Atoms and Elements: Structured Study Notes for GOB Chemistry

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').

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; elements in the same group share similar chemical properties.

• Periods: Horizontal rows; elements are arranged by increasing atomic number.

• Group Numbering: Two systems: 1A–8A (representative elements), 3B–12B (transition elements), or 1–18 (modern system).

• 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 (Mg) 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 of heat and electricity; mostly solids (except mercury).

• Nonmetals: Dull, brittle, poor conductors, often good insulators; low densities and melting points.

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

Example: Silicon (Si) is a metalloid, sodium (Na) is a metal, and chlorine (Cl) is a nonmetal.

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 cellular metabolism.

4.3 The Atom

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

• Proton: Positive charge, located in the nucleus.

• Neutron: No charge, located in the nucleus.

• Electron: Negative charge, found outside the nucleus.

Dalton’s Atomic Theory: Atoms are indivisible, combine to form compounds, and are rearranged in chemical reactions.

Historical Models:

• Thomson’s Plum-Pudding Model: Electrons and protons scattered in a positively charged cloud.

• Rutherford’s Gold Foil Experiment: Discovery of the dense, positively charged nucleus.

Structure and Mass of the Atom

The nucleus contains protons and neutrons and accounts for most of the atom’s mass. Electrons occupy the space around the nucleus. Atomic mass is measured in atomic mass units (amu), where 1 amu is defined as 1/12 the mass of a carbon-12 atom.

4.4 Atomic Number and Mass Number

Each element is defined by its atomic number (number of protons). The mass number is the sum of protons and neutrons in the nucleus.

• Atomic Number (Z): Number of protons; unique for each element.

• Mass Number (A): Number of protons plus neutrons.

• Neutral Atom: Number of protons equals 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 each isotope’s mass by its percent abundance, then sum.

Example: Gallium atomic mass = (0.6010 × 68.926) + (0.3990 × 70.925).

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 travels as waves; wavelength determines energy.

• Atomic Spectrum: Unique set of lines for each element, related to electron transitions.

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

• Sublevels: Each energy level contains sublevels (s, p, d, f).

• Orbitals: Regions of high probability for finding electrons; s orbitals are spherical, p orbitals are dumbbell-shaped.

Orbital Capacity:

• s: 1 orbital, 2 electrons

• p: 3 orbitals, 6 electrons

• d: 5 orbitals, 10 electrons

• f: 7 orbitals, 14 electrons

4.7 Electron Configurations

Electron configurations describe the arrangement of electrons in an atom. Electrons fill orbitals in order of increasing energy, following the Pauli exclusion principle and Hund’s rule.

• Orbital Diagram: Boxes and arrows show electron arrangement and spin.

• Electron Configuration Notation: Indicates filled sublevels (e.g., 1s2 2s2 2p2 for carbon).

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

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

Example: Selenium (Se): [Ar] 4s2 3d10 4p4

4.8 Trends in Periodic Properties

Periodic trends arise from electron configurations and nuclear charge. Key properties include atomic size, ionization energy, and metallic character.

• Valence Electrons: Outermost electrons; determine chemical properties.

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

• 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: Tendency to lose electrons; increases down a group, decreases across a period.

Example: Carbon is larger than nitrogen and fluorine; fluorine has the highest ionization energy.

Summary Table: Periodic Trends

Property	Down a Group	Across a Period
Atomic Size	Increases	Decreases
Ionization Energy	Decreases	Increases
Metallic Character	Increases	Decreases

Concept Map

Atoms and elements are foundational to understanding chemical reactions, periodic properties, and the structure of matter. Mastery of atomic structure, electron configuration, and periodic trends is essential for further study in general, organic, and biological chemistry.