sharing one electron and fluorine is sharing one electron to enable both atoms to have their full octet.

Nonpolar Example - Cl2

In the covalent compound Cl2, two chlorine atoms bond together to share the final electron that they need to have their full octet.

https://firebasestorage.googleapis.com/v0/b/fiveable-92889.appspot.com/o/images%2F-bZUIw9uaulzh.JPG?alt=media&token=84d9c704-c7c3-49ff-86fb-77e01cd49c99

Image courtesy of Wayne Breslyn

Just like in a polar covalent bond, they are sharing electrons with each other to obtain the full octet. However, note here that two atoms of the same element are bonding. In other words, their electronegativities are the same, so they would have the same pull on the electrons they share.

This does not mean that all are between two atoms of the same nonmetal. The carbon-oxygen bonds in carbon dioxide (CO2), as well as the carbon-hydrogen bonds in methane (CH4) are also nonpolar covalent. Take a look at their electronegativities:

  • of hydrogen: 2.2

  • of carbon: 2.55

  • of oxygen: 3.44

Charges and Partial Charges

When two atoms interact with each other in a bond, there is often a distribution of charge. Let's take a look at the different charge distributions we can see.

In a nonpolar covalent bond, the electronegativities of the two atoms are very similar, if not the same. Therefore, the electrons in the bond will be shared equally, resulting in a neutral charge distribution. The positive charge of the two nuclei is balanced by the negative charge of the shared electrons.

When there is a large gap in electronegativities between the two atoms in a bond, we start to see charges. In a polar covalent bond, two atoms, such as fluorine and hydrogen, share electrons unevenly. Fluorine has an of 4.0, while hydrogen has that of 2.2. The atom with the higher (F) will have a partial negative charge, while the atom with the lower (H) will have a partial positive charge. This is because the very negative electrons are shifted towards the fluorine, creating an abundance of negative charge, and away from the hydrogen, creating a "loss" of positive charge.

https://firebasestorage.googleapis.com/v0/b/fiveable-92889.appspot.com/o/images%2F-ntsmC6FyFKmM.png?alt=media&token=23180858-ccec-40d4-9666-e1e1ad3c42b2

Image Courtesy of Chem LibreTexts

The Greek letter delta (δ) is used to indicate an atom that has either a positive or negative partial charge.

In an ionic bond, the transfer of electrons forms ions with complete opposite charges. Take a look at the transfer of electrons between the sodium and chlorine atoms above.

The loss of an electron causes the sodium to be a little extra positive (hence its +1 charge), whereas the gain of an electron causes the chlorine to be a little extra negative (hence its -1 charge). The reason why are typically the atoms to donate an electron is that they have very low electronegativities. Sodium has an of 0.93, while chlorine has an of 3.16. Compare this difference to that between hydrogen and fluorine! This is why the charges are not partial.

Check your Understanding

Try these little questions on your own and see how you do! It's just to see how well you understood this key topic:

Atoms of Ca combine with atoms of Br to form an ionic bond.

  1. What ratio would they combine in?

  2. What other compounds have this same ratio with Ca?

  3. What elements could form an ionic bond with sulfur?

Answers

  1. Calcium and bromine bond in a 1:2 ratio, creating the compound CaBr2. This is because Ca has a +2 charge and Br has a -1 charge. In order for the two to bond together and form a neutral compound, there must be two Br atoms.

  2. We need other elements that have a -1 charge in order for them to bond in this 1:2 ratio with calcium. This includes all of the group 17 elements on the , such as fluorine, chlorine, bromine, and iodine.

  3. In a 1-to-1 ratio, any elements in group two would form an ionic bond with sulfur. Some examples include MgS, CaS, and BaS. In a 2-to-1 ratio, any elements in group one would form an ionic bond with sulfur. An example is Na2S.

🎥 Watch AP Chemistry teacher Mónica Gracida discuss how to do electron configurations of transition metals and how to quickly know the valence electrons for atoms.

Key Terms to Review (17)

Anion

: An anion is an atom or group of atoms that has gained one or more electrons, giving it a negative charge.

Cation

: A cation is an atom or group of atoms that has lost one or more electrons, giving it a positive charge.

Charges of Ions

: The charge of an ion is a measure of the number of electrons that an atom has gained or lost in order to achieve a full outer shell. If an atom loses electrons, it becomes positively charged (cation), and if it gains electrons, it becomes negatively charged (anion).

Covalent Bonds

: A covalent bond is a type of chemical bond where two atoms share one or more pairs of electrons, resulting in a stable balance of attractive and repulsive forces.

Electron Configuration

: Electron configuration is the arrangement of electrons in an atom, molecule, or other physical structure.

Electronegativity

: Electronegativity is a measure of an atom's ability to attract shared electrons in a chemical bond. Elements with high electronegativity tend to pull electrons towards themselves more strongly.

Ionic Bonds

: Ionic bonds are chemical bonds formed through the electrostatic attraction between oppositely charged ions (cations and anions).

Ionization Energies

: Ionization energy is the minimum amount of energy required to remove an electron from a neutral atom in its gaseous state.

Metalloids

: Metalloids are elements that have properties of both metals and non-metals. They can act as a conductor or insulator depending on the conditions.

Metals

: Metals are elements that typically have luster, conduct heat and electricity well, are malleable (can be hammered into thin sheets), ductile (can be drawn into wires), and tend to lose electrons during chemical reactions becoming cations.

Nonmetals

: Nonmetals are elements that typically lack luster, are poor conductors of heat and electricity, brittle in solid form, and tend to gain electrons during chemical reactions becoming anions.

Nonpolar Covalent Bonds

: A type of chemical bond where two atoms share a pair of electrons with each other equally because they have similar or same electronegativities.

Octet Rule

: A chemical rule of thumb that states atoms tend to combine in such a way that they each have eight electrons in their valence shells, making them as stable as noble gases.

Partial Charges

: Partial charges occur when atoms share electrons unequally in covalent bonds, leading to regions with slightly positive or negative charges within molecules.

Periodic Table

: The periodic table is a tabular arrangement of chemical elements, organized based on their atomic number, electron configuration, and recurring chemical properties. Elements are listed in order of increasing atomic number.

Polar Covalent Bonds

: A type of chemical bond where a pair of electrons is unequally shared between two atoms. This happens when one atom is more electronegative than the other, causing an uneven distribution of charge.

Valence Electrons

: Valence electrons are the outermost electrons in an atom that participate in chemical reactions.

1.8 Valence Electrons and Ionic Compounds

8 min readdecember 22, 2022

Dalia Savy

Dalia Savy

Dalia Savy

Dalia Savy

We've discussed several times in this unit's study guides already! Here is a quick recap regarding what you should know so far:

  • are the outermost electrons in an atom.

  • are found in the s and p orbital of the outermost shell.

  • A gap in could tell us how many an element has.

  • Elements in the same group on the have the same number of .

Foundational Knowledge of Bonding

Valence Electrons

From the , you could tell how many an element has.

https://firebasestorage.googleapis.com/v0/b/fiveable-92889.appspot.com/o/images%2Fv.e-n0ilH50Y3JRF.JPG?alt=media&token=553d90a5-f882-4597-90b2-5dc771a3a6b6

Image Courtesy of ck12

The graph above skips over the transition because those are much less predictable. The AP Chemistry exam is going to primarily focus on the other elements, but it is good to be familiar with several transition (like Co, Cu, Zn).

Just by looking at the , you could tell that oxygen has six and carbon has four . Remember that the number of an atom has affects the way the element bonds with others. Therefore, elements in the same group tend to bond with similar elements and form similar compounds. Here are some examples:

  • Elements in group one can all bond with chlorine: LiCl, NaCl, KCl, RbCl.

  • Elements in group two can all bond with O: MgO, CaO, SrO, BaO.

Charges of Ions

Before getting into bonds, it is also good to memorize the charges of most elements on the when they bond with another element. This will make more sense once we look into what happens when elements form a bond.

Essentially, ions are charged atoms or molecules that have gained or lost electrons. Atoms may become ions in order to achieve a more stable . Remember, most things in chemistry try to become more stable! ⚖️

https://firebasestorage.googleapis.com/v0/b/fiveable-92889.appspot.com/o/images%2F-XjZLpkhZBYeM.jpg?alt=media&token=547b25f5-f128-43b4-9ec4-ece985a05e0a

Image Courtesy of Chemistry Land

Again, the transition aren't included here because they have various charges and very different properties. Don't worry though, the AP won't question you about them. At most, you would have to write their electron configurations.

Types of Elements

There are three different types of elements: , , and .

https://firebasestorage.googleapis.com/v0/b/fiveable-92889.appspot.com/o/images%2F-YaEAnnIYHe7H.JPG?alt=media&token=c6ff0009-3fdd-467e-9c22-59b3318b2938

Image Courtesy of ck12

are good conductors of heat and electricity🔥⚡, shiny, malleable (can bend), and ductile (can be made into a wire).

are the complete opposite: bad conductors of heat and electricity and brittle.

have properties of both and and there are only seven of them.

Electronegativity

refers to how strongly a nucleus attracts electrons of another atom. This comes into play when two atoms are sharing since the pull of the electrons depends on how electronegative the atom is!

👉 Want a quick refresh on electronegativity? Check out our study guide on "Periodic Trends."

Types of Bonds

Elements bond to achieve the lowest possible energy to reach the highest stability. There are two different types of bonds you should be familiar with: and .

Ionic Bonds

are formed by the transfer of electrons from one atom to another, usually from a metal to a nonmetal.

The atom that loses an electron will gain a positive charge and is called a (usually a metal).

The atom that gains an electron will gain a negative charge and is called an (usually a nonmetal).

Some properties of ionic compounds include very strong bonds, solubility in water, and the ability to strongly conduct heat and electricity.

Example of an Ionic Bond - NaCl

In the ionic compound NaCl, sodium (Na) loses an electron and obtains a positive charge, while chlorine (Cl) gains an electron and therefore obtains a negative charge.

https://firebasestorage.googleapis.com/v0/b/fiveable-92889.appspot.com/o/images%2F-Tc86ZSRT3dxx.jpg?alt=media&token=241f7ed9-e23f-475a-8838-bedd9e3e2ffd

The one valence electron that was in Na was transferred to the chlorine atom in order for both ions to have a full octet. The is based on the idea that atoms are most stable when they have eight in their valence shell.

Group one elements and group 17 elements (halogens) often bond this way to reach stability. They have the proper number of that enable them to both reach a full octet. In fact, when forming an ionic bond with halogens, group one elements lose an entire electron shell.

When sodium and chlorine become ions, their actually matches that of the noble gas closest to it. Here is an example:

https://firebasestorage.googleapis.com/v0/b/fiveable-92889.appspot.com/o/images%2F-9LTdbrg1X4Ir.JPG?alt=media&token=a7326149-1b7a-4bd7-83ef-e203b518d229

Covalent Bonds

are formed when atoms share electrons (usually two ). Some properties of include low melting points and weak electroconductivity abilities.

There are actually two types of : and

  • are a type of bonding where electrons are unequally shared between two different . This is due to the two atoms having significantly different electronegativities.

  • are a type of bonding where electrons are equally shared between, usually, two of the same nonmetal. This bond typically forms between two atoms that have a similar tendency to attract electrons, or rather, two atoms that have very similar electronegativities.

are strong and stable, and they are actually responsible for the properties of many common substances that we interact with on a daily basis. Some examples include water, methane, carbon dioxide, and even proteins, DNA, and carbohydrates! 🤯

Polar-Covalent Example - HF

Hydrogen and fluorine create a polar covalent bond. Fluorine attracts electrons more strongly due to its high of 4.0, resulting in an unequal distribution of electrons. Think about it this way: fluorine is super greedy to fulfill its full octet of eight , so it more strongly attracts them towards it.

https://firebasestorage.googleapis.com/v0/b/fiveable-92889.appspot.com/o/images%2F-ZxQxjkGNkVZ8.png?alt=media&token=9fc71ebf-4662-425e-bc17-9df18c1ff311

Image Courtesy of Study

Note the difference in representation between a covalent bond and an ionic bond. The dash here represents two shared electrons. Rather than a transfer of electrons, hydrogen is sharing one electron and fluorine is sharing one electron to enable both atoms to have their full octet.

Nonpolar Example - Cl2

In the covalent compound Cl2, two chlorine atoms bond together to share the final electron that they need to have their full octet.

https://firebasestorage.googleapis.com/v0/b/fiveable-92889.appspot.com/o/images%2F-bZUIw9uaulzh.JPG?alt=media&token=84d9c704-c7c3-49ff-86fb-77e01cd49c99

Image courtesy of Wayne Breslyn

Just like in a polar covalent bond, they are sharing electrons with each other to obtain the full octet. However, note here that two atoms of the same element are bonding. In other words, their electronegativities are the same, so they would have the same pull on the electrons they share.

This does not mean that all are between two atoms of the same nonmetal. The carbon-oxygen bonds in carbon dioxide (CO2), as well as the carbon-hydrogen bonds in methane (CH4) are also nonpolar covalent. Take a look at their electronegativities:

  • of hydrogen: 2.2

  • of carbon: 2.55

  • of oxygen: 3.44

Charges and Partial Charges

When two atoms interact with each other in a bond, there is often a distribution of charge. Let's take a look at the different charge distributions we can see.

In a nonpolar covalent bond, the electronegativities of the two atoms are very similar, if not the same. Therefore, the electrons in the bond will be shared equally, resulting in a neutral charge distribution. The positive charge of the two nuclei is balanced by the negative charge of the shared electrons.

When there is a large gap in electronegativities between the two atoms in a bond, we start to see charges. In a polar covalent bond, two atoms, such as fluorine and hydrogen, share electrons unevenly. Fluorine has an of 4.0, while hydrogen has that of 2.2. The atom with the higher (F) will have a partial negative charge, while the atom with the lower (H) will have a partial positive charge. This is because the very negative electrons are shifted towards the fluorine, creating an abundance of negative charge, and away from the hydrogen, creating a "loss" of positive charge.

https://firebasestorage.googleapis.com/v0/b/fiveable-92889.appspot.com/o/images%2F-ntsmC6FyFKmM.png?alt=media&token=23180858-ccec-40d4-9666-e1e1ad3c42b2

Image Courtesy of Chem LibreTexts

The Greek letter delta (δ) is used to indicate an atom that has either a positive or negative partial charge.

In an ionic bond, the transfer of electrons forms ions with complete opposite charges. Take a look at the transfer of electrons between the sodium and chlorine atoms above.

The loss of an electron causes the sodium to be a little extra positive (hence its +1 charge), whereas the gain of an electron causes the chlorine to be a little extra negative (hence its -1 charge). The reason why are typically the atoms to donate an electron is that they have very low electronegativities. Sodium has an of 0.93, while chlorine has an of 3.16. Compare this difference to that between hydrogen and fluorine! This is why the charges are not partial.

Check your Understanding

Try these little questions on your own and see how you do! It's just to see how well you understood this key topic:

Atoms of Ca combine with atoms of Br to form an ionic bond.

  1. What ratio would they combine in?

  2. What other compounds have this same ratio with Ca?

  3. What elements could form an ionic bond with sulfur?

Answers

  1. Calcium and bromine bond in a 1:2 ratio, creating the compound CaBr2. This is because Ca has a +2 charge and Br has a -1 charge. In order for the two to bond together and form a neutral compound, there must be two Br atoms.

  2. We need other elements that have a -1 charge in order for them to bond in this 1:2 ratio with calcium. This includes all of the group 17 elements on the , such as fluorine, chlorine, bromine, and iodine.

  3. In a 1-to-1 ratio, any elements in group two would form an ionic bond with sulfur. Some examples include MgS, CaS, and BaS. In a 2-to-1 ratio, any elements in group one would form an ionic bond with sulfur. An example is Na2S.

🎥 Watch AP Chemistry teacher Mónica Gracida discuss how to do electron configurations of transition metals and how to quickly know the valence electrons for atoms.

Key Terms to Review (17)

Anion

: An anion is an atom or group of atoms that has gained one or more electrons, giving it a negative charge.

Cation

: A cation is an atom or group of atoms that has lost one or more electrons, giving it a positive charge.

Charges of Ions

: The charge of an ion is a measure of the number of electrons that an atom has gained or lost in order to achieve a full outer shell. If an atom loses electrons, it becomes positively charged (cation), and if it gains electrons, it becomes negatively charged (anion).

Covalent Bonds

: A covalent bond is a type of chemical bond where two atoms share one or more pairs of electrons, resulting in a stable balance of attractive and repulsive forces.

Electron Configuration

: Electron configuration is the arrangement of electrons in an atom, molecule, or other physical structure.

Electronegativity

: Electronegativity is a measure of an atom's ability to attract shared electrons in a chemical bond. Elements with high electronegativity tend to pull electrons towards themselves more strongly.

Ionic Bonds

: Ionic bonds are chemical bonds formed through the electrostatic attraction between oppositely charged ions (cations and anions).

Ionization Energies

: Ionization energy is the minimum amount of energy required to remove an electron from a neutral atom in its gaseous state.

Metalloids

: Metalloids are elements that have properties of both metals and non-metals. They can act as a conductor or insulator depending on the conditions.

Metals

: Metals are elements that typically have luster, conduct heat and electricity well, are malleable (can be hammered into thin sheets), ductile (can be drawn into wires), and tend to lose electrons during chemical reactions becoming cations.

Nonmetals

: Nonmetals are elements that typically lack luster, are poor conductors of heat and electricity, brittle in solid form, and tend to gain electrons during chemical reactions becoming anions.

Nonpolar Covalent Bonds

: A type of chemical bond where two atoms share a pair of electrons with each other equally because they have similar or same electronegativities.

Octet Rule

: A chemical rule of thumb that states atoms tend to combine in such a way that they each have eight electrons in their valence shells, making them as stable as noble gases.

Partial Charges

: Partial charges occur when atoms share electrons unequally in covalent bonds, leading to regions with slightly positive or negative charges within molecules.

Periodic Table

: The periodic table is a tabular arrangement of chemical elements, organized based on their atomic number, electron configuration, and recurring chemical properties. Elements are listed in order of increasing atomic number.

Polar Covalent Bonds

: A type of chemical bond where a pair of electrons is unequally shared between two atoms. This happens when one atom is more electronegative than the other, causing an uneven distribution of charge.

Valence Electrons

: Valence electrons are the outermost electrons in an atom that participate in chemical reactions.


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AP® and SAT® are trademarks registered by the College Board, which is not affiliated with, and does not endorse this website.


© 2024 Fiveable Inc. All rights reserved.

AP® and SAT® are trademarks registered by the College Board, which is not affiliated with, and does not endorse this website.