Overview
This lecture covers the structure of the modern periodic table, periodicity, trends in Groups 2 and 7, and how to deduce element information from electronic configurations.
The Periodic Table Structure
- The modern periodic table arranges elements by increasing atomic number.
- Vertical columns are called groups; elements in the same group have similar chemical properties.
- Horizontal rows are called periods; atomic number increases from left to right across a period.
- Early classification attempts included Dobereiner's triads and Mendeleev's table, who left gaps for undiscovered elements and predicted their properties.
Group 2: Alkaline Earth Metals
- Group 2 elements have two electrons in their outermost shell (e.g., beryllium, magnesium, calcium).
- Atomic radius increases down the group as outer electrons are farther from the nucleus.
- Ionization energy decreases down the group; outer electrons are lost more readily, so reactivity increases.
- Reactions with water: magnesium reacts slowly with cold water, vigorously with steam; calcium reacts vigorously with cold water.
- Reactions with oxygen: elements form oxide layers, requiring scraping before burning.
- Reactions with hydrochloric acid: form metal chlorides and hydrogen gas; reactions become more rapid down the group.
Group 7: Halogens
- Group 7 elements (fluorine, chlorine, bromine, iodine) have seven electrons in their outermost shell.
- Physical state shifts from gas (fluorine/chlorine) to liquid (bromine) to solid (iodine) down the group.
- Exist as diatomic molecules (e.g., Cl₂, Br₂, I₂).
- Weak van der Waals forces between molecules increase down the group, raising melting and boiling points.
- Act as oxidizing agents; oxidizing strength decreases down the group.
- Higher group members can displace lower ones from solutions (e.g., Cl₂ displaces Br₂ from NaBr).
Periodic Trends and Electronic Configuration
- Period 3 includes sodium, magnesium, aluminum, silicon, phosphorus, sulfur, chlorine, and argon.
- Metallic character decreases and non-metallic character increases across a period.
- Group number is determined by the number of electrons in the outer shell; period number is by the number of electron shells.
- Atomic number can be determined by summing electrons in the configuration (e.g., 2+8+1=11 for sodium).
- Group 8 (noble gases) have full outer shells and are very stable.
Key Terms & Definitions
- Group — Vertical column in the periodic table with elements of similar properties.
- Period — Horizontal row where atomic number increases from left to right.
- Atomic number — Number of protons in an atom, determines element identity.
- Ionization — Process of losing electrons from the outer shell.
- Diatomic molecule — Molecule made of two atoms of the same element (e.g., Cl₂).
- van der Waals forces — Weak intermolecular forces between molecules.
- Oxidizing agent — Substance that gains electrons and is reduced in a chemical reaction.
- Noble gases — Elements in Group 8 with full outer electron shells, very stable.
Action Items / Next Steps
- Memorize the first 20 elements of the periodic table, organized by group.
- Practice writing chemical equations for Group 2 and Group 7 reactions.
- Review how to determine group, period, and atomic number from electronic configurations.