General Chemistry

Chapter 2 DefinitionsLeft click your mouse to continue.

DIRECTIONS

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ALLOTROPIC MODIFICATIONS (ALLOTROPES)

Different forms of the same element in the same physical state

ANHYDROUS

Without water

ANION

A negative ion

ATOM

• The smallest particle of an element that maintains its chemical identity through all chemical and physical changes.

• Fundamentally consists of: electrons, protons and neutrons

• Atom contains equal numbers of protons and electrons

• Atoms are components of molecules

ATOMIC MASS UNIT (AMU)

One twelfth of the mass of an atom of the carbon-12 isotope; a unit used for stating atomic and formula weights

ATOMIC NUMBER

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

• An atom contains an equal number of electrons and protons.

ATOMIC WEIGHT

Weighted average of the masses of the constituent isotopes of an element; the relative mass of atoms of different

elements.

AVOGADRO’S NUMBER

6.02 x 1023 of the specified items.

CATION

A positive ion.

CHEMICAL FORMULA

Combination of symbols that indicates the chemical composition of

a substance.

COMPOSITION STOICHIOMETRY

Describes the quantitative (mass) relationships among elements in

compounds.

EMPIRICAL FORMULA

The smallest whole-number ratio of atoms present in a compound;

Also known as the simplest formula

FORMULA

Combination of symbols that indicates the chemical composition of

a substance

FORMULA UNIT

The smallest repeating unit of a substance- for non-ionic substances,

the molecule

FORMULA WEIGHT

The mass, in atomic mass units, of one formula unit of substance.

Numerically equal to the mass, in grams, of one mole of the substance. This number is obtained by adding

the atomic weights of the atoms specified in the formula.

HYDRATE

A crystalline sample that contains water and another compound in a

fixed mole ratio.

Ex: CuCO4.5H2O

ION

• An atom or group of atoms that carries an electrical charge.

• A positive ion is a cation; a negative ion is an anion

• Ions are formed when neutral atoms lose or gain electrons

IONIC COMPOUND

A compound that is composed of cations and anions.

EX: NaCl

LAWS OF CONSTANT COMPOSITION

Different samples of a pure compound always contain the same elements in the same proportions by mass; this corresponds to atoms of these elements in fixed numerical ratios.

Also known the Law of Definite Proportions

LAW OF MULTIPLE PROPORTIONS

When two elements, A and B, form more than one compound, the ratio of the masses of element B that combine

with a given mass of element A in each of the compounds can be

expressed by small whole numbers.

MOLAR MASS

The mass of substance in one mole of the substance; numerically equal to the formula weight of the substance

Units= g/mol

MOLE

6.02 x 1023 formula units (or molecules, for a molecular substance)

of the substance under discussion. The mass of one mole, in grams, is numerically equal to the formula

(molecular) weight of the substance.

MOLECULAR FORMULA

A formula that indicates the actual number of atoms present in a

molecule of a molecular substance.

MOLECULE

• The smallest particle of an element or compound that can have a stable independent existence.

• Atoms are components of molecules, and molecules are the components of many elements and most compounds

MOLECULAR WEIGHT

The mass, in atomic mass units, of one molecule of a nonionic

(molecular) substance. Numerically equal to the mass, in grams, of one

mole of such a substance. This number is obtained by adding the

atomic weights of the atoms specified in the formula.

MONATOMIC

Consisting of a single atom. The noble gases exist as monatomic molecules. Chlorine gas is not

monatomic Cl2

PERCENT COMPOSITION

The mass percentage of each element in a compound.

PERCENT PURITY

The percentage of a specified compound or element in an impure

sample.

POLYATOMIC

Consisting of more than one atom. Elements such as Cl2, P4 and S8 exist as polyatomic molecules. Examples of polyatomic ions are ammonium

ion, NH4+, and sulfate ion, SO4

-2

STOICHIOMETRY

Description of the quantitative relationships among elements in

compounds and among substances as they undergo chemical changes

STRUCTURAL FORMULA

A representation that shows how atoms are connected in a compound.

DALTON’S ATOMIC THEORY1. An element is composed if extremely small indivisible

particles called atoms.

2. All atoms of a given element have identical properties, which differ from those of other elements.

3. Atoms cannot be created, destroyed, or transformed into atoms of another element.

4. Compounds are formed when atoms of different elements combine with each other in small whole-number ratios.

5. The relative numbers and kinds of atoms are constant in a given compound.

Diatomic Molecules

O2, H2, N2, F2, Cl2, Br2, I2

There are two atoms in each of these molecules.

1 mole =? particles

• 6.02 x 1023 particles

• 1 mol He = 6.02 x 1023 He atoms

• 1 mol H2 = 6.02 x 1023 H2 molecules

– Because this is a diatomic molecule, 1 mol H2 = 2(6.02 x 1023) H2 atoms