Form 4 Chemistry Bab 2: Matter and the Atomic Structure
2.1 Basic Concepts of Matter
Matter is something that has mass and occupies space, and can exist as solid, liquid or gas. (p.24)
Plasma is the fourth state of matter (an ionised gas) and is the state of matter most found in the universe. (p.24)
State changes: melting/freezing (solid↔liquid), boiling/condensation (liquid↔gas), sublimation/deposition (solid↔gas), caused by heat absorbed or released. (p.25)
Melting point: the constant temperature when a substance changes from solid to liquid at a specific pressure. Freezing point: the constant temperature when a substance changes from liquid to solid at a specific pressure. (p.26)
Matter is classified as elements (atoms or molecules) or compounds (molecules or ions). (p.26)
During heating/cooling curves, temperature stays constant during melting/freezing because heat energy is used to overcome (or is released from) the attraction force between particles, not to change kinetic energy. (p.28)
2.2 The Development of the Atomic Model
Subatomic particles: electron (charge −1, relative mass 1/1840), proton (charge +1, relative mass 1), neutron (charge 0, relative mass 1). (p.29)
Dalton’s model: atoms are the smallest indivisible spherical particles. Thomson’s model: atom is a positively-charged sphere with embedded electrons. Rutherford’s model: atom has a small positively-charged nucleus (protons) with electrons moving outside it. (p.30)
Chadwick’s model: discovered neutral particles (neutrons) in the nucleus. Bohr’s model: electrons move in shells around the nucleus. (p.31)
2.3 Atomic Structure
Proton number = number of protons in the nucleus of an atom. Nucleon number = number of protons + number of neutrons. (p.32)
An atom is neutral when the number of electrons equals the number of protons. (p.33)
Standard representation of an atom: AZX, where A = nucleon number, Z = proton number, X = symbol of element. (p.34)
Electrons fill the shell closest to the nucleus first; the outermost shell filled with electrons is the valence shell, and its electrons are valence electrons. Elements with the same number of valence electrons have similar chemical properties. (p.35)
2.4 Isotopes and Its Uses
Isotopes are atoms of the same element with the same number of protons but a different number of neutrons. (p.37)
Relative atomic mass = Σ(% isotope × mass of isotope) / 100, based on natural abundance. (p.37)
Uses of isotopes: cobalt-60 and iodine-131 in medicine (radiotherapy, thyroid treatment); phosphorus-32 in agriculture (plant metabolism); uranium-235 in nuclear power generation; carbon-14 in archaeology (dating artifacts); sodium-24 in engineering (detecting pipe leaks). (p.38)