🧪AP Chemistry
Verified for the 2025 AP Chemistry exam•7 min read•Last Updated on June 18, 2024
Now that you've learned about the structure of an atom and the properties of electrons, we have to discuss how to draw molecules! By doing this, we can observe how the structure of an atom impacts the way it bonds.
Lewis diagrams, or Lewis structures, are a way of drawing molecular structures and showing the present valence electrons and bonds. Lewis structures serve as one of the most important topics in this unit and the course as a whole, with the ability to draw out any molecule opening the door to thousands of other possibilities.
Lewis diagrams are used to predict the shape of a molecule and the types of chemical reactions it can undergo. They are based on the octet rule, which states that atoms tend to form bonds in such a way that they have a full valence shell of eight electrons.
Lewis diagrams are a type of localized electron model, meaning the electrons do not move freely throughout the structure. In a Lewis structure, the valence electrons are divided into two categories:
Ionic Lewis dot structures are very easy to draw out since ionic bonds form due to a transfer of electrons!🥳
Sodium Bromide
** Try drawing the lewis dot structure of magnesium chloride. We'll give you the answer at the end!**
Covalent bonds are a little more difficult to draw out because the electrons are shared.
👉Here are some steps to guide you:
There CAN be exceptions to the rules, so be careful when drawing Lewis dot structures.
Let's go over some relatively straightforward compounds first!
Here is what you should be thinking as you get used to drawing these:
Looking at the periodic table, we can notice that oxygen is in group 16. This means it has six valence electrons and since there are two oxygen atoms, there should be 12 valence electrons in this diagram in total.
Since there are only two oxygen atoms, we could just draw them side by side (there is technically no central atom here).
Connect the two oxygen atoms with a single dash, which represents two valence electrons.
Draw 3 lone pairs on both of the oxygen atoms so that they both have a full octet. Here is what you should have so far:
Count the number of valence electrons in the diagram above. There are 14 of them right now, but we only want 12. Since there are too many electrons, we can convert this single bond into a double bond by erasing lone pairs from each atom.
Now to check our work, we can count the number of valence electrons. Since there are 12 total and the octet rule is fulfilled on both atoms, this is the proper lewis dot structure of O2.
Try drawing the lewis dot structure of N2. The answer will be provided at the end.
Looking at the periodic table, we know that C has 4 v.e. and S has 6 v.e.. This accounts for a total of 16 valence electrons since the carbon atom has four and each of the two sulfur atoms have six.
Draw carbon, the central atom.
Draw two sulfur atoms, connecting them to the carbon atom with a single bond (4 electrons so far out of 16).
Draw full octets on all three atoms. Here's what it should look like so far:
In this current diagram, there are a total of 20 valence electrons, but we need 16. Therefore, we should form two double bonds. Here is what the final LDS looks like:
Xe has 8 v.e. and F has 7 each. Therefore, there is a total of 22 valence electrons in this compound.
Xe is the central atom since there is only one atom of xenon.
Draw two fluorine atoms on either side and connect them to xenon with a single bond. Draw 3 full octets again. Here's what it looks like so far:
There is a total of 20 electrons; we need two more! This is where breaking the octet rule might need to happen. Since Xe has an atomic number of 54, which is much greater than 14, we can break the octet rule and add the necessary number of electrons to Xe. Here is what the final LDS looks like:
When you break the octet rule and have three lone pairs and two bonds, make sure that your lone pairs stay together. For example, you cannot have three valence electrons on one side of the xenon atom and three on the other side. They must remain in pairs of two. The following diagram is incorrect:
** Try drawing the lewis dot structure of the polyatomic ion NH4+.**
You always want to draw out the empirical formula first and make sure the charges cancel out to be 0 because magnesium chloride actually has 2 Cl atoms! Mg has a +2 charge while Cl has a -1 charge, so the compound is MgCl2. Here is the lewis dot structure:
You could also draw only one Cl atom, with a 2 coefficient outside of the brackets (indicating there are two chlorine ions). Don't confuse the term "coefficient" with "subscript" or "superscript."
Once you go through all the steps, you'll notice that there are 14 valence electrons. We only need 10 though since each nitrogen atom has five valence electrons, so we have to form double or triple bonds. If you draw a double bond, you'd still notice that we don't have 14 valence electrons, so there should be a triple bond.
Since the compound has a charge, we would just have to take one electron away. A positive charge indicates an absence of electrons, while a negative charge indicates an addition of electrons.
Here, it looks like there would be 9 valence electrons but since there is a +1 charge, there should only be 8 valence electrons total.
These lewis dot structures get slightly more complex in the next key topic, but practice makes perfect! Try to master these examples before moving forward. 😀
The atomic number is equal to the number protons found in an atom's nucleus. It defines what element an atom is and its place on the periodic table.
Term 1 of 19
The atomic number is equal to the number protons found in an atom's nucleus. It defines what element an atom is and its place on the periodic table.
Term 1 of 19
In chemistry, bonds refer to forces holding together atoms within molecules. There are three main types - covalent (sharing), ionic (transferring), and metallic (pooling).
Polar Bond: A type of covalent bond between two atoms where the electrons forming the bond are unequally distributed.
Nonpolar Bond: A type of covalent bond in which electrons are shared equally by both atoms.
Hydrogen Bond: A weak bond between two molecules resulting from an electrostatic attraction between a proton in one molecule and an electronegative atom in the other.
Lewis diagrams, also known as Lewis dot structures, are graphical representations that illustrate the bonding between atoms of a molecule and the lone pairs of electrons that may exist in the molecule.
Electron Configuration: This is the distribution of electrons of an atom or molecule (or other physical structure) in atomic or molecular orbitals.
Covalent Bonding: This is a type of chemical bond involving the sharing of electron pairs between atoms.
Resonance Structures: These are several structures that represent some average of the actual distribution of electrons in molecules with multiple bonds or unshared electron pairs.
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!
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.
Valence Electrons: The electrons present in the outermost shell of an atom which take part in bonding. These are the "items" you're packing into your suitcase!
Lewis Dot Structure: Diagrams that show the bonding between atoms and lone pairs of electrons that may exist in a molecule. It’s like a map showing where all your items (electrons) are packed inside your suitcase (atom).
Noble Gases: Elements with full electron shells which make them very stable and unreactive. They’re like perfectly packed suitcases ready for any trip!
The localized electron model is a model which assumes that an electron belongs to one or two atoms and its movement can be described accurately only relative to these atoms.
Molecular Orbital Theory: A method for determining molecular structure in which electrons are not assigned to individual bonds between atoms, but are treated as moving under the influence of the nuclei in whole molecule.
Chemical Bonding: The joining together of atoms to form new substances; chemical bonding involves either transferring or sharing electrons and can be explained using models like localized electron model.
Lewis Structures: Diagrams that show how valence electrons are arranged among atoms in a molecule; they help predict many properties of molecules and ions including shape, polarity, and reactivity.
Lone pairs refer to pairs of valence electrons that are not involved in covalent bond formation. They belong entirely to one atom.
Bonding Pair: This is a pair of electrons shared between two atoms, forming a covalent bond.
VSEPR Theory: The Valence Shell Electron Pair Repulsion theory predicts the shape of individual molecules based on the extent of electron-pair electrostatic repulsion.
Electronegativity: A measure of how strongly an atom can attract a bonding pair; it often determines whether the pair remains shared (bonding) or becomes unshared (lone).
Bonding pairs are pairs of electrons that are shared between atoms in a covalent bond.
Covalent Bonds: These are chemical bonds formed by the sharing of electron pairs between atoms.
Electron Pair: This is a term used in chemistry to describe two electrons that occupy the same orbital but have opposite spins.
Valence Electrons: These are the outermost electrons in an atom and are involved in forming bonds with other atoms.
The empirical formula is the simplest positive integer ratio of atoms present in a compound.
Mole Ratio: The ratio between the amounts in moles of any two compounds involved in a chemical reaction.
Stoichiometry: The calculation involving the masses (in grams) or moles of reactants and products in a chemical reaction based on the balanced equation.
Chemical Formula: A representation using symbols and numbers to show what elements are present in a compound and their ratios.
In molecular geometry, central atom is an atom in a molecule or ion that has bonds with two or more other atoms.
Lone Pair Electrons: These are pairs of valence electrons that are not used in bonding but belong exclusively to one atom.
Octet Rule: A rule stating that atoms lose, gain, or share electrons so as to have eight valence electrons.
VSEPR Theory (Valence Shell Electron Pair Repulsion): A model used to predict molecular shapes based on minimizing repulsion between electron pairs around a central atom.
Hydrogen is the first element on the periodic table with atomic number 1. It's also the lightest and most abundant element in the universe, making up about 75% of its elemental mass.
Proton: A subatomic particle found in the nucleus of every atom. The number of protons in an atom determines what element it is.
Electron: A subatomic particle that carries a negative electric charge. It orbits around the nucleus of an atom.
Atomic Number: This is defined as the number of protons found in the nucleus of an atom, which also defines its chemical properties.
Beryllium is a chemical element with the symbol Be and atomic number 4. It's a relatively rare element in both the universe and on Earth, and it's known for its stiffness, light weight, and resistance to heat.
Atomic Mass: The mass of an atom of a chemical element expressed in atomic mass units. It is approximately equivalent to the number of protons and neutrons in the atom.
Alkaline Earth Metals: A group of chemical elements in the periodic table with very similar properties. They are all shiny, silvery-white metals that includes beryllium.
Periodic Table: A tabular arrangement of chemical elements, organized based on their atomic number, electron configuration, and recurring properties.
Boron is a chemical element with the symbol B and atomic number 5. It's a metalloid, which means it has properties of both metals and non-metals.
Metalloid: A type of chemical element which has properties in between or that are a mixture of those of metals and nonmetals.
Atomic Number: The number of protons found in the nucleus of an atom, which determines the chemical properties of an element and its place in the periodic table.
Chemical Element: A species of atoms having the same number of protons in their atomic nuclei.
The atomic number is equal to the number protons found in an atom's nucleus. It defines what element an atom is and its place on the periodic table.
Protons: Positively charged particles found in the nucleus of an atom.
Electrons: Negatively charged particles that orbit around the nucleus of an atom.
Neutrons: Neutral particles (no charge) found in the nucleus of an atom.
A double bond involves four bonding electrons between two atoms rather than two as found in single covalent bonds.
Single Bonds: A single bond is a chemical bond in which one pair of electrons is shared between two atoms.
Triple Bonds: A triple bond is a chemical bond in which three pairs of electrons are shared between two atoms.
Covalent Bond: This type of bond occurs when pairs of electrons are shared by atoms.
A polyatomic ion is a charged particle which has two or more atoms held together by covalent bonds.
Monatomic Ion: This is an ion formed from only one atom. It's like the solo athlete in our sports analogy.
Covalent Bond: This is the type of bond that holds the atoms within a polyatomic ion together. It's like the teamwork and strategies that hold a sports team together.
Electron Configuration: This refers to the distribution of electrons among energy levels in an atom, which influences how it forms ions and bonds with other atoms. It's like the positions and roles each player takes on in our sports team.
A triple bond is a chemical bond between two atoms involving six bonding electrons instead of the usual two in a covalent single bond. It's the strongest type of covalent bond.
Single Bond: This is a chemical bond where one pair of electrons is shared between two atoms.
Double Bond: This is a chemical bond where four bonding electrons are shared between two atoms.
Covalent Bond: This is a type of chemical bond where electron pairs are shared between atoms.