Biology Chapter: Elements and Compounds

Common Elements in Living Organisms

• Four most common elements in living organisms are carbon (C), hydrogen (H), oxygen (O), and nitrogen (N).

Trace Elements

• Zinc (Zn) is a trace element in the human body.

Matter and Elements

• All living things are composed of matter.
• All matter has mass and is composed of elements.
• Not all matter exists as compounds.

Compounds

• Compounds are formed when two or more different elements combine in a fixed ratio.
• Sodium chloride (table salt) is an example of a compound.

Atomic Structure

• The nucleus of an atom contains protons and neutrons.
• Electrons orbit the nucleus.

Atomic Mass and Number

• Atomic mass is the sum of protons and neutrons in an atom.
• Mass number is the same as the atomic mass of an atom.

Chemical Bonds

• A covalent bond forms when two atoms share electrons.
• An ionic bond forms when one atom donates an electron to another atom.

Formation of Table Salt

• Table salt (NaCl) forms when sodium donates its outer electron to chlorine.

Water Molecules

• Water molecules are held together by polar covalent bonds.

Weak Bonds

• Hydrogen bonds are weak bonds that hold molecules together.

Chemical Reactions

• In a chemical reaction, reactants form products.
• The equation 2H₂ + O₂ → 2H₂O shows reactants (H₂ and O₂) forming products (H₂O).

Water Properties

• Cohesion is the tendency of water molecules to stick together.

pH Scale

• A pH of 7 is neutral.
• A pH of 3 is strongly acidic.

This quiz covers fundamental concepts about the common elements in living organisms, trace elements, and the basic structures of matter. It explores the formation of compounds, atomic structure, and chemical bonds. Test your understanding of these essential topics in biology.