Let’s start by thinking about what inter and intra mean because that’s the key to understanding the difference between intermolecular and intramolecular forces.

• Inter means between, like interacting between two things.

• Intra means within, like staying inside one thing.

Now, apply that to molecules:

Intermolecular forces:

These are the forces that occur between different molecules. They don’t form actual bonds like intramolecular bonds, but they’re still super important because they affect how molecules behave around each other. These forces are weaker than intramolecular bonds but still control things like whether a substance is a solid, liquid, or gas at room temperature. Examples include:

• Hydrogen bonding: This is the strong attraction between water molecules. It’s why water sticks to itself and forms drops.

• Van der Waals forces: These are weak attractions that happen between all molecules, even if they’re not as strong as hydrogen bonds.

Intramolecular forces:

These are the forces that hold atoms together within a single molecule. They’re the chemical bonds—covalent, ionic, or metallic—that keep the atoms connected and form the actual structure of the molecule. For example:

In a water molecule (H₂O), the hydrogen and oxygen atoms are held together by intramolecular forces (specifically covalent bonds).

Ionic Bonds

Ionic bonds form when atoms transfer electrons, typically between metals and nonmetals. In this process, one atom (usually a metal) loses electrons, becoming a positively charged ion, while the other (usually a nonmetal) gains electrons, becoming negatively charged. These oppositely charged ions then attract each other, creating a strong bond.

We will discuss the characteristics of ionic compounds later, but generally you can determine if a compound has ionic bonds if the compound is a metal bonded to a nonmetal.  Also if you have a polyatomic ion, it holds to other ions with ionic bonds.  We can calculate if a bond is ionic by looking at the difference in electronegativity.

Covalent Bonds - Polar and Nonpolar

Polar and nonpolar covalent bonds differ primarily in how the electrons are shared between atoms in a molecule.  In a nonpolar covalent bond, the electrons are shared equally between the two atoms. This usually happens when the atoms have similar electronegativities (the tendency to attract electrons).

In a polar covalent bond, the electrons are shared unequally due to a significant difference in electronegativity between the atoms. The more electronegative atom pulls the electrons closer to itself, creating a slight charge imbalance.  This unequal sharing leads to a partial negative charge (δ-) on the more electronegative atom and a partial positive charge (δ+) on the less electronegative atom, making the molecule polar.

Molecular Bonds and Phases

The differences in the properties of a solid, liquid, or gas reflect the strengths of the attractive forces between the atoms, molecules, or ions that make up each phase. The phase in which a substance exists depends on the relative extent of its intermolecular force (IMFs) and its molecules' kinetic energies (KE). IMFs are the various forces of attraction that may exist between the atoms and molecules of a substance due to electrostatic phenomena, as will be detailed in this chapter. These forces hold particles close together, whereas the particles’ KE provides the energy required to overcome the attractive forces and thus increase the distance between particles. 

Section Questions