Chemical Substances: Elements, Compounds, and Their Properties

Fundamental Chemical Substances: Elements and Compounds

Defining Simple Substances: Elements

Simple substances, or elements, are the basic building blocks of matter. They combine in fixed proportions by mass to form compounds.

Classification of Elements

• Metallic Elements

  Located on the left side of the periodic table. All are solids at room temperature, except mercury (Hg), which is a liquid with a melting point of -39 °C.

• Nonmetals

  Found on the right side of the periodic table. Examples include carbon (C), nitrogen (N), phosphorus (P), oxygen (O), sulfur (S), selenium (Se), and the halogens: fluorine (F), chlorine (Cl), bromine (Br), and iodine (I). They exist in various physical states; for instance, sulfur is solid, chlorine is gaseous, and bromine is liquid. Their melting and boiling points are generally lower than those of metals.

• Metalloids (Semimetals)

  These elements are few and are situated between metals and nonmetals on the periodic table, exhibiting intermediate properties. Examples include boron (B), silicon (Si), germanium (Ge), arsenic (As), antimony (Sb), tellurium (Te), and astatine (At). They possess properties that are intermediate between those of metals and nonmetals.

Understanding Composite Substances: Compounds

Composite substances, or compounds, are formed by two or more elements chemically united in fixed proportions by mass. Compounds are abundant in nature and can also be synthesized in the laboratory.

Decomposition of Composite Substances

• Thermal Decomposition

  This process involves applying heat to a pure substance. An example is the decomposition of mercury(II) oxide (HgO), which, when heated, releases oxygen gas, leaving a residue of mercury metal at the bottom of the container.

• Electrical Decomposition (Electrolysis)

  This involves passing an electric current through a pure substance to decompose it. A classic example is the electrolysis of water, which liberates two gases: oxygen, a combustible gas that intensifies a match flame and makes it burn very blue, and hydrogen, a non-combustible gas that extinguishes a match flame.

Key Inorganic Compound Types

• Oxides

  Formed by the combination of oxygen with a metal or a nonmetal. Example: mercury(II) oxide (HgO).

• Acids

  Typically formed from the union of a nonmetal oxide with water, or a nonmetal with hydrogen. Example: hydrochloric acid (HCl).

• Bases (Hydroxides)

  Formed from the union of a metal with water. Example: sodium hydroxide (NaOH).

• Salts

  Result from the union of an acid with a base, a metal with an acid, or a metal with a nonmetal (excluding oxygen). Examples: sodium carbonate (Na₂CO₃), sodium chloride (NaCl).