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 in the same period have the same number of electron shells.

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

• Group Names:

  • Alkali Metals: Group 1A (Li, Na, K, Rb, Cs)

  • Alkaline Earth Metals: Group 2A (Be, Mg, Ca, Sr, Ba, Ra)

  • Halogens: Group 7A (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, mostly solids (except Hg).

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

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

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

Chemistry Link to Health: Elements Essential to Health

Twenty elements are essential for human health. Four elements—oxygen, carbon, hydrogen, and nitrogen—make 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 distributed in a positively charged cloud.

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

Atomic Mass Unit (amu): 1 amu = 1 Dalton (Da); used to express masses of subatomic particles.

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 + neutrons; always a whole number.

• 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: $\text{Atomic mass} = \sum (\text{isotope mass} \times \text{percent abundance})$

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

4.6 Electron Energy Levels

Electrons occupy energy levels, sublevels, and orbitals. The arrangement of electrons determines the chemical properties of an element.

• Energy Levels: Principal quantum numbers (n = 1, 2, ...); higher n means higher energy and greater distance from nucleus.

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

• Orbitals: Regions of high probability for finding electrons; s (sphere), p (dumbbell), d (complex), f (more complex).

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

Electron 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 show the arrangement of electrons in an atom. Electrons fill orbitals in order of increasing energy.

• Orbital Diagram: Boxes and arrows represent orbitals and electron spins.

• Electron Configuration Notation: Indicates placement of electrons in energy levels and sublevels (e.g., 1s2 2s2 2p2).

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

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

Example: Carbon (C): 1s2 2s2 2p2

4.8 Trends in Periodic Properties

Periodic trends arise from electron configurations and the structure of the periodic table.

• Valence Electrons: Outermost electrons; determine chemical properties.

• 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.

Lewis Symbols: Dots around element symbol represent valence electrons.

Example: Chlorine (Cl) has seven valence electrons, represented as seven dots around the symbol.

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 the foundation of chemistry. The periodic table organizes elements by atomic number and properties. Atoms consist of protons, neutrons, and electrons. Electron configurations and periodic trends explain chemical behavior.