The Periodic Table and the Logic Behind It
Every general chemistry question on the MCAT ultimately traces back to one 1869 insight: Russian chemist Dmitri Mendeleev noticed that when he arranged the 63 elements then known by increasing atomic weight, their chemical properties repeated in a predictable pattern. He organized them into a table — and then did something that made the periodic table more than a filing system: he left gaps. Mendeleev predicted that undiscovered elements would eventually fill those gaps, and even forecasted their approximate properties. When gallium was discovered in 1875 and germanium in 1886, both matched his predictions closely enough that the periodic table’s underlying logic — not just its layout — was validated. Modern versions of the table, reorganized by atomic number rather than atomic weight after the 1913 work of physicist Henry Moseley, still follow Mendeleev’s core principle: elements in the same column, or group, share a number of valence electrons and therefore similar chemical behavior, from reactivity to the charge of the ions they tend to form.
That group-based logic explains most of what the MCAT tests under “periodic trends”: atomic radius shrinks moving left to right across a period because each added proton pulls the same number of electron shells in tighter, while ionization energy — the energy required to remove an electron — generally increases across that same span for the identical reason. Electronegativity, the tendency of an atom to attract shared electrons in a bond, follows the same pattern, peaking at fluorine. None of these trends are arbitrary memorization; they all fall out of the same underlying relationship between nuclear charge and electron shell distance that Mendeleev’s original table only hinted at.
Acids, Bases, and the Language Chemists Still Use
The vocabulary MCAT test-takers use to describe acids and bases layers three separate historical definitions on top of each other, each broader than the last. Svante Arrhenius offered the first formal definition in 1884: an acid increases hydrogen ion concentration in water, a base increases hydroxide ion concentration. It worked, but only for reactions happening in water. In 1923, chemists Johannes Brønsted and Thomas Lowry — working independently in Denmark and England — proposed a broader definition: an acid is any substance that donates a proton, a base any substance that accepts one, a framework that no longer required water as the solvent. That same year, American chemist Gilbert Lewis proposed an even broader definition still in use today: an acid accepts an electron pair, a base donates one, a definition that covers reactions with no protons involved at all.
The pH scale that quantifies all of this — a logarithmic measure of hydrogen ion concentration running from 0 to 14 — was introduced in 1909 by Danish biochemist Søren Sørensen, who developed it while working at the Carlsberg Laboratory to standardize quality control in beer brewing, an origin far removed from the medical and biological contexts where the scale is used today. Sørensen’s scale, and the equilibrium math behind buffer systems and titration curves that the MCAT builds on top of it, all rest on the same underlying reality: at any given moment in a solution, acid-base reactions are in dynamic equilibrium, constantly shifting in response to whatever is added — which is exactly why buffer solutions can resist pH changes up to a point, then fail abruptly once that buffering capacity is exceeded.
Source: National Institutes of Health (NIH) and Encyclopaedia Britannica.