Anatomy and Physiology 2e · The Chemical Level of Organization
Elements and Atoms: The Building Blocks of Matter
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In 30 seconds
Every structure in the body — bones, muscles, nerves, blood — is built from chemicals. The chemical level of organization is the body's most basic level, and it begins with two ideas: matter and the elements that compose it. Matter is anything that has mass and takes up space. Elements are the simplest pure substances, made of tiny particles called atoms, whose properties determine how each Element A pure substance that cannot be broken down chemically into simpler substances Full entry → behaves.
The body does not need all of the roughly 90 naturally occurring elements equally. Most of its mass comes from just four — oxygen, carbon, hydrogen, and nitrogen. A handful of others (calcium, phosphorus, potassium, sulfur, sodium, chlorine, magnesium) are needed in smaller amounts, and many more (iron, iodine, zinc, copper) only in trace amounts. Understanding what an Atom The smallest unit of an element retaining its properties Full entry → is, what its parts are, and how those parts give each element its identity is the foundation for everything that follows in this chapter: chemical bonds, chemical reactions, and the molecules that make life possible.
Why this matters
Physiology is chemistry in action. The water filling your cells, the oxygen your blood carries, the sodium and potassium driving your nerves, the calcium building your bones, and the iron in your red blood cells are all elements behaving by chemical rules. When clinicians order tests for "electrolytes" or "calcium," they are measuring elements; when iodine runs short, the thyroid cannot make its hormones.
Diseases and treatments are described in the language of elements: iron-deficiency anemia, potassium imbalance affecting heart rhythm, iodine-fortified salt. Grasping elements and atoms lets you understand why these substances matter and why the body regulates their amounts tightly — and how atoms combine, the subject of the next topic.
The college version
Core Concepts
Matter and elements
Matter is anything that occupies space and has mass; it includes your entire body and the air you breathe. An element is a substance that cannot be broken down into simpler substances by ordinary chemical means. About 92 elements occur naturally on Earth, each identified by a one- or two-letter symbol such as O (oxygen), C (carbon), and Na (sodium, from its Latin name natrium).
Elements are organized in the periodic table by increasing Atomic number The number of protons in an atom Full entry →; elements in the same column share similar chemical behavior because their outermost electrons are arranged similarly.
The structure of an atom
An atom is the smallest unit of an element that still has that element's properties. It contains three main subatomic particles:
- Protons — positively charged, in the central nucleus.
- Neutrons — uncharged, also in the nucleus; they add mass and stabilize it.
- Electrons — negatively charged, far lighter, in regions around the nucleus called Electron Negatively charged particle in shells around the nucleus Full entry → shells (energy levels).
In a neutral atom, the number of protons equals the number of electrons, so charges balance. The atomic number — the number of protons — makes each element unique: every atom with 6 protons is carbon, every atom with 8 is oxygen. The mass number is protons plus neutrons.
Isotopes and ions
Atoms of an element always have the same number of protons but may differ in neutrons. These variants are isotopes, named by mass number — carbon-12, carbon-13, and carbon-14 all have 6 protons but 6, 7, and 8 neutrons. Isotopes behave almost identically in chemical reactions, which is why trace amounts of radioactive isotopes are used in medicine: they react like normal atoms but emit detectable signals (as in PET imaging).
If an atom gains or loses electrons, it becomes an Ion Atom that gained or lost electrons, giving it a charge Full entry → — an atom with a net charge. Losing electrons leaves more protons than electrons, producing a positively charged cation (Na⁺, Ca²⁺); gaining electrons produces a negatively charged anion (Cl⁻). Ions are the workhorses of physiology: nerve signals, muscle contraction, and fluid balance all depend on ions moving across cell membranes.
Electron shells and chemical behavior
Electrons occupy shells at increasing distances from the nucleus: the innermost holds up to 2 electrons, the next up to 8, and so on (a simple teaching model). The outermost electrons, called valence electrons, determine how an atom interacts with others. Atoms with a full outer shell, like the noble gases, are very stable and rarely react; atoms with incomplete shells gain, lose, or share electrons to reach a more stable arrangement — the drive behind every chemical bond, covered in the next topic.
The elements of the human body
The body is mostly CHON: oxygen (~65%), carbon (~18%), hydrogen (~10%), and nitrogen (~3%) make up roughly 96% of body mass (percentages vary somewhat among sources). Oxygen and hydrogen form water, which dominates body mass; carbon is the backbone of organic molecules; nitrogen appears in proteins and nucleic acids.
The remaining few percent matter enormously. Calcium and phosphorus build bone; potassium, sodium, and chlorine control fluid balance and nerve and muscle function; sulfur appears in some amino acids; magnesium supports enzyme reactions. Trace elements — needed in tiny amounts — include iron (oxygen transport in hemoglobin), iodine (thyroid hormones), zinc, copper, and selenium. Deficiencies or excesses cause disease, which is why the body regulates them carefully.
Common Confusions
| Do not confuse | With | Difference |
|---|---|---|
| An atom | A molecule | An atom is a single element particle; a molecule is two or more atoms bonded (e.g., O₂) — covered in the next topic |
| Atomic number | Mass number | Atomic number counts protons (identity); mass number counts protons + neutrons (weight) |
| An ion's charge | An isotope's mass | Ions differ in electron count (charge); isotopes differ in neutron count (mass) — both can apply to the same atom |
| A cation | An anion | Cations lost electrons and are positive ("ca+tion"); anions gained electrons and are negative |
| The simple shell model | Exact electron structure | The 2-8-8 rule is a teaching approximation; real configurations are more nuanced |
| "Trace" meaning unimportant | Trace elements in the body | Trace means small quantity only — iron and iodine deficiencies cause serious disease |
Eli explains
The same idea, in plain words
Explain it like I’m 10
Everything — including your body — is made of tiny building blocks called atoms. Atoms come in different kinds called elements, like oxygen, carbon, and iron, just like LEGO bricks come in different colors. An atom has a heavy middle part with protons (positive) and neutrons (neutral), and much lighter electrons spinning around it. If you have six protons, you're carbon; eight, you're oxygen — the proton count is the atom's ID card. Your body is mostly four kinds: oxygen, carbon, hydrogen, and nitrogen.
Worked example
Consider a patient who feels weak and tired. The clinician orders a basic metabolic panel measuring sodium, potassium, chloride, and calcium. Those are not abstract lab values — they are ions of elements: Na⁺, K⁺, Cl⁻, and Ca²⁺.
Sodium and potassium are cations (they lost an electron); chloride is an anion (it gained one). The proton count — 11 for sodium, 19 for potassium, 17 for chlorine, 20 for calcium — determines each element's identity, while electron gain or loss determines its charge. The lab measures them because imbalances affect nerve signaling, muscle contraction, and heart rhythm. Meanwhile, a separate test might check hemoglobin, whose oxygen-carrying ability depends on iron — a trace element — nestled inside each molecule. One patient, one set of tests, and every result is a story about atoms, ions, and elements.
Key takeaways
- Matter → elements → atoms: matter has mass and volume; elements are the simplest substances; atoms are their smallest units.
- Atomic number = number of protons — it defines the element. Mass number = protons + neutrons.
- Protons (+) balance electrons (−) in a neutral atom; gaining or losing electrons creates ions (cation = positive, anion = negative).
- Isotopes = same protons, different neutrons — chemically similar; some have medical uses (e.g., PET imaging).
- Valence electrons (outermost shell) determine chemical behavior; atoms seek full outer shells — the reason they bond.
- The body's big four: O, C, H, N (~96% of body mass) — plus Ca, P, K, S, Na, Cl, Mg and trace elements (Fe, I, Zn, Cu) for specialized jobs.
Check yourself
6 review questions from the chapter. Try each one, then open the answer.
What determines whether an atom is carbon, oxygen, or iron?
Show answer
The number of protons (atomic number): 6 = carbon, 8 = oxygen, 26 = iron.
An atom has 11 protons, 12 neutrons, and 10 electrons. What element is it, what is its mass number, and is it an ion?
Show answer
Sodium (11 protons). Mass number = 11 + 12 = 23. It has 10 electrons vs. 11 protons, so it lost an electron and is a cation (Na⁺).
How do isotopes of the same element differ, and why do they behave similarly in reactions?
Show answer
Same protons, different neutrons (carbon-12 vs. carbon-14). They react similarly because chemical behavior depends mainly on electrons, especially valence electrons, which are the same.
Which four elements make up about 96% of body mass, and what is a major role of each?
Show answer
Oxygen (water and energy reactions), carbon (backbone of organic molecules), hydrogen (water and most organic compounds), nitrogen (proteins and nucleic acids).
Why do noble gases rarely form bonds, and how does that relate to valence electrons?
Show answer
Their outermost shell is already full, so they have little tendency to gain, lose, or share electrons — the drive that makes other atoms bond.
Give two examples of ions essential to nerve and muscle function, with their charges.
Show answer
Sodium (Na⁺), potassium (K⁺), chloride (Cl⁻) — these drive nerve impulses and muscle contraction; calcium (Ca²⁺) is also essential.
Study tools & related lessonsKey vocabulary · Related
Key vocabulary
- Element
- A pure substance that cannot be broken down chemically into simpler substances
- Atom
- The smallest unit of an element retaining its properties
- Proton
- Positively charged particle in the nucleus
- Neutron
- Uncharged particle in the nucleus
- Electron
- Negatively charged particle in shells around the nucleus
- Atomic number
- The number of protons in an atom
- Isotope
- Same element, different neutron number
- Ion
- Atom that gained or lost electrons, giving it a charge
- Valence electron
- Electron in the outermost shell
Sources & references
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