14.1 Atomic Structure & the Periodic Table

14.1 Atomic Structure & the Periodic Table

Quick Answer: An atom is defined by its atomic number (Z) = number of protons. Neutrons add mass; electrons determine charge and bonding. Isotopes differ in neutrons; ions differ in electrons. Elements are one atom type; compounds are fixed-ratio chemical combinations. The periodic table groups elements by shared valence patterns—metals, nonmetals, and noble gases. This is foundational Chemistry for NLN NEX Science (~9% of scored items) and underpins meds, body fluids, and pH later in this chapter.

Chemistry on the NEX is conceptual, not a deep physics detour. You need a clear mental model of what atoms are made of, how the periodic table predicts behavior, and why that matters when you read a drug label, an electrolyte panel, or a radiology order that mentions an isotope.

Subatomic Particles

Every atom has three kinds of particles you must know by charge, relative mass, and location.

• The nucleus holds nearly all the atom’s mass (protons + neutrons).
• Electrons occupy regions of space called orbitals and are responsible for chemical bonding and most chemical reactions.
• In a neutral atom, the number of electrons equals the number of protons, so net charge is zero.

Atomic Number and Mass Number

Two numbers identify an atom on exams:

• Atomic number (Z) = number of protons. This defines the element. Carbon always has 6 protons; if it had 7, it would be nitrogen.
• Mass number (A) = protons + neutrons. It is a whole-number count for a specific atom or isotope (not the same as average atomic mass on the periodic table, which is a weighted average of isotopes).

Shorthand nuclear notation is often written as (^{A}_{Z}\mathrm{X}) or simply “carbon-14” for the isotope with mass number 14.

Example: Sodium (Na) has Z = 11. A common isotope is sodium-23 → 11 protons + 12 neutrons. A neutral sodium atom also has 11 electrons.

Isotopes

Isotopes are atoms of the same element (same Z / same proton count) with different numbers of neutrons and therefore different mass numbers.

• Chemically, isotopes of an element behave nearly alike because electron configuration (chemistry) is set by proton/electron count, not neutron count.
• Some isotopes are stable; others are radioactive (decay and emit radiation).

Nursing and clinical links you should recognize conceptually:

• Iodine-131 and other radioisotopes are used in thyroid imaging/therapy.
• Technetium-99m is widely used in nuclear medicine scans.
• Carbon-14 dating is more archaeology than nursing, but the isotope idea is the same: same element, different neutrons.

On the NEX, the trap is thinking isotopes are different elements. They are not—same protons, different neutrons.

Ions: When Electron Count Changes

An ion is an atom (or group of atoms) with a net electric charge because electrons were gained or lost.

Key exam rules:

• Changing electrons → ion (charge changes).
• Changing protons → different element.
• Changing neutrons → different isotope (same element).

Body fluids are full of ions—electrolytes. Sodium, potassium, calcium, magnesium, chloride, bicarbonate, and phosphate ions drive nerve impulses, muscle contraction, acid–base balance, and fluid distribution. When a lab reports “Na 138 mEq/L,” you are reading ion chemistry applied to nursing.

Elements vs Compounds

• An element is a pure substance made of only one type of atom (e.g., O₂ is still the element oxygen; Fe is iron). Elements cannot be broken into simpler substances by ordinary chemical means.
• A compound is a substance formed when two or more different elements are chemically bonded in a fixed ratio (e.g., H₂O, NaCl, CO₂, C₆H₁₂O₆). Compounds have properties different from their component elements (sodium metal + chlorine gas → table salt).
• A molecule is two or more atoms bonded together (can be elemental like O₂ or compound like H₂O).

Mixtures (covered in Section 14.3) are physical blends that keep the identities of their components and can often be separated by physical means—unlike compounds, which require chemical change to separate into elements.

The Periodic Table: Organization

The modern periodic table lists elements in order of increasing atomic number.

• Periods = horizontal rows. Period number relates to the highest principal energy level occupied by electrons in that row’s atoms.
• Groups (families) = vertical columns. Elements in a group often share similar valence electron counts and therefore similar chemical behavior.

Valence Electrons (Intro)

Valence electrons are the outermost electrons—the ones involved in bonding. For many main-group elements, the group number (in the older A-group numbering, or the ones place of the IUPAC group number for groups 1–2 and 13–18) predicts valence count:

• Group 1 (alkali metals): 1 valence electron → tend to form +1 cations (Na⁺, K⁺)
• Group 2 (alkaline earth): 2 valence electrons → often +2 cations (Ca²⁺, Mg²⁺)
• Group 17 (halogens): 7 valence electrons → tend to gain 1 electron → −1 anions (Cl⁻, F⁻)
• Group 18 (noble gases): full outer shell → very low reactivity

The octet rule (detailed in Section 14.2) says many atoms “prefer” eight valence electrons—like a noble-gas configuration.

Metals, Nonmetals, and Noble Gases

Hydrogen sits atop Group 1 but behaves as a nonmetal chemically—do not assume it is a metal because of its table position.

High-Yield Elements for Nursing Context

You do not need to memorize the whole table, but these appear constantly in health science:

• H, C, N, O — backbone of water, organics, proteins, nucleic acids
• Na, K, Ca, Mg — major cations / electrolytes
• Cl, P (as phosphate), S — anions and biomolecules
• Fe — hemoglobin; I — thyroid hormone; F — dental/enamel chemistry

Why This Matters for Nursing and the NEX

• Electrolyte panels are ion chemistry: Na⁺, K⁺, Cl⁻, HCO₃⁻.
• Drug formulas (e.g., NaHCO₃, KCl, Ca gluconate) are compounds built from elements and ions.
• Isotopes appear in imaging and radiation safety contexts.
• Periodic trends explain why alkali metals and halogens form salts so readily—the same salts dissolved in IV bags.

Exam Traps

• Atomic number vs mass number: Z = protons (identity); A = protons + neutrons (isotope mass count).
• Isotope ≠ ion: Isotopes change neutrons; ions change electrons.
• Neutral atom electron count: Equals protons—not mass number.
• Compound vs mixture: Compounds have fixed ratios and chemical bonds; mixtures do not.
• Noble gases: Full valence shells → chemically unreactive under ordinary conditions—not “metals.”
• Mass on the periodic table: Average atomic mass reflects isotope abundance; mass number of one isotope is a whole number.

Build the chain: protons define the element → neutrons make isotopes → electrons make ions and bonds → the periodic table predicts valence patterns. That chain unlocks bonding, solutions, and acid–base chemistry in the next sections.