3.2 Chemistry Basics

Chemistry on the NEX

Chemistry is the smallest science subsection — roughly 5 items (~8%) of the 60-question Science section. Do not skip it: a few certain points (blood pH, bond types) are easy marks, and the concepts return in nursing pharmacology and acid-base nursing.

Atomic Structure

An atom is the smallest unit of an element that keeps the element's properties. It has three subatomic particles:

• Atomic number = number of protons (this defines the element). Carbon is always 6 protons.
• Mass number = protons + neutrons.
• A neutral atom has equal protons and electrons.
• An ion has a charge: losing electrons makes a cation (+), gaining electrons makes an anion (-).
• Isotopes are atoms of the same element with different neutron counts (same atomic number, different mass number).

Worked example: An atom with mass number 23 and atomic number 11 (sodium) has 11 protons, 11 electrons (if neutral), and 23 - 11 = 12 neutrons.

The Periodic Table

High-yield groups: Group 1 alkali metals (Li, Na, K) are highly reactive; Group 17 halogens (F, Cl, Br, I) are reactive nonmetals; Group 18 noble gases (He, Ne, Ar) are inert because their valence shell is full. Trap: elements in the same group (column) — not the same period — share chemical properties.

Chemical Bonding

Ionic compounds dissolve into charged electrolytes in water (Na+ and Cl-), which is why saline conducts electricity — a bridge to fluid-and-electrolyte nursing. Hydrogen bonds are weak individually but collectively give water its cohesion and high boiling point.

Acids, Bases, and pH

The pH scale measures hydrogen-ion (H+) concentration from 0 to 14. Each whole number is a 10-fold change: pH 4 has ten times more H+ than pH 5.

Critical nursing value: normal arterial blood pH is 7.35-7.45 (slightly alkaline). Below 7.35 = acidosis; above 7.45 = alkalosis. The body buffers this narrow range using the bicarbonate (HCO3-) buffer system, the lungs (exhaling CO2), and the kidneys. Trap: blood is not neutral 7.0 — choosing 7.0 is the classic distractor.

The Four Macromolecules

Lipids are the exception — they are not true polymers, but the monomer-style pairing (fatty acids + glycerol) is what the NEX expects.

Water and Solutions

Water's polarity (slightly + hydrogens, slightly - oxygen) makes it the universal solvent — it dissolves more substances than any other liquid, though not nonpolar oils. Its high specific heat resists temperature swings, helping the body hold a stable core temperature. In a solution, the solute is dissolved (salt), the solvent does the dissolving (water), and together they form a homogeneous solution (saline).

Tonicity and IV Fluids

Because nursing students immediately apply solution concepts to intravenous therapy, the NEX may frame chemistry questions in clinical terms. Concentration describes how much solute sits in a given volume of solvent; a more concentrated solution has more dissolved particles. Tonicity compares a solution to the inside of a body cell. An isotonic fluid such as 0.9 percent normal saline has the same particle concentration as the cell, so water does not shift and the cell stays the same size. A hypotonic fluid is more dilute, so water moves into the cell and it swells.

A hypertonic fluid is more concentrated, so water leaves the cell and it shrinks. This is osmosis from the biology section applied to patient care, and recognizing the link earns points in both subsections.

States of Matter and Physical vs. Chemical Change

Matter exists as solid (fixed shape and volume), liquid (fixed volume, variable shape), and gas (variable shape and volume). Adding heat drives melting, evaporation, and sublimation; removing heat drives freezing and condensation. A physical change alters form without creating a new substance, such as ice melting into water or salt dissolving in water. A chemical change produces a new substance with new properties, such as iron rusting, wood burning, or an acid neutralizing a base. Telltale signs of a chemical change include a color change, gas bubbles, a temperature shift, light, or a precipitate forming.

Trap: boiling water and dissolving sugar are physical changes, not chemical ones, because no new substance is formed. Distinguishing the two is a recurring NEX chemistry item and a useful frame for understanding how the body chemically transforms nutrients during digestion and metabolism.