1. Science, Matter, Energy, and Systems2
Science, Matter, Energy, and Systems
Photo © Borut Trdina/iStockphoto

2. 2.2 What Is Matter?
Matter is anything that has mass and takes up space.
Exists in three physical states: solid, liquid, gas
Two chemical forms:
1. elements (all the same atoms)
2. compounds (combinations of more than one type of atom)

3. Elements and Compounds
Elements cannot be broken down into simpler forms by chemical means.
Represented by a one- or two-letter symbol (C=carbon, Au=gold)
Arranged in periodic table based on chemical behavior
Currently 118 elements, not all of which occur naturally
Compounds: two or more different elements held together in fixed proportions

4. Atom: Basic Building Block of Matter
An atom is the smallest building block of matter that an element can have that will still retain its chemical properties.
Each atom has three subatomic particles:
Neutrons (no electric charge, inside nucleus)
Protons (positive electric charge, inside nucleus)
Electrons (negative electric charge, move rapidly outside nucleus)
Each atom has a small central nucleus that contains its protons and neutrons.

5. Atomic Number, Atomic Mass, and Isotopes
Atomic number: the number of protons in an atom’s nucleus
Mass number: the total number of neutrons and protons in an atom’s nucleus
Isotopes of an element have the same atomic number, but a different mass number (the same number of protons but a different number of neutrons).

6. Structure of Atoms
Atom: This simplified model depicts a carbon-12 atom. The atom’s nucleus consists of six protons, each with a positive electrical charge, and six neutrons with no electrical charge. Six negatively charged electrons move rapidly outside its nucleus.

7. Molecule: Second Building Block of Matter
A molecule is a combination of two or more atoms of the same or different elements held together by forces known as chemical bonds.

8. Ion: Third Building Block of Matter
An ion is an atom or group of atoms with a net positive or negative charge.
Subscript number: number of positive/negative electrical charges
Important for measuring a substance’s acidity in a water solution (pH)

9. Physical and Chemical Changes
Physical changes, such as changes in size or state (ice to water), do not involve changes in chemical composition.
Chemical changes (reactions) involve changes in chemical composition of the substances involved.
Chemists use a chemical equation to show how the chemicals involved are arranged in a chemical reaction.
Chemists use a process of balancing the equation to account for all atoms involved in any reaction.

10. We Cannot Create or Destroy Atoms
Law of conservation of matter
Physical or chemical changes can take place in chemical reactions, but no atoms are created or destroyed in the process.