Chapter 4: Atoms and Elements

4.1 Elements and Symbols

Elements are the fundamental substances from which all matter is constructed. They cannot be broken down into simpler substances and are represented by unique chemical symbols.

• Element: A pure substance consisting of only one type of atom.

• Chemical Symbol: One- or two-letter abbreviation for an element, always starting with a capital letter (e.g., C for carbon, Ca for calcium).

• Origin of Names: Element names may derive from planets, mythological figures, minerals, colors, geographic locations, or famous people.

• Latin Names: Some symbols are based on Latin names (e.g., Ag for silver from 'argentum', Au for gold from 'aurum').

Example: The symbol for iron is Fe (from 'ferrum'), and for iodine is I.

4.2 The Periodic Table

The periodic table organizes all known elements by increasing atomic number and groups elements with similar properties together.

• Groups: Vertical columns containing elements with similar properties (numbered 1–18 or 1A–8A for representative elements).

• Periods: Horizontal rows numbered 1–7.

• Group Names:

  • Alkali Metals: Group 1A (e.g., Li, Na, K)

  • Alkaline Earth Metals: Group 2A (e.g., Be, Mg, Ca)

  • Halogens: Group 7A (e.g., F, Cl, Br, I)

Example: Magnesium is in Group 2A, Period 3.

Metals, Nonmetals, and Metalloids

Elements are classified based on their physical and chemical properties.

• Metals: Shiny, ductile, good conductors, mostly solids (except Hg).

• Nonmetals: Dull, brittle, poor conductors, low density and melting points.

• Metalloids: Properties intermediate between metals and nonmetals; used as semiconductors.

Example: Sodium is a metal, chlorine is a nonmetal, silicon is a metalloid.

Chemistry Link to Health: Elements Essential to Health

Twenty elements are essential for human health, with oxygen, carbon, hydrogen, and nitrogen making up 96% of body mass. Macrominerals (Ca, P, K, Cl, S, Na, Mg) are vital for bone formation, heart function, muscle contraction, nerve impulses, and metabolic regulation.

4.3 The Atom

An atom is the smallest unit of an element, retaining its chemical properties. Atoms consist of subatomic particles: protons, neutrons, and electrons.

• Proton: Positive charge, located in nucleus.

• Neutron: No charge, located in nucleus.

• Electron: Negative charge, found outside nucleus.

Dalton’s Atomic Theory: Atoms are indivisible, combine to form compounds, and are rearranged in reactions but not created or destroyed.

Example: Aluminum foil contains aluminum atoms.

Atomic Models

• Thomson’s Model: "Plum-pudding" model with electrons and protons distributed in a positive cloud.

• Rutherford’s Model: Gold foil experiment showed a dense, positively charged nucleus.

Structure and Mass of the Atom

The nucleus contains protons and neutrons, accounting for most of the atom’s mass. Electrons occupy the space around the nucleus.

• Atomic Mass Unit (amu): 1 amu = 1 Dalton (Da); based on carbon-12.

• Electrons have negligible mass compared to protons and neutrons.

4.4 Atomic Number and Mass Number

Each element has a unique atomic number (number of protons). The mass number is the sum of protons and neutrons in the nucleus.

• Atomic Number: Appears above the element symbol; equals number of protons.

• Mass Number: Equals protons + neutrons; always a whole number.

• Neutral Atom: Number of protons = number of electrons.

Example: Lead (Pb) with atomic number 82 and mass number 207 has 82 protons, 82 electrons, and 125 neutrons.

4.5 Isotopes and Atomic Mass

Isotopes are atoms of the same element with different numbers of neutrons and thus different mass numbers. Atomic mass is the weighted average of all naturally occurring isotopes.

• Isotope: Same number of protons, different number of neutrons.

• Atomic Mass Calculation: Multiply percent abundance by isotope mass and sum for all isotopes.

Example: Chlorine has two isotopes; atomic mass is 35.45 amu.

Formula:

4.6 Electron Energy Levels

Electrons occupy energy levels, sublevels, and orbitals. Energy levels are quantized and described by principal quantum numbers (n).

• Electromagnetic Radiation: Energy moves as waves; wavelength determines energy.

• Atomic Spectrum: Unique lines for each element, related to electron energy changes.

• Energy Levels: Electrons absorb energy to move to higher levels; emit energy when returning to lower levels.

• Sublevels: s, p, d, f; number of sublevels equals n.

• Orbitals: s (spherical), p (dumbbell-shaped), d (complex shapes), f (even more complex).

• Pauli Exclusion Principle: Each orbital holds max two electrons with opposite spins.

Electron Capacity:

• s: 2 electrons

• p: 6 electrons

• d: 10 electrons

• f: 14 electrons

4.7 Electron Configurations

Electron configurations show the arrangement of electrons in energy levels and sublevels. Orbital diagrams use boxes and arrows to represent electron placement and spin.

• Notation: Indicates order of filling (e.g., 1s2 2s2 2p2).

• Blocks: s, p, d, f blocks correspond to periodic table sections.

• Exceptions: Some elements (e.g., Cr, Cu) have unusual electron arrangements for stability.

Example: Carbon: 1s2 2s2 2p2

4.8 Trends in Periodic Properties

Periodic trends are explained by electron configurations and atomic structure.

• Valence Electrons: Outermost electrons; group number indicates number for representative elements.

• Lewis Symbols: Dots represent valence electrons around element symbol.

• Atomic Size: Increases down a group, decreases across a period.

• Ionization Energy: Energy to remove an electron; decreases down a group, increases across a period.

• Metallic Character: Increases down a group, decreases across a period.

Summary Table:

Concept Map

A concept map visually organizes the relationships between atomic structure, periodic properties, and chemical behavior.