United Grade 9 Physical Science
Chapters: 4
1. Matter
Atomic structure and periodic table · Bonding basics · Chemical reactions
- Structure of the Atom – An atom has a tiny, heavy, positive nucleus made of protons and neutrons. Electrons move around it in fixed shells K, L, M, N. The number of protons (Z) tells the element; protons + neutrons give the mass number (A). Outer electrons decide valency. Isotopes share Z; isobars share A.
- Chemical Bonding: Ionic, Covalent and Metallic Bonds – Atoms join together (bond) to become more stable. Only their outer electrons take part. Most atoms are most stable with 8 outer electrons: the octet rule. There are three main ways to reach it. In an ionic bond, a metal gives electrons to a non-metal, making oppositely charged ions that attract. In a covalent bond, two non-metals share pairs of electrons to make molecules. In a metallic bond, metal atoms release outer electrons into a shared 'sea' that holds positive ions together. The difference in electronegativity (how strongly an atom pulls shared electrons) tells us which kind of bond forms. The type of bond explains melting points, whether a substance conducts electricity, and whether it dissolves in water.
- Kossel-Lewis Approach and the Ionic Bond – Atoms join so that each gets a stable outer shell of 8 electrons (an octet), like a noble gas. Kossel said atoms can give or take electrons to make ions (ionic bond). Lewis said atoms can also share pairs of electrons (covalent bond). Lewis structures show these electrons as dots and lines. Formal charge (V − L − B/2) helps pick the best Lewis structure. Ions pack into a crystal, and the energy released is linked to the lattice enthalpy.
- Chemical Reactions and Balancing Equations – In a chemical reaction atoms are not made or destroyed; they only change partners. So a chemical equation must have the same number of each kind of atom on both sides. We balance it by changing the numbers in front of formulas, never the formulas themselves.
2. Motion and forces
Speed, velocity, acceleration · Newton's laws · Momentum
- Motion: Distance, Speed, Velocity, Acceleration and Graphs – An object is in motion when its position changes with time. Distance is the full path length (a scalar); displacement is the straight gap from start to finish with a direction (a vector). Speed = distance ÷ time; velocity = displacement ÷ time. Acceleration = change in velocity ÷ time. The slope of an s–t graph gives velocity, the slope of a v–t graph gives acceleration, and the area under a v–t graph gives the distance. For uniform acceleration: v = u + at, s = ut + ½at², v² = u² + 2as.
- Force and Laws of Motion – A force is a push or a pull. Balanced forces (net force zero) do not change motion; an unbalanced force changes speed or direction. Friction opposes sliding. First law: a body keeps its state of rest or uniform motion unless an unbalanced force acts (inertia; heavier bodies have more inertia). Momentum p = mv. Second law: F = ma (rate of change of momentum), 1 N = 1 kg m/s². Third law: forces come in equal and opposite pairs acting on two different bodies. For a system with no outside force, internal forces cancel and total momentum is conserved.
3. Energy
Energy forms and conservation · Electricity and circuits
- Work, Energy and Power – Work is done when a force moves an object: W = F × s, measured in joules (J). Energy is the ability to do work. A moving body has kinetic energy ½mv²; a raised body has potential energy mgh. Energy is never made or destroyed, only changed from one form to another. Power is how fast work is done: P = W ÷ t, in watts. Simple machines like levers and pulleys let a small effort move a big load.
- Electric Current, Potential Difference and Electric Circuits – Electric current is the rate of flow of charge, I = Q/t, measured in amperes with an ammeter joined in series. Potential difference is the work done to move a unit charge between two points, V = W/Q, measured in volts with a voltmeter joined in parallel. Charge flows only in a closed circuit.
4. Waves
Waves and electromagnetic spectrum
- Sound – Sound is made by vibrating objects. It travels through a medium (air, water, solids) as a longitudinal wave: particles move back and forth, making crowded parts (compressions) and spread-out parts (rarefactions). Frequency (Hz) sets the pitch, amplitude sets the loudness, and speed v = f × λ. Sound cannot travel in vacuum. Humans hear 20 Hz to 20,000 Hz; below is infrasound, above is ultrasound. Reflected sound gives echoes, used and controlled in buildings.