Anatomy & Physiology I · Basic Chemistry for A&P
Matter, Elements, and Atomic Structure
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In 30 seconds
Everything in the body is made of Matter anything that has mass and takes up space; exists as solid, liquid, or gas., and matter is built from atoms of a limited set of elements. This section explains what matter and Energy the capacity to do work; includes kinetic (energy of motion) and potential (stored) energy. are, which elements dominate the human body, and how the subatomic particles — protons, neutrons, and electrons — are arranged. It also covers Atomic number the number of protons; defines the element., Mass number protons + neutrons., and Isotopes atoms of the same element with different numbers of neutrons..
Why this matters
Physiology is chemistry in action. Nerve signals, muscle contraction, oxygen transport, and energy production all come down to how atoms behave and combine. You cannot understand pH, ions, or metabolism without first knowing what an Atom the smallest unit of an element that retains its properties. is. This chapter is the foundation the rest of A&P quietly stands on.
The college version
Matter and energy. Matter is the "stuff" of the body — bone, blood, air in the lungs — and it exists in three physical states. Energy has no mass; it is the ability to move or change matter. Energy comes in forms the body constantly converts: chemical (stored in molecular bonds, released during metabolism), electrical (ion movement in nerves and muscle), mechanical (muscle movement), and radiant (light, used in vitamin D synthesis). A recurring theme in physiology is stored (potential) energy in chemical bonds being released to do work.
Elements of the body. Of the naturally occurring elements, just four make up about 96% of body mass: oxygen (O), carbon (C), hydrogen (H), and nitrogen (N). Another group — including calcium (Ca), phosphorus (P), potassium (K), sulfur (S), sodium (Na), chlorine (Cl), and magnesium (Mg) — makes up most of the remaining mass. Trace elements such as iron and iodine are needed in tiny amounts but are essential (iron for hemoglobin, iodine for thyroid hormone).
Atomic structure. An atom has a dense central nucleus containing protons (positive) and neutrons (neutral), surrounded by electrons (negative) moving in energy levels (shells). Because protons and electrons carry equal and opposite charges, a neutral atom has equal numbers of each. Almost all of an atom's mass is in the nucleus; almost all of its volume is the electron cloud — an atom is mostly empty space.
Atomic number and mass number. The atomic number equals the number of protons and is what makes an Element a pure substance made of one type of atom; cannot be broken down by ordinary chemical means. that element — every carbon atom has 6 protons, every oxygen atom has 8. The mass number is the sum of protons and neutrons (electrons are too light to count meaningfully). For example, a typical carbon atom has 6 protons and 6 neutrons, giving a mass number of 12.
Isotopes. Atoms of one element always share the same proton count but can differ in neutrons; these variants are isotopes. Carbon-12 and carbon-14 are both carbon (6 protons) but have 6 and 8 neutrons. Some isotopes are radioisotopes, which are unstable and emit radiation as they decay — the basis for many medical imaging and treatment tools (for example, radioactive iodine used to image or treat the thyroid).
How it works
To describe any atom:
- Protons = atomic number → identifies the element.
- Electrons = protons in a neutral atom → determines chemical behavior.
- Neutrons = mass number − atomic number → varies among isotopes.
Comparisons
| Particle | Charge | Location | Relative mass |
|---|---|---|---|
| Proton | +1 | Nucleus | ~1 amu |
| Neutron | 0 | Nucleus | ~1 amu |
| Electron | −1 | Energy levels | ~0 (negligible) |
| Quantity | Equals | Tells you |
|---|---|---|
| Atomic number | # protons | Which element |
| Mass number | protons + neutrons | Atomic "weight" of that atom |
| Isotope difference | # neutrons | Same element, different mass |
Common confusions
- Atomic number vs mass number. Atomic number = protons only (identity); mass number = protons + neutrons.
- Isotopes are still the same element. Changing neutrons changes mass, not identity; changing protons would make it a different element.
- Element vs atom. An element is the type; an atom is one particle of it.
- Mass vs weight. Mass is amount of matter (constant); weight depends on gravity.
Memory aids
- "PEN": Protons, Electrons, Neutrons — the three particles.
- Protons are Positive (both start with P).
- Atomic number = "the ID number" (protons never change for an element).
Quick review
- Matter has mass and volume; energy is the capacity to do work (chemical, electrical, mechanical, radiant).
- Four elements — O, C, H, N — make up ~96% of body mass; trace elements (iron, iodine) are essential in small amounts.
- Atoms have a nucleus (protons + neutrons) surrounded by electrons.
- Atomic number = protons (element identity); mass number = protons + neutrons; isotopes differ in neutrons.
Eli explains
The same idea, in plain words
Explain it like I’m 10
Simple idea
Everything is built from tiny building blocks called atoms, and each atom is made of even tinier pieces.
Analogy
Think of an atom like a tiny solar system. In the center is the "sun" — the nucleus — packed with heavy pieces called protons and neutrons. Zipping around it, like planets, are lightweight electrons. Just as our solar system is mostly empty space between the sun and planets, an atom is mostly empty space too.
What is actually happening
The number of protons in the center is like the atom's ID badge: 6 protons always means carbon, 8 always means oxygen — that's the atomic number. Neutrons add weight but no charge; add up protons and neutrons and you get the mass number. Sometimes the same element has a few extra neutrons — those versions are called isotopes, and some of them are slightly unstable and give off energy, which doctors actually use for scans and certain treatments.
Where the analogy stops
Electrons don't really travel on neat planet-like paths — they exist in fuzzy "clouds" of likely locations, not tidy orbits. The solar-system picture is a helpful cartoon, not a photograph.
Key takeaway
Trace-element deficiencies produce recognizable conditions — iron deficiency causes anemia, iodine deficiency impairs thyroid function. Radioisotopes appear throughout diagnostics and therapy (PET scans, radioactive iodine, radiation oncology). Understanding that electrons drive chemical behavior sets up the next section on bonding and, later, the electrolytes (Na⁺, K⁺, Ca²⁺) central to nerve, muscle, and cardiac function.
Study tools & related lessonsYou’ll learn to · Key vocabulary · Related
You’ll learn to
- Define matter and energy and distinguish their major forms.
- Identify the major and trace elements of the human body.
- Describe the structure of an atom: protons, neutrons, electrons.
- Explain atomic number, mass number, and isotopes.
Key vocabulary
- Matter
- anything that has mass and takes up space; exists as solid, liquid, or gas.
- Energy
- the capacity to do work; includes kinetic (energy of motion) and potential (stored) energy.
- Element
- a pure substance made of one type of atom; cannot be broken down by ordinary chemical means.
- Atom
- the smallest unit of an element that retains its properties.
- Proton (p⁺)
- positively charged particle in the nucleus.
- Neutron (n⁰)
- uncharged particle in the nucleus.
- Electron (e⁻)
- negatively charged particle orbiting the nucleus in energy levels.
- Atomic number
- the number of protons; defines the element.
- Mass number
- protons + neutrons.
- Isotopes
- atoms of the same element with different numbers of neutrons.
Sources & references
- OpenStax, *Anatomy and Physiology 2e*, Chapter 2 (The Chemical Level of Organization): Elements and Atoms. https://openstax.org/details/books/anatomy-and-physiology-2e
- U.S. National Library of Medicine, MedlinePlus — Minerals. https://medlineplus.gov/minerals.html
This lesson was adapted from the open educational references above; their licenses and attributions are preserved. See Copyright & Licensing.
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