Atomic Theories, Periodic Trends and Matter

• These notes cover atomic theories, periodic trends, phases of matter, and properties of matter

Atomic Theories

• Democritus (400 BC) proposed that matter consists of indivisible particles called atomos
• John Dalton (1803) stated in his atomic theory that atoms are solid, indivisible spheres that combine in fixed ratios to form compounds
• J.J. Thomson (1897) discovered electrons using the cathode ray tube experiment and proposed the Plum Pudding Model, which consists of negative electrons in a positive sphere
• Ernest Rutherford (1911) conducted the gold foil experiment and discovered the nucleus containing positive protons, with electrons moving around it
• Niels Bohr (1913) proposed that electrons orbit the nucleus in specific energy levels, known as the Planetary Model
• Erwin Schrödinger, along with the Quantum Model (1926), suggested that electrons are found in electron clouds (probability regions) around the nucleus, not in fixed orbits

Periodic Trends

• Groups (columns) in the periodic table contain elements with similar properties
• Periods (rows) in the periodic table contain elements with increasing atomic number and energy levels
• Atomic radius is the size of an atom
• Atomic radius increases down a group because there are more energy levels
• Atomic radius decreases across a period because more protons pull electrons closer
• Ionization energy is the energy needed to remove an electron
• Ionization energy decreases down a group, making it easier to remove electrons
• Ionization energy increases across a period, making it harder to remove electrons
• Electronegativity is the ability to attract electrons
• Electronegativity decreases down a group
• Electronegativity increases across a period
• Reactivity is the tendency to react
• Metals become more reactive down a group
• Nonmetals become less reactive down a group

Phases of Matter

• The four states of matter are solid, liquid, gas, and plasma
• Solids have a definite shape and volume with tightly packed particles
• Liquids have a definite volume but take the shape of their container, with particles close but able to move past each other
• Gases have no definite shape or volume, and their particles move freely
• Plasma is an ionized gas with electrons removed, found in stars and lightning
• Melting is the phase change from solid to liquid
• Freezing is the phase change from liquid to solid
• Evaporation is the phase change from liquid to gas
• Condensation is the phase change from gas to liquid
• Sublimation is the phase change from solid to gas, like dry ice
• Deposition is the phase change from gas to solid, like frost formation

Properties of Matter

• Physical properties can be observed without changing the substance
• Mass measures the amount of matter in an object
• Volume is the space occupied by matter
• Density is mass per unit volume, calculated as Density = Mass ÷ Volume
• Solubility is the ability to dissolve in a substance
• Conductivity is the ability to transfer heat or electricity
• Malleability is the ability to be shaped or hammered into thin sheets
• Ductility is the ability to be stretched into a wire
• Chemical properties are observed when a substance undergoes a chemical change
• Flammability is the ability to burn
• Reactivity is the ability to react with other substances such as acids or oxygen
• Corrosion is the gradual destruction of materials, like the rusting of iron