Atoms and the Periodic Table: Foundations for Anatomy & Physiology

Atomic Theory and the Structure of Atoms

Introduction to Atomic Theory

Atomic theory is a fundamental concept in chemistry and biology, describing the nature and behavior of matter at its most basic level. Understanding atoms is essential for grasping molecular interactions in anatomy and physiology.

• Atom: The smallest particle of an element that retains its chemical properties. The term comes from the Greek atomos, meaning "indivisible."

• Atomic Theory: Chemistry is founded on four main assumptions:

  1. All matter is composed of atoms.

  2. Atoms of a given element differ from those of other elements.

  3. Chemical compounds consist of atoms combined in specific ratios; only whole atoms can combine.

  4. Chemical reactions change only the way atoms are combined in compounds.

Subatomic Particles

Atoms are made up of three types of subatomic particles:

• Protons: Positively charged particles found in the nucleus.

• Neutrons: Electrically neutral particles with a mass similar to protons, also located in the nucleus.

• Electrons: Negatively charged particles with a much smaller mass (about 1/1,836 that of a proton), found in orbitals around the nucleus.

Atomic Structure

• The nucleus is the dense core of the atom, containing protons and neutrons.

• Electrons move rapidly in a large volume of space surrounding the nucleus.

• Opposite charges attract (electrons are held near the nucleus), while like charges repel.

Elements and Atomic Number

Defining Elements

An element is defined by the number of protons in its atoms, known as the atomic number (Z).

• All atoms of a particular element have the same number of protons.

• Mass number (A): The sum of protons and neutrons in an atom.

• In a neutral atom, the number of electrons equals the number of protons.

Example Calculation

• Phosphorus (Z = 15, A = 31):

  • Protons: 15

  • Electrons: 15

  • Neutrons: $31 - 15 = 16$

• Nickel (Z = 28, A = 60):

  • Protons: 28

  • Electrons: 28

  • Neutrons: $60 - 28 = 32$

Isotopes and Atomic Weight

Isotopes

Isotopes are atoms of the same element (same atomic number) but with different mass numbers due to varying numbers of neutrons.

• Example: Hydrogen has three isotopes:

  • Protium: 1 proton, 0 neutrons (A = 1)

  • Deuterium: 1 proton, 1 neutron (A = 2)

  • Tritium: 1 proton, 2 neutrons (A = 3)

• Isotope notation: AZSymbol (e.g., 3H for tritium)

Atomic Weight

The atomic weight of an element is the weighted average mass of its naturally occurring isotopes.

• Formula: $\text{Atomic weight} = \sum [(\text{isotopic abundance}) \times (\text{isotopic mass})]$

• Example (Gallium):

  • Ga-69: 60.4% at 68.9257 amu

  • Ga-71: 39.6% at 70.9248 amu

  • Calculation: $\text{Atomic weight} = (0.604 \times 68.9257) + (0.396 \times 70.9248) = 69.7$ amu

The Periodic Table

Organization and Classification

The periodic table arranges elements by increasing atomic number and groups elements with similar properties together.

• Metals: Malleable, lustrous, good conductors; found on the left side.

• Nonmetals: Poor conductors; found on the upper-right side.

• Metalloids: Intermediate properties; located in a zigzag band between metals and nonmetals.

Chemical Groups

• Main group elements

• Transition metals

• Inner transition metals

Periodic Trends

• Periodicity: Repeating patterns in properties such as atomic radius, ionization energy, and reactivity.

Characteristics of Selected Groups

• Group 1A (Alkali metals): Highly reactive, soft metals, react with water, never found pure in nature.

• Group 2A (Alkaline earth metals): Less reactive than alkali metals, lustrous, silvery, not found pure in nature.

• Group 7A (Halogens): Colorful, corrosive nonmetals, found in compounds, name means "salt-former."

• Group 8A (Noble gases): Colorless, inert gases, very low chemical reactivity.

Electronic Structure of Atoms

Quantum Mechanical Model

The arrangement of electrons determines the chemical properties of elements. The quantum mechanical model describes electrons as occupying quantized energy levels.

• Shells: Main energy levels (numbered 1, 2, 3, 4...)

• Subshells: Types s, p, d, f within each shell, increasing in energy.

• Orbitals: Regions of space where electrons are likely found; each orbital holds two electrons with opposite spins.

Shell Capacities

Shell Number	Electron Capacity
1	2
2	8
3	18
4	32

Subshell and Orbital Details

Subshell	Number of Orbitals	Electron Capacity
s	1	2
p	3	6
d	5	10
f	7	14

Electron Configurations

Rules for Electron Arrangement

• Electrons occupy the lowest energy orbitals available.

• Each orbital holds two electrons with opposite spins.

• Orbitals of equal energy are half-filled before any is fully filled.

Notation

• Electron configuration is written as a sequence of subshells with superscripts indicating the number of electrons (e.g., 1s2 2s2 2p6).

• Shorthand notation uses noble gas symbols to represent filled inner shells (e.g., [Ne] 3s1).

Examples

• Magnesium (Z = 12): 1s2 2s2 2p6 3s2 or [Ne] 3s2

• Phosphorus (Z = 15): 1s2 2s2 2p6 3s2 3p3 or [Ne] 3s2 3p3

Electron Configurations and the Periodic Table

Periodic Table Blocks

The periodic table is divided into blocks (s, p, d, f) based on the subshell being filled last. Elements in the same group have similar valence shell configurations.

• Valence shell: The outermost electron shell.

• Valence electrons: Electrons in the valence shell, important for chemical bonding.

General Valence-Shell Configuration

• Group 6A: ns2 np4

• Tin (Sn, Z = 50):

  • Shells: 2, 8, 18, 18, 4 electrons

  • Valence electrons: 4 (in 5s and 5p subshells)

  • Valence-shell configuration: 5s2 5p2

Electron-Dot (Lewis) Symbols

Lewis Dot Symbols

Electron-dot symbols represent the number of valence electrons as dots around the atomic symbol.

• First four dots are placed singly around the symbol; remaining dots are paired.

• Example: Group 5A element X has five valence electrons, shown as five dots around the symbol.

Summary Table: Subatomic Particles

Particle	Symbol	Mass (amu)	Charge
Proton	p	1	+1
Neutron	n	1	0
Electron	e	~0.0005	-1

Additional info: These chemistry concepts are foundational for understanding molecular and cellular processes in anatomy and physiology, such as ion transport, chemical signaling, and metabolic reactions.