Atoms and the Periodic Table

Elements and Chemical Symbols

Elements are pure substances that cannot be broken down into simpler substances and are the fundamental building blocks of all matter. Each element is represented by a unique chemical symbol, typically consisting of one or two letters. The first letter is always capitalized, and the second letter, if present, is lowercase. Some symbols are derived from Latin or Greek names.

• Examples: - Sodium: Na (from Latin "natrium") - Silver: Ag (from Latin "argentum") - Gold: Au (from Latin "aurum") - Iron: Fe (from Latin "ferrum")

• Application: Elements are essential in biological functions (e.g., calcium and phosphorus in bones, iron in blood) and in industry (e.g., aluminum in cans, titanium in sports equipment).

The Periodic Table: Structure and Classification

The periodic table organizes elements by increasing atomic number into horizontal rows called periods (1–7) and vertical columns called groups (1–18). Elements in the same group have similar chemical properties due to similar valence electron configurations.

• Groups: Also called families; numbered 1–18 or 1A–8A (representative elements) and 1B–8B (transition elements).

• Periods: Horizontal rows; period number indicates the highest occupied energy level for the element's electrons.

• Special Groups:

  • Group 1 (1A): Alkali metals (e.g., Li, Na, K) – soft, highly reactive metals.

  • Group 2 (2A): Alkaline earth metals (e.g., Mg, Ca) – shiny, less reactive than alkali metals.

  • Group 17 (7A): Halogens (e.g., F, Cl, Br) – highly reactive nonmetals.

  • Group 18 (8A): Noble gases (e.g., He, Ne, Ar) – very unreactive gases.

• Metals, Nonmetals, Metalloids:

  • Metals: Left of the zigzag line; shiny, malleable, ductile, good conductors.

  • Nonmetals: Right of the zigzag line; dull, brittle, poor conductors.

  • Metalloids: Along the zigzag line; properties intermediate between metals and nonmetals (e.g., Si, As).

Subatomic Particles and Atomic Structure

Atoms are composed of three main subatomic particles:

• Protons (p or p+): Positive charge (+1), mass ≈ 1 amu, located in the nucleus.

• Neutrons (n or n0): No charge, mass ≈ 1 amu, located in the nucleus.

• Electrons (e-): Negative charge (–1), mass ≈ 0.00055 amu, located outside the nucleus in energy levels.

The nucleus is extremely small and dense, containing nearly all the atom's mass. The atom is mostly empty space, with electrons occupying the vast region around the nucleus.

Atomic Number, Mass Number, and Isotopes

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

  • In a neutral atom, number of electrons = number of protons.

  • Atomic number is the whole number above the element symbol on the periodic table.

• Mass Number (A): Total number of protons and neutrons in the nucleus.

  • Mass number is not found on the periodic table; it is specific to each isotope.

• Isotopes: Atoms of the same element (same Z) with different numbers of neutrons (different A).

  • Isotopes are written as: (e.g., for magnesium-24).

  • Atomic mass is the weighted average of all naturally occurring isotopes.

  • To find neutrons:

Electron Arrangement and Energy Levels

Electrons occupy specific energy levels (shells) around the nucleus. The arrangement of electrons determines the chemical properties of an element.

• Energy Levels:

  • First level: up to 2 electrons

  • Second level: up to 8 electrons

  • Third level: up to 8 electrons (for first 20 elements)

  • Fourth level: up to 2 electrons (for first 20 elements)

• Electron Configuration Examples (First 20 Elements):

  • Hydrogen (H): 1

  • Helium (He): 2

  • Lithium (Li): 2,1

  • Neon (Ne): 2,8

  • Sodium (Na): 2,8,1

  • Calcium (Ca): 2,8,8,2

• Valence Electrons: Electrons in the outermost energy level; determine chemical reactivity and group number for representative elements.

Periodic Trends

Periodic trends are recurring patterns in element properties across periods and groups, explained by electron arrangements and nuclear charge.

• Atomic Size (Radius):

  • Increases down a group (more energy levels).

  • Decreases across a period (left to right) due to increasing nuclear charge pulling electrons closer.

• Ionization Energy: Energy required to remove an outer electron.

  • Decreases down a group (outer electrons farther from nucleus, less attraction).

  • Increases across a period (greater nuclear charge, harder to remove electrons).

• Metallic Character: Tendency to lose electrons easily.

  • Increases down a group.

  • Decreases across a period.

• Valence Electrons and Lewis Symbols:

  • Group number (for Groups 1A–8A) = number of valence electrons.

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

Sample Table: Classification of Elements

Key Formulas and Equations

Examples and Applications

• Isotope Calculation: For (chlorine-37):

  • Protons: 17

  • Neutrons:

  • Electrons: 17 (neutral atom)

• Electron Arrangement: Sulfur (S), atomic number 16: 2,8,6

• Periodic Trend: Atomic size: K > Na > Li (increases down Group 1)

• Lewis Symbol: Oxygen (O), Group 16: six dots around O

Summary Table: Periodic Trends

Key Terms

• Atom: Smallest particle of an element retaining its properties.

• Isotope: Atoms of the same element with different numbers of neutrons.

• Ionization Energy: Energy required to remove an electron from an atom.

• Valence Electrons: Electrons in the outermost energy level.

• Lewis Symbol: Representation of valence electrons as dots around the element symbol.

• Metals/Nonmetals/Metalloids: Classification based on physical and chemical properties.

Additional info:

• Atomic mass unit (amu) is defined as one-twelfth the mass of a carbon-12 atom.

• Electromagnetic radiation (e.g., visible light, UV, X-rays) is involved in electron transitions between energy levels.

• Periodic trends are crucial for predicting element behavior in chemical reactions and bonding.