Overview
This lecture covers the structure of atoms, the periodic table, chemical and physical changes, separation techniques, and how elements form ions.
Atoms, Elements, and Compounds
- Substances are made of atoms, represented on the periodic table by symbols.
- Elements are different types of atoms.
- A compound is two or more different atoms chemically bonded, e.g., water (H₂O).
- The chemical formula shows the ratio of atoms; a missing number implies "1".
- Atoms change bonds via chemical reactions but are not created or destroyed.
Chemical Equations and Balancing
- Chemical reactions are shown with word or symbol equations.
- The number of atoms for each element must be equal on both sides (conservation of mass).
- Balance equations by adjusting coefficients, not subscripts.
- Start with atoms only in compounds, finish with elements like O₂.
Mixtures and Separation Techniques
- A mixture is a combination of substances not chemically bonded, e.g., air.
- Filtration separates insoluble solids from liquids.
- Crystallization leaves a solid solute after evaporating the solvent.
- Distillation heats a solution and condenses the vapor; fractional distillation separates liquids by boiling point.
- Physical changes do not create new substances.
States of Matter and Physical Changes
- Solid, liquid, and gas are main states; particles behave differently in each.
- Melting or evaporating requires energy to overcome attraction between particles but doesn’t break chemical bonds.
- Changing state is a physical, not chemical, change.
- State symbols: (s) = solid, (l) = liquid, (g) = gas, (aq) = aqueous/dissolved.
Atomic Structure and History
- JJ Thomson: atoms contain positive and negative charges (plum pudding model).
- Rutherford: nucleus is a tiny, dense positive core (most mass), electrons orbit outside.
- Bohr: electrons in shells/orbitals.
- Chadwick: nucleus contains neutrons (neutral).
Protons, Neutrons, Electrons, and Isotopes
- Protons (+1 charge), electrons (-1), neutrons (0).
- Protons and neutrons have relative mass of 1; electrons almost zero.
- Atomic number = number of protons; mass number = protons + neutrons.
- Isotopes: atoms of same element with different neutrons.
- Relative atomic mass is a weighted average of isotopes.
The Periodic Table Structure
- Early tables arranged elements by atomic weight; modern table by atomic number.
- Mendeleev organized elements by properties and predicted missing elements.
- Electron shells fill 2, 8, 8, 2 (up to calcium, atomic number 20).
Metals, Non-metals, Groups, and Reactivity
- Metals (left of staircase) donate electrons; non-metals accept electrons.
- Group number = electrons in outer shell (except transition metals).
- Group 1: alkali metals, lose one electron, more reactive down the group.
- Group 7: halogens, gain one electron, less reactive down the group, boiling point increases.
- Group 0: noble gases, very unreactive, full outer shell.
Ions and Charges
- Atoms become ions by gaining/losing electrons.
- Metals form positive ions, non-metals form negative ions.
- Group 1 ions: +, Group 2: 2+, Group 7: -, Group 6: 2-.
- Transition metals can have multiple charges (e.g., Fe²⁺, Fe³⁺).
- Transition metals are harder, less reactive, and form colored compounds.
Key Terms & Definitions
- Element — substance made of one type of atom.
- Compound — substance with two/more different atoms chemically bonded.
- Mixture — combination of substances not chemically bonded.
- Isotope — atoms of the same element with different numbers of neutrons.
- Ion — atom that gained or lost electrons.
- Relative Atomic Mass — average mass of isotopes, weighted by abundance.
- Electron Shell — energy level where electrons orbit the nucleus.
Action Items / Next Steps
- Practice balancing chemical equations.
- Memorize state symbols and common group charges.
- Review periodic table layout and group properties.