ASVAB Science Study Guide Part 4



Atoms: The smallest unit of ordinary matter which has the properties of a chemical element. Atoms are made of protons, neutrons and electrons.

Elements: A chemical element is a substance that cannot be broken down into any simpler chemical substances. Each element is distinguished by its atomic number (number of protons). There are 118 elements and they are usually organized on a chart called the periodic table.

Molecules: A molecule is a particle made up of two or more atoms that are chemically bonded together.

Compounds: Compounds are molecules made up of two or more elements.

States of Matter

States of matter can be determined by a substance’s shape and volume.

Solid materials have a definite shape and volume. Cinderblocks are solid.

Liquid materials have a definite volume but an indefinite shape. If you pour milk from the carton into your cereal bowl, the milk’s shape will change, but the amount (volume) will not.

Gases have neither a definite shape nor a definite volume. Gases take up the shape and volume of their container. For example, helium fills the shape and size of a balloon when inflated.

When a substance changes from one state of matter to another, it is referred to as a phase change. A phase change is reversible. For example, water can easily change between being a solid, liquid and gas an infinite number of times. There are six common phase changes: melting, freezing, vaporization, condensation, sublimation and deposition.

Atomic Structure

Atoms are made of smaller, subatomic particles – protons, electrons and neutrons.

Protons are positively charged subatomic particles found in the nucleus of an atom. An atom’s nucleus may contain numerous protons. Each proton is assigned a 1+ charge.

Electrons are negatively charged subatomic particles, found outside the nucleus. Each electron has a 1- charge.

Neutrons are found in the nucleus, and they do not have a charge (they are neutral).

Protons and neutrons have almost exactly the same mass. Electrons have a much smaller mass.

Atomic Number and Mass Number

The atomic number of an element is defined by the number of protons in one atom of the element. Each element has a different number of protons. For example, hydrogen (H) has an atomic number 1 because it has one proton, and is the only element with one proton.

The mass number is the number of protons plus the number of neutrons in the nucleus of an atom. If you know the mass number and the atomic number, you can calculate the number of neutrons in an atom.

Number of neutrons = Mass number – Atomic number


A molecule is a group of atoms bonded together. Molecules may contain one or more elements bonded together.

Chemical Equations

Chemical equations describe a chemical reaction.

Reactants are the substances in a chemical reaction that undergo a change. They are shown on the left side of the arrow.

Products are the new substances formed as a result of the reaction. They are shown on the right side of the arrow.

Reactants → Products

Example of a Chemical Equation:

Carbon + Oxygen → Carbon Dioxide
C + O2 → CO2

The pH Scale

The pH scale is a number scale that describes the concentration of hydronium ions in a solution; it is used to show if a solution is acidic or basic. The pH scale runs from 0 to 14.

Acids have a pH less than 7, and a greater concentration of hydronium ions. Examples include vinegar and tomatoes.

Bases have a pH greater than 7, and a lower concentration of hydronium ions. Examples include baking soda and ammonia.

A neutral substance has a pH equal to 7. Distilled water is neutral.


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