Matter 2.1

Element• A substance that cannot be broken down

into simpler substances by ordinary chemical or physical means.

Atomic Number

• The number of protons in the nucleus of an atom.

Energy Level

• One of several distinct regions around the nucleus of an atom where electrons are located.

Isotope

• An atom with the same number of protons but different numbers of neutrons for a given element; an isotope’s mass number is different from that of the given element.

Mass Number

• The number of neutrons and protons in the nucleus of an atom.

Compound

• A substance formed by the chemical combination of two or more elements in definite proportions and usually having properties different from those of its constituent elements.

Chemical Bond• A force that holds together atoms that

form a compound.

Ion

• An atom or a molecule that possesses an electrical charge.

Ionic Bond

• A bond that forms between negative and positive ions.

Covalent Bond

• A bond that forms when atoms share electrons.

Metallic Bond

• A bond that forms when electrons are shared by metal ions.

Key Concept

• What is an element?–An element is a substance that cannot

be broken down into simpler substances by chemical or physical means.

Key Concept

• What particles make up atoms?–An atom is the smallest particle of

matter that contains the characteristics of an element.

Key Concept

• What are isotopes?–Atoms with the same number of

protons but different numbers of neutrons are isotopes of an element.

Key Concept

• What are compounds and why do they form?–A compound is a substance that

consists of two or more elements that are chemically combined in specific proportions.

Key Concept

• How do chemical bonds differ?–When an atom’s outer-most energy

level does not contain the maximum number of electrons, the atom is likely to form a chemical bond with one or more other atoms.