🧪AP Chemistry
Verified for the 2025 AP Chemistry exam•7 min read•Last Updated on June 18, 2024
Atoms are the most basic unit of matter. When several atoms interact with each other, they can form molecules. Depending on how the electrons of these atoms interact, a covalent bond or an ionic bond could form.
Everything in chemistry strives to become the most stable possible. Atoms do the same thing! Atoms bond in order to reach a more stable, lower energy state. ⚖️
Valence electrons are typically involved in bonding, not core electrons! They play a crucial role in the bonding process and determine the chemical properties of the resulting molecule. Here is a quick review of everything we learned about valence electrons in unit one:
As we discussed in unit one, electronegativity is one of the five periodic trends you should be familiar with. Here is what you should know before learning about bonding:
Coulomb's Law comes in handy when you want to measure the attraction between two atoms. It states that the strength of forces that hold atoms together depends on two factors:
Ionic bonds are formed by the transfer of valence electrons from atom to atom, usually from a metal to a nonmetal. Let's take a look at an example of an ionic bond between a sodium and chloride atom! 🤓
Na(s) + ½ Cl2(g) → NaCl(s)
NaCl, a brittle salt with a high melting point was formed in this chemical reaction. Ionic bonds are held together not by shared electrons or a direct bond, but rather through electromagnetic forces that hold positive and negative ions together. These electromagnetic forces are so strong that it takes lots of energy to break them apart, hence the high boiling and melting points.
Ionic compounds also form a crystal lattice of ions, giving them their rigidity. In a crystal lattice, ions are arranged in a repeating, three-dimensional pattern. This crystal lattice is, again, held by strong electrostatic forces. This characteristic of ionic molecules makes them generally good electrical conductors. When an ionic compound is melted or dissolved in water, the ions can move freely within the crystal lattice structure, producing electricity! ⚡
Note that when sodium and chlorine interact to form an ionic bond, sodium gives a valence electron to chlorine. This gain and loss of an electron produces ions, hence the name "ionic bond." The atom that loses an electron, sodium, will gain a positive charge and is called a cation. The atom that gains an electron, chlorine, will gain a negative charge and is called an anion.
Remember that Coulomb's Law states that greater charges and smaller distances lead to the strongest attractions.
When asked which ionic compound would have a higher melting point, always look for differences in charge and size. The higher the charge of the ion, the stronger the negative-positive attraction is and the more energy it takes to break the bond. This increases both the melting and boiling point. Same goes for size!
TIP - Always look for differences in charge first; they have a greater impact on melting points.
In covalent bonds, electrons are shared between two or more atoms (typically nonmetals). There are two different types of covalent bonds based on the electronegativities of the atoms involved:
To analyze the difference between the two types of covalent bonds, we would need to look at electronegativity. Valence electrons shared between atoms of similar electronegativity constitute a nonpolar covalent bond. This is why the two oxygen atoms above form a nonpolar covalent bond. Since the electronegativity is the same, the oxygen nuclei pull on the other oxygen atom's electrons with the same strength. Think "nonpolar = balance!"
Valence electrons shared between atoms of unequal electronegativity constitute a polar covalent bond, like in a water molecule. Hydrogen has an electronegativity of 2.2 while oxygen has an electronegativity of 3.44. Therefore, oxygen attracts electrons more strongly and will pull the electrons towards it. This unequal distribution of charge leads to oxygen developing a partial negative charge. This difference in electronegativity leads to bond dipoles, which are covered more in the next unit.
For now, just remember that greater differences in electronegativity within a bond lead to greater bond dipoles!
Here are some principles of ionic and covalent bonds that you should look out for when deciding what chemical bond will form:
If a solid has a high melting point and is a good conductor of heat and electricity when dissolved in water, it is most likely an ionic compound. This is because of the concept of a free-flowing ion that generates electricity.
If a solid has a low melting point and doesn't conduct electricity in any state, it is most likely a molecular compound (which has covalent bonds).
There is one more circumstance: If a solid has a high melting point and doesn't conduct electricity in any state, it is a network solid made up of covalent bonds. Don't worry about this yet, it's covered in future units :).
An anion is an atom or group of atoms that has gained one or more electrons, giving it a negative charge.
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An anion is an atom or group of atoms that has gained one or more electrons, giving it a negative charge.
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Valence electrons are the outermost electrons in an atom that participate in chemical reactions.
Electron Shell: This is like a layer of an onion, where each shell can hold a certain number of electrons. The outermost shell is where you'll find the valence electrons.
Covalent Bond: This is when two atoms share their valence electrons, kind of like how best friends might share their favorite toys.
Ionization Energy: This is the energy required to remove a valence electron from an atom. It's like how much effort it would take for someone to pry your favorite toy out of your hands!
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.
Polarity: A property of molecules resulting from the uneven distribution of charges due to differences in electronegativity; leads to regions of partial positive and negative charge within the molecule.
Ionization Energy: The energy required to remove an electron from an atom or ion; generally increases with increasing electronegativity because these atoms hold their electrons more tightly.
Electron Affinity: The amount of energy released when an electron is added to a neutral atom; elements with high electronegativity typically have high electron affinity as they readily accept additional electrons.
Coulomb's Law describes the force between two charged objects. It states that this force is directly proportional to the product of their charges and inversely proportional to the square of the distance between them.
Electric Charge: This is a fundamental property of matter that can be either positive or negative, with like charges repelling and opposite charges attracting each other.
Inverse Square Law: This law states that a specified physical quantity or intensity is inversely proportional to the square of the distance from the source of that physical quantity.
Force Field: A region around a charged particle within which a force would be exerted on other charged particles or objects.
A crystal lattice is a symmetrical, three-dimensional arrangement of atoms or ions in a crystal. It's the repeating pattern of particles that forms the solid structure of a crystal.
Lattice Energy: This is the energy required to break apart an ionic compound and convert its component atoms into gaseous ions. It's like how much effort it would take to dismantle your LEGO structure into individual bricks.
Unit Cell: This is the smallest repeating unit in a crystal lattice that shows the entire pattern of atoms. It's like identifying one small section of your LEGO structure that, if repeated, could recreate the whole thing.
Ionic Bonding: This refers to when positively charged ions (cations) and negatively charged ions (anions) are attracted to each other and form bonds. Like magnets with opposite poles attracting each other.
A cation is an atom or group of atoms that has lost one or more electrons, giving it a positive charge.
Ionization Energy: The amount of energy required to remove an electron from an atom or ion. It’s like how much persuasion it would take for you to leave that party early!
Metallic Bonding: Metals often lose electrons easily, forming cations which then bond together through their shared sea of electrons. It's like a group of friends (the metal atoms) all deciding to leave the party early (lose electrons) and hang out together instead.
Electrolyte: A substance that produces an electrically conducting solution when dissolved in water. Cations are often part of electrolytes, like how you might be a key player in starting a new trend at school.
An anion is an atom or group of atoms that has gained one or more electrons, giving it a negative charge.
Electron Affinity: The amount of energy released when an electron is added to a neutral atom. It’s like how happy you would feel if someone gave you free pizza at the party!
Covalent Bonding: This type of bonding happens when two atoms share electrons. In some cases, one atom pulls on the shared electron(s) harder and becomes slightly negative, acting as an anion. It's like sharing your pizza with someone else but still holding onto most of it yourself.
Salt Bridge: In electrochemistry, this is a laboratory device used to connect the oxidation and reduction half-cells of a galvanic cell (a type of battery), allowing ion flow without extensive mixing of different solutions. Anions move towards the positive electrode through this bridge – think about it as if they’re drawn towards their favorite band playing on stage!
A polar covalent bond is a type of chemical bond where a pair of electrons is unequally shared between two atoms. This happens when one atom has higher electronegativity than the other, causing an uneven distribution of charge.
Electronegativity: The measure of the ability of an atom in a chemical compound to attract electrons from another atom in the compound.
Covalent Bond: A type of strong chemical bond where two atoms share one or more pairs of valence electrons.
Dipole Moment: A measure of the polarity in a system of charges; for molecules, it's created by differences in electronegativity.
A nonpolar covalent bond is a type of chemical bond where two atoms share a pair of electrons equally because they have similar electronegativities.
Bond Length: The average distance between nuclei of two bonded atoms in a molecule.
Single Bond: A single covalent bond that involves only one pair (two) shared electrons.
Double Bond: A double covalent bond that involves two pairs (four) shared electrons.
Bond dipoles occur when there is a difference in electronegativity between two atoms bonded together, causing an uneven distribution of electron density and creating partial positive and negative charges at each end.
Electronegativity: The measure of an atom's ability to attract shared electrons in a chemical bond.
Dipole Moment: A measure of the polarity of a molecule, calculated as the product of charge and distance between charges.
Ionic Bond: A type of chemical bond where one atom completely transfers its valence electrons to another atom.
A chemical bond is the force of attraction between two atoms or ions that binds them together as a unit. This occurs due to the sharing, donation, or acceptance of electrons.
Covalent Bond: A type of chemical bond where electrons are shared equally between two atoms.
Ionic Bond: A type of chemical bond where one atom donates one or more electrons to another atom creating positive and negative ions which attract each other.
Polar Covalent Bond: A type of covalent bond where electrons are not shared equally causing one part of the molecule to be slightly negative and the other slightly positive.
A network solid is a substance in which all the atoms are covalently bonded to each other forming a large network structure.
Covalent Network Crystal: A crystal in which all atoms are interconnected by covalent bonds forming an extended network throughout the crystal.
Diamond Structure: An example of network solid where carbon atoms are linked by covalent bonds in tetrahedral arrangement giving it extreme hardness.
Silicon Dioxide (SiO2): Another example of network solid commonly known as quartz or sand.