Elements, Atoms, and the Periodic Table

Introduction

• Tooth enamel is the hardest material in the human body but can be attacked by acids.
• Fluorine in water discovered to reduce tooth decay while excess causes stains.
• Fluorapatite forms from fluorine and hydroxyapatite, making tooth enamel more resistant to decay.
• Fluoridation of water and toothpaste has significantly reduced tooth decay in children.

The Elements

Key Concepts

• Element: A substance that cannot be broken down into simpler substances.
• Chemical Symbol: A one or two-letter representation of an element.

Abundance

• **Universe: ** Hydrogen (90%), Helium (~10%), others in trace amounts.
• Earth's Crust: Oxygen (46.1%), Silicon (28.5%), with others like Aluminum, Iron, and Calcium.
• Human Body: Oxygen (61%), Carbon (23%), followed by Hydrogen, Nitrogen, Calcium.
• Discrepancy in element abundance in Earth vs. the human body.
• Phosphorus as a limiting element for life due to its scarcity and essential role.

Names and Symbols

• Derived from various languages, sometimes Latin.
• International Union of Pure and Applied Chemistry approves new element names.
• Symbols often not directly derived from the English name (e.g., Au for Gold from Latin 'Aurum').

Atomic Theory

Key Concepts

• Proposed by John Dalton in 1803.
• Atoms: Smallest part maintaining identity of an element.
• Diatomic Molecules: Some elements like H2, O2 exist naturally as two-atom molecules.

The Structure of Atoms

Subatomic Particles

• Protons (p+): Positive charge, mass = 1.673 x 10^-27 kg.
• Neutrons (n0): Neutral, similar mass to protons.
• Electrons (e-): Negative charge, much smaller mass.

The Nucleus

• Protons and neutrons concentrated in the nucleus.
• Electrons orbit nucleus in electron clouds.
• Rutherford's Model: Foundation for modern atomic theory.

Nuclei of Atoms

Atomic Number and Isotopes

• Atomic Number: Number of protons in the nucleus, defining element identity.
• Isotopes: Atoms of the same element with different numbers of neutrons.
• Mass Number: Total protons and neutrons in an atom.

Atomic Masses

Key Concepts

• Atomic Mass Unit (amu): 1/12th of the mass of a 12C atom.
• Atomic Mass: Weighted average of an element’s isotopes.

Arrangements of Electrons

Shells and Subshells

• Quantum Mechanics: Electrons exist in quantized energy levels (shells).
• Subshells: s, p, d, f, with increasing numbers of electrons.
• Electron Configuration: Describes the arrangement of electrons in an atom.

The Periodic Table

Organization

• Elements arranged by atomic number.
• Groups: Columns with similar chemical properties (e.g., Alkali metals, Halogens).
• Periods: Rows indicating levels of electron shells.
• Metals, Nonmetals, Semimetals: Classification based on physical and chemical properties.

Trends

• Atomic Radius: Decreases across a period, increases down a group.
• Reflects electron shell and subshell filling order.

Summary

• Elements are fundamental substances, each with a unique atomic number.
• Atomic theory explains matter as being composed of atoms.
• The periodic table organizes elements by properties and electron configurations.