Overview
This lecture covers the fundamentals of atoms and elements, including their structure, behavior, and trends on the periodic table, which are crucial for understanding chemistry.
Elements, Symbols, and the Periodic Table
- A chemical symbol is a one or two-letter abbreviation for an element.
- Columns (groups) on the periodic table are vertical and contain elements with similar chemical properties.
- Rows (periods) are horizontal and categorize elements without necessarily similar properties.
- Major element groups: Alkali Metals (1A), Alkaline Earth Metals (2A), Transition Metals (B groups), Halogens (7A), Noble Gases (8A), Lanthanides, and Actinides.
- Most elements are metals; non-metals and metalloids also exist on the table.
Structure of the Atom
- Dalton’s Theory: All matter is made of atoms; atoms of each element are identical; compounds are combinations of elements; chemical reactions rearrange atoms.
- An atom's nucleus contains protons (positive) and neutrons (neutral); electrons (negative) are outside the nucleus.
- Proton mass: 1.67 × 10⁻²⁴ g; electron mass: 9.11 × 10⁻²⁸ g; atomic mass unit (amu) simplifies mass calculations.
Atomic Number and Mass Number
- Atomic number = number of protons in the nucleus (defines the element).
- Mass number = protons + neutrons.
- In neutral atoms, number of electrons equals number of protons.
- Example: For zinc (Zn), atomic number = 30, mass number = 68, neutrons = 68 – 30 = 38.
Isotopes and Atomic Mass
- Isotopes are atoms of the same element with different numbers of neutrons.
- Atomic mass reflects the weighted average of all isotopes based on their percent abundance.
- Calculation example: Chlorine's atomic mass is weighted using the abundances and masses of Cl-35 and Cl-37.
Electron Energy Levels and Subshells
- Electrons exist in quantized energy levels (principal quantum number n).
- Subshells: s (1 orbital), p (3), d (5), f (7), each with characteristic shapes.
- Electrons fill the lowest available energy subshells first (Aufbau principle).
- Electron configuration notation shows electron distribution in subshells (e.g., 1s² 2s² 2p¹ for boron).
Periodic Trends and Valence Electrons
- Valence electrons are outermost electrons, determined by group number (e.g., Group 1 = 1 valence electron).
- Lewis dot structures represent valence electrons as dots around the element symbol.
- Atomic size decreases left to right across a period and increases down a group.
- Ionization energy (energy to remove an electron) increases left to right, decreases down a group.
- Metallic character decreases left to right, increases down a group.
Key Terms & Definitions
- Element — Pure substance consisting of one type of atom.
- Isotope — Atoms of the same element with different neutron numbers.
- Atomic Number — Number of protons in an atom’s nucleus.
- Mass Number — Total number of protons and neutrons in the nucleus.
- Valence Electrons — Electrons in the outermost shell.
- Periodic Trend — Patterns in elemental properties across the periodic table.
- Subshell — A set of orbitals within a principal energy level (s, p, d, f).
- Electron Configuration — Distribution of electrons in atom’s energy levels and subshells.
Action Items / Next Steps
- Complete homework problems on element symbols, atomic numbers, mass numbers, isotopes, electron configurations, and periodic trends.
- Practice using the periodic table for determining element properties and electron arrangements.
- Review textbook tables and diagrams for further clarification of subshell filling and periodic trends.