Chapter 2: The Chemical Basis of Life – Study Notes

Nature’s Chemical Language

Pheromones and Chemical Communication

Chemicals play a vital role in the lives of organisms, not only in their structure but also in communication and defense. For example, the rattlebox moth produces chemicals important for mating and defense. These chemicals, called pheromones, are released outside the body and serve as stimuli to other individuals of the same species, triggering varied behavioral responses.

• Pheromones: Chemical signals used for communication among members of the same species.

• Example: Rattlebox moths use pheromones during mating.

• Additional info: Chemical signaling is also crucial in other species for territory marking, alarm signaling, and social organization.

Matter and Elements

Basic Definitions and Importance

All living organisms are composed of matter, which is anything that occupies space and has mass. Matter is made up of chemical elements, substances that cannot be broken down into other substances by ordinary chemical means.

• Matter: Anything that occupies space and has mass.

• Element: A pure substance consisting of only one type of atom; cannot be broken down by chemical means.

• There are 92 naturally occurring elements; about 25 are essential to life.

Major Elements in the Human Body

Four elements—carbon, hydrogen, oxygen, and nitrogen—make up about 96% of the human body.

Symbol	Element	Percentage of Human Body Weight
O	Oxygen	65.0
C	Carbon	18.5
H	Hydrogen	9.5
N	Nitrogen	3.3
Ca	Calcium	1.5
P	Phosphorus	1.0
K	Potassium	0.4
S	Sulfur	0.3
Na	Sodium	0.2
Cl	Chlorine	0.2
Mg	Magnesium	0.1

• Other elements, called trace elements, are essential but present in minute amounts (less than 0.01%).

Trace Elements and Health

Trace elements are often added to food and water to prevent deficiencies that can cause health problems.

• Iodine deficiency: Causes goiter (enlarged thyroid gland).

• Iron deficiency: Causes anemia.

• Fluoride: Prevents tooth decay.

Element, Molecule, and Compound

Definitions and Examples

• Element: All atoms in an element are similar.

• Molecule: Formed when two or more atoms combine. Atoms may be of the same or different elements. Example: $O_2$ (oxygen molecule), $H_2O$ (water molecule)

• Compound: Formed when two or more atoms of different elements combine. Example: $H_2O$ (water)

Elements Can Combine to Form Compounds

Properties and Examples

• Compounds contain two or more elements in a fixed ratio.

• Unique properties of compounds arise from different combinations of atoms.

• Example: Table salt ($NaCl$) is formed from sodium and chlorine in equal amounts.

• Most elements do not exist in pure form in nature.

Structure of an Atom

Subatomic Particles

An atom is the smallest particle of matter that retains the properties of an element. Atoms consist of three types of subatomic particles:

• Protons: Positively charged, located in the nucleus.

• Neutrons: No charge, located in the nucleus.

• Electrons: Negatively charged, move rapidly around the nucleus in an electron cloud.

Atomic Number

Definition and Importance

• Atomic number: The number of protons in an atom; unique for each element.

• In a neutral atom: $\text{Atomic number} = \text{Number of protons} = \text{Number of electrons}$

Atomic Weight / Atomic Mass / Mass Number

Calculation and Significance

• Atomic mass: Number of protons plus number of neutrons. $\text{Atomic mass} = \text{Number of protons} + \text{Number of neutrons}$

• Electrons have negligible mass (1 electron ≈ 1/2000 the mass of a proton).

Isotopes

Definition and Examples

• All atoms of an element have the same atomic number but may have different atomic masses due to varying numbers of neutrons. These are called isotopes.

	Carbon-12	Carbon-13	Carbon-14
Protons	6	6	6
Neutrons	6	7	8
Electrons	6	6	6

Radioactive Isotopes

Stability and Applications

• Stable isotopes have nuclei that remain intact permanently.

• Unstable (radioactive) isotopes decay spontaneously, emitting particles and energy.

• Radioactive isotopes can be both beneficial and harmful.

Research Uses of Isotopes

Scientific Applications

• Isotopes of the same element behave identically in chemical reactions.

• Radioactive isotopes are used as tracers to study the fate of elements and molecules in living systems.

• Example: Botanists use $^{14}C$-labeled $CO_2$ to trace photosynthesis in plants.

Medical Diagnosis and Treatment

Diagnostic and Therapeutic Uses

• Radioactive elements are used in diagnostic procedures; their decay is monitored by instruments to diagnose diseases such as cancer and kidney failure.

• Radiation therapy is used to treat cancer by killing cancer cells.

• Radioactive tracers are used in combination with imaging instruments for diagnosis.

Dangers of Radioactive Isotopes

Health Risks

• Long-term or high-energy radiation exposure can cause diseases such as cancer.

• Example: Radon contamination in homes can cause lung cancer; nuclear accidents (e.g., Chernobyl) have led to increased cancer rates and fatalities.

Additional info:

• The Periodic Table of Elements organizes all known elements by atomic number and properties, providing a framework for understanding chemical behavior.