diff --git a/docs/sch3uz.md b/docs/sch3uz.md
index d548632..425429c 100644
--- a/docs/sch3uz.md
+++ b/docs/sch3uz.md
@@ -125,7 +125,7 @@ Please see [SL Physics#Uncertainty of gradient and intercepts](/sph3u7/#uncertai
 
 ## 11.3 - Spectroscopic identification of organic compounds
 
-## 12 - Atomic structure
+## 2 - Atomic structure
 
 !!! definition
     - The **effective nuclear charge** ($Z_\text{eff}$) is the net positive charge (attraction to the nucleus) experienced by an electron in an atom.
@@ -247,6 +247,12 @@ A chemical bond consists of the strong electronic interactions of the **valence*
     - Electrons to be transferred must have arrows pointing to their destination.
     - x'es are used to represent additional electrons that have an unknown source.
 
+### Nomenclature
+
+### Types of reactions
+
+### Total and net ionic equations
+
 ### Percentage ionic character
 
 Bonding is a spectrum. The percentage ionic character of a chemical bond shows roughly the amount of time valence electrons spend near an atom or ion in a bond. The difference between two elements' electronegativity (ΔEN) indicates how covalent and how ionic the bond **behaves**.
diff --git a/docs/sph3u7.md b/docs/sph3u7.md
index a5bc4e9..aac890b 100644
--- a/docs/sph3u7.md
+++ b/docs/sph3u7.md
@@ -394,9 +394,22 @@ If an object as a **zero net force** in all dimensions, the forces acting on it
 
 ### Newton's laws of motion
 
-Newton's first law of motion (also known as the **law of inertia**) states that every object will remain at rest or at constant velocity unless acted upon by an unbalanced force.
+Newton's first law, also known as the **law of inertia**) states that every object will remain at rest or at constant velocity unless acted upon by an unbalanced force.
 
-Newton's second law of motion states that the **net** force on an object is equal to its mass multiplied by its acceleration, such that $\Sigma \vec{F} = m\vec{a}$.
+Newton's second law states that the **net** force on an object is equal to its mass multiplied by its acceleration, such that $\Sigma \vec{F} = m\vec{a}$.
+
+Newton's third law states that every force applied on an object as an equal and opposite reaction force on another object. These pairs of forces are known as **action/reaction pairs**. Forces are *only* part of the same pair when they:
+
+ - have the same magnitude but opposite directions
+ - are of the same type (e.g., both are weight or normal force)
+ - start and stop at the same time
+ - act on **two different objects**, thus requiring **two** force diagrams to show both of them
+
+!!! example
+    If a block is lying on the ground, the force from the weight of the Earth on the block is the same as the force of the block on the Earth.
+
+!!! warning
+    The weight of an object and the normal force resisting that weight is *not* an action/reaction pair as they appear on the *same* force diagram and act on the *same* object.
 
 ## 2.3 - Work, energy, and power