To understand the complex biological systems of the human body—from the beating of a heart to the firing of a neuron—we must first look at the microscopic level of atoms and molecules. At the heart of all matter is the atom, a tiny particle composed of protons, neutrons, and electrons. While protons and neutrons reside in the central nucleus, it is the behavior of the electrons that dictates how atoms interact, react, and ultimately form the bonds that create everything from simple salt to complex DNA.
The Drive for Stability: Electron Shells and Reactivity
Every atom has "shells" or rings around its nucleus that hold electrons. The most critical shell for understanding chemistry is the outermost one, often called the valence shell. For an atom to be chemically stable and "unreactive," it typically needs to have a full outer shell—usually consisting of eight electrons.
If an atom's outer shell is incomplete, the atom becomes "reactive," meaning it is unstable and will actively seek out other atoms to trade or share electrons with in an attempt to reach that stable state of eight.
Exam Tip: If you are asked why an atom is chemically reactive, look for the answer involving an "incomplete electron shell." Having too many neutrons or protons does not determine chemical reactivity; it is the electron configuration that matters.
Ionic Bonds: The Transfer of Electrons
An ionic bond occurs when one atom "donates" or transfers one or more electrons to another atom. This transfer creates a fundamental change in the atom's electrical charge:
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