Study Notes: Chemical Elements, Atomic Structure, and Chemical Bonds in Biology

2.1 Matter: Elements and Compounds

Elements and Compounds in Biology

All matter is composed of chemical elements, which may exist in pure form or combine to form compounds. Understanding the distinction between elements and compounds is fundamental in biology, as living organisms are made up of a limited number of key elements.

• Element: A substance that cannot be broken down into other substances by chemical means. Each element is defined by its number of protons.

• Compound: A substance formed when two or more elements are chemically bonded in fixed ratios.

• Key Biological Elements: Four elements—carbon (C), hydrogen (H), oxygen (O), and nitrogen (N)—make up about 96% of living matter.

Example: Water (H2O) is a compound made from the elements hydrogen and oxygen.

2.2 Atomic Structure and Properties

Atoms: Structure and Terminology

Atoms are the smallest units of elements that retain their chemical properties. Their structure determines the properties of elements and their behavior in chemical reactions.

• Atomic Model: Atoms consist of a nucleus (containing protons and neutrons) surrounded by electrons.

• Key Terms:

  • Atomic Number: Number of protons in the nucleus; defines the element.

  • Mass Number: Sum of protons and neutrons in the nucleus.

• Isotopes: Atoms of the same element with different numbers of neutrons, resulting in different mass numbers.

• Radioactive Isotopes: Unstable isotopes that decay over time, emitting radiation. Used in biological applications such as medical imaging and dating fossils.

Example: Carbon-12 and Carbon-14 are isotopes of carbon; Carbon-14 is radioactive and used in radiometric dating.

Equation:

$\text{Mass Number} = \text{Number of Protons} + \text{Number of Neutrons}$

Additional info: Subatomic particles involved in chemical reactions are typically electrons, which participate in bond formation.

2.3 Chemical Bonds and Molecule Formation

Covalent and Ionic Bonds

Chemical bonds form between atoms to create molecules, which are essential for biological structure and function. The two main types of bonds are covalent and ionic.

• Covalent Bond: Atoms share pairs of electrons. Can be nonpolar (equal sharing) or polar (unequal sharing).

• Ionic Bond: Atoms transfer electrons, resulting in oppositely charged ions that attract each other.

• Hydrogen Bond: Weak attraction between a hydrogen atom (in a polar covalent bond) and another electronegative atom.

• Biological Importance: The shape of molecules, determined by the arrangement of bonds, affects their biological function (e.g., enzyme-substrate specificity).

Example: Water molecules are held together by polar covalent bonds and interact via hydrogen bonds.

2.4 Chemical Reactions and Equilibrium

Chemical Reactions and Equilibrium in Biology

Chemical reactions involve the making and breaking of chemical bonds, transforming reactants into products. In biological systems, many reactions reach a state of equilibrium.

• Chemical Reaction: Process in which substances (reactants) are converted into new substances (products).

• Chemical Equilibrium: State in which the rate of the forward reaction equals the rate of the reverse reaction; concentrations of reactants and products remain constant.

Example: The equilibrium between carbonic acid and bicarbonate in blood helps maintain pH balance.

Table: Comparison of Bond Types

Bond Type	Description	Example
Covalent	Atoms share electrons	H2O (water)
Ionic	Atoms transfer electrons, forming ions	NaCl (table salt)
Hydrogen	Weak attraction between polar molecules	Between water molecules