DAT Biology: Matter, Elements, and Atoms

DAT High-Yield Concept

The "Big Four" Elements: Carbon (C), Hydrogen (H), Oxygen (O), and Nitrogen (N) make up approximately 96% of living matter.
Structure Determines Function: The number of protons (Atomic Number) defines the element's identity, while the number of valence electrons determines its chemical reactivity and bonding behavior.
Distinction of Terms: Be able to distinguish between an atom (smallest unit), a molecule (bonded group), and a compound (bonded group of different elements).
Conservation: Matter is conserved in chemical reactions; it is neither created nor destroyed, only rearranged.

1. Matter

Biology is the study of life, but life is composed of matter. Matter is defined as anything that takes up space (has volume) and has mass.

Key Definition: Mass vs. Weight
On the DAT, do not confuse these terms.

Mass is the amount of matter in an object (constant regardless of location).
Weight is the measure of the force of gravity on that mass (changes if you go to the moon).

Matter exists in states: solid, liquid, and gas. In biological systems, matter is frequently found in aqueous solutions (dissolved in water).

Three beakers showing particles: packed tight in solid, loose in liquid, and floating freely in gas.

Particle diagrams showing the arrangement of atoms/molecules in Solid, Liquid, and Gas states. (Source: Wikimedia Commons)

2. Elements

An element is a substance that cannot be broken down to other substances by chemical reactions. It is the purest form of a chemical substance.

There are 92 naturally occurring elements.
Essential Elements: About 20-25% of the 92 natural elements are essential to life.
Trace Elements: Required by an organism in only minute quantities (e.g., Iron (Fe) for hemoglobin, Iodine (I) for thyroid function).

Four quadrants showing particle diagrams: top left shows single atom type (Element), top right shows two atom types bonded (Compound), bottom quadrants show mixtures.

Comparison of matter classifications: Elements (pure substances with one type of atom) vs. Compounds (pure substances with bonded atoms) vs. Mixtures. (Source: Wikimedia Commons)

3. Atoms

An atom is the smallest unit of matter that still retains the properties of an element. Atoms are composed of subatomic particles.

Subatomic Particles

Particle	Charge	Mass (Daltons/amu)	Location	Function
Proton	Positive (+)	~1	Nucleus	Determines element identity (Atomic Number)
Neutron	Neutral (0)	~1	Nucleus	Determines isotope (stability/radioactivity)
Electron	Negative (-)	~0 (negligible)	Orbitals/Shells	Determines chemical reactivity

Mnemonic: PEN
Protons, Electrons, Neutrons are the building blocks. Remember: Protons = Positive, Neutrons = Neutral.

Diagram of a helium atom with a nucleus of red protons and green neutrons, surrounded by two blue electrons in a circular orbit.

A Bohr model of a Helium atom, showing the central nucleus (protons and neutrons) orbited by electrons. (Source: Wikimedia Commons)

Atomic Terminology for DAT

Atomic Number: The number of protons (unique to each element). In a neutral atom, this also equals the number of electrons.
Mass Number: The sum of protons plus neutrons in the nucleus.
Isotopes: Atoms of the same element (same protons) with different numbers of neutrons (different mass). Example: Carbon-12 vs. Carbon-14.

4. Molecules and Compounds

Atoms combine to form molecules and compounds to achieve stability (usually filling their valence electron shells).

Molecule: Two or more atoms held together by covalent bonds. The atoms can be the same (e.g., O₂) or different (e.g., H₂O).
Compound: A substance consisting of two or more different elements combined in a fixed ratio (e.g., NaCl, H₂O).

Venn Diagram Concept:
All compounds are molecules (generally speaking, in loose biological terms regarding covalent compounds), but not all molecules are compounds.

Example: O₂ (Oxygen gas) is a molecule because two atoms are bonded. It is NOT a compound because both atoms are the same element.

3D diagram of a water molecule with one large red oxygen atom bonded to two smaller white hydrogen atoms.

A space-filling model of a water molecule (H2O). Water is both a molecule and a compound. (Source: Wikimedia Commons)

Summary Comparison

Term	Definition	Example
Atom	Smallest unit of an element	Carbon (C), Oxygen (O)
Element	Pure substance made of one type of atom	Gold foil (Au), Oxygen gas (O₂)*
Molecule	Two or more atoms bonded together	O₂, H₂O, CO₂
Compound	Two or more different elements bonded	H₂O, Glucose (C₆H₁₂O₆)

*Note: O₂ is an elemental molecule.

Critical Thinking Questions

Application: If an atom of Carbon-12 gains one neutron, does its chemical identity change? Why or why not?
Hint: Consider what defines an element versus an isotope.
Analysis: Why are electrons the only subatomic particles involved in chemical bonding?
Hint: Think about the location of the nucleus versus the electron shells and the forces involved.
Synthesis: A researcher identifies a substance composed of Carbon and Hydrogen atoms bonded together. Is this substance an atom, an element, a molecule, or a compound? (Select all that apply).
Answer: It is a molecule and a compound.

Matter, Elements, and Atoms

Summary

Practice Questions

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