Atoms, Elements, and Chemical Bonds: Foundations of General Biology

Atoms, Elements, and Matter

Definitions and Key Concepts

Understanding the basic building blocks of matter is essential in biology, as all living things are composed of atoms and molecules. Below are foundational definitions and concepts:

• Matter: Anything that takes up space and has mass.

• Atoms: The smallest unit of matter that retains the properties of an element. Example: Oxygen atom.

• Molecules: Two or more atoms held together by chemical bonds. Example: H2O (water).

• Elements: Pure substances that cannot be broken down by chemical means. Example: Carbon, Oxygen.

Compounds are substances formed when two or more elements combine in fixed ratios. Example: NaCl (table salt).

Major Elements in Living Matter

Four elements make up most of the mass of living organisms:

• Oxygen (O)

• Carbon (C)

• Hydrogen (H)

• Nitrogen (N)

These elements are essential for biological molecules and processes.

Atomic Structure

Subatomic Particles

Atoms are composed of three main subatomic particles:

• Protons: Positively charged particles found in the nucleus. Mass ≈ 1.7 × 10-24 g.

• Neutrons: Neutral particles also found in the nucleus. Mass ≈ 1.7 × 10-24 g.

• Electrons: Negatively charged particles orbiting the nucleus. Mass ≈ 1/2000 that of a proton or neutron.

The nucleus contains protons and neutrons, while electrons move in energy levels (shells) around the nucleus.

Atomic Number, Atomic Mass, and Isotopes

• Atomic Number (Z): Number of protons in the nucleus. Determines the element's identity.

• Atomic Mass: Sum of protons and neutrons in the nucleus. Expressed in Daltons (Da).

• Isotopes: Atoms of the same element with different numbers of neutrons. Example: Carbon-12 vs. Carbon-14.

Radioactive isotopes are unstable and decay, emitting radiation. They are used in medicine for imaging and treatment.

Example: Periodic Table Entries

Element	Atomic Number	Atomic Mass	Number of Protons	Number of Neutrons	Number of Electrons
Hydrogen	1	1	1	0	1
Helium	2	4	2	2	2
Nitrogen	7	14	7	7	7
Phosphorus	15	30.97	15	16	15

Additional info: Atomic mass is often not a whole number due to the presence of isotopes.

Electron Arrangement and Energy Levels

Electron Shells and Energy

Electrons occupy specific energy levels (shells) around the nucleus:

• Innermost shell: Lowest energy

• Outer shells: Higher energy

Electrons must gain or lose a specific amount of energy to move between shells.

Energy Level Diagram

• Moving toward the nucleus: energy lost

• Moving away from the nucleus: energy gained

When a plant absorbs light, electrons are excited to higher energy levels, initiating photosynthesis.

Valence Electrons and Chemical Behavior

Valence electrons are electrons in the outermost shell. They determine an atom's chemical properties and reactivity.

• Atoms are most stable when their outer shell is full (usually 8 electrons: the octet rule).

• Atoms will gain, lose, or share electrons to achieve a full outer shell.

Valence Electrons in Common Elements

Element	Valence Electrons	Electrons Needed to Fill Shell
Oxygen (O)	6	2
Carbon (C)	4	4
Hydrogen (H)	1	1
Nitrogen (N)	5	3

Chemical Bonds and Interactions

Types of Chemical Bonds

Atoms form chemical bonds to achieve stability. The main types of bonds and interactions in biological systems are:

• Covalent bonds: Atoms share electrons. Strongest type of bond. Example: H2O.

• Ionic bonds: Atoms transfer electrons, resulting in charged ions that attract each other. Example: NaCl.

• Hydrogen bonds: Weak attractions between a hydrogen atom and an electronegative atom (like oxygen or nitrogen).

• Van der Waals interactions: Weak, temporary attractions due to fluctuating electron distributions.

Order of bond strength (strongest to weakest): Covalent bonds > Ionic bonds > Hydrogen bonds > Van der Waals interactions.

Summary Table: Key Atomic Properties

Property	Definition
Atomic Number	Number of protons in the nucleus
Atomic Mass	Sum of protons and neutrons
Isotope	Atoms of the same element with different numbers of neutrons
Valence Electron	Electron in the outermost shell
Compound	Substance formed by two or more elements in fixed ratio

Key Equations

• Atomic Mass Calculation:

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

• Number of Electrons in a Neutral Atom:

$\text{Number of Electrons} = \text{Atomic Number}$

Applications and Examples

• Radioactive Isotopes in Medicine: Used for imaging (e.g., PET scans) and cancer treatment.

• Photosynthesis: Involves electron excitation in chlorophyll molecules.

Conclusion

Understanding atoms, elements, and chemical bonds is fundamental to biology. These concepts explain the structure and behavior of all matter, including living organisms, and set the stage for exploring more complex biological molecules and processes.