Units of Measurement in Chemistry

Units of Measurement

• Units are standard quantities used to specify measurements. They are critical in chemistry.
• The English system, used in the United States.
• The Metric system, used in most countries.
• The International System of Units (SI), used by scientists, is based on the metric system.

Units in Metric and SI Systems

• The metric and SI systems use one unit for each type of measurement:
  • Length: meter (m)
  • Volume: liter (L) or cubic meter (m³)
  • Mass: gram (g) or kilogram (kg)
  • Temperature: Celsius (°C) or Kelvin (K)
  • Time: second (s)

Meter: Measure of Length

• Length is measured using a meter stick.
• The meter (m) is the unit of length used in both the metric and SI systems.
• Centimeters (cm) are used for smaller lengths.

Kilogram: Measure of Mass

• Mass is a measure of the quantity of matter in an object.
• The Law of Definite Proportions (Proust): A chemical compound always contains the same elements in the exact same proportions by mass, regardless of its source or how it was prepared.

Law of Multiple Proportions

• The Law of Multiple Proportions (Dalton): When two elements combine to form two or more compounds, the ratios of the masses of those elements in the formed compounds are simple whole numbers.
  • For example, carbon dioxide (CO₂) has a ratio of 1 carbon atom to 2 oxygen atoms (1:2). Carbon monoxide (CO) has a ratio of 1 carbon atom to 1 oxygen atom (1:1).

Elements: Defined by Number of Protons

• Each element has a unique name, symbol, and atomic number.
• The elements are arranged on the periodic table in order of increasing atomic number.
• Atomic number (Z): The number of protons in an atom’s nucleus, it identifies the element.

Isotopes: Varying Number of Neutrons

• Isotopes: Atoms of the same element that have the same number of protons (atomic number) but different numbers of neutrons.
• They differ in mass number (A) because they have different numbers of neutrons.
• Isotopes are chemically identical, but may be physically different.
• Mass Number (A) = Protons + Neutrons
• Isotopes are identified by their mass numbers (e.g. C-12, C-13, C-14).

Molecules

• Molecules : An electrically neutral group of two or more atoms held together by chemical bonds.
• Homonuclear molecules: consist of atoms of the same chemical element (e.g., oxygen, O2).
• Heteronuclear molecules: consist of atoms of more than one element (e.g., water, H2O).

Ions: Charged Atoms

• Ions: Formed when an atom loses or gains one or more electrons.
• Cations: formed when an atom loses one or more electrons, they are positively charged (more protons than electrons).
• Anions: formed when an atom gains one or more electrons, they are negatively charged (fewer protons than electrons).

Major Families: Halogens (Group 7A)

• Halogens: Group 7A elements, they are very reactive nonmetals.
• Always found in nature as salts.
  • Examples: Chlorine (Cl2), Bromine (Br2), Iodine (I2), Fluorine (F2)

Major Families: Noble Gases (Group 8A)

• Noble Gases: Group 8A elements, they are mostly unreactive (inert).
  • Examples: Helium (He), Neon (Ne), Argon (Ar), Krypton (Kr), Xenon (Xe)

Elements, Compounds and Mixtures

• Elements: The simplest form of matter.
• Compounds: Formed when two or more elements combine in a fixed ratio.
• Mixtures: Two or more substances physically combined but not chemically bonded.

Molecular Elements

• Molecular Elements: Elements whose particles are multi-atom molecules, having the same type of atoms (e.g., H2, O2, N2, Cl2, P4, S8, Se8).

Molecular Compounds

• Molecular Compounds: Compounds whose particles are molecules made of only nonmetals (e.g., H2O, NH3, HCl, CH4)

Ionic Compounds

• Ionic Compounds: Compounds whose particles are composed of cations (metals) and anions (nonmetals) (e.g., NaCl, AlF3, Fe2O3, Mg2S).

Formula Mass & Molar Mass

• Formula Mass (amu): The mass of an individual molecule or formula unit, expressed in atomic mass units (amu). Also known as molecular mass or molecular weight.
• Molar Mass (g/mol): The mass of one mole of a substance.