Difficulty: Introductory | Prerequisites: Measurement and unit conversion basics
This section covers how matter is organised (elements, compounds, mixtures), how it behaves in different physical states (solid, liquid, gas), and what atoms are made of (protons, neutrons, electrons, isotopes). It also introduces the periodic table as the map that ties all of this together.
Element
A pure substance composed of only one type of atom, represented by a one- or two-letter symbol (e.g. H, O, Fe).
In simple terms, this means: elements are the simplest building blocks. You cannot break gold into anything simpler by chemical means.
Compound
A pure substance made of two or more elements chemically bonded in a fixed ratio (e.g. H2O, CO2).
Think of it as a recipe with exact proportions: water is always two hydrogens to one oxygen, never three to one.
Homogeneous mixture
A mixture with uniform composition throughout. Also called a solution.
Think of it as: if you take a sample from anywhere in the mixture, it looks and measures the same. Saltwater is the classic example.
Heterogeneous mixture
A mixture with non-uniform composition. You can see or distinguish the different components.
In simple terms, this means: a salad, trail mix, or oil and vinegar dressing before shaking.
Atom
The smallest unit of an element that retains that element's chemical properties.
Molecule
A group of two or more atoms bonded together. Can be the same element (O2) or different elements (H2O).
Proton
A positively charged subatomic particle found in the nucleus. The number of protons defines the element (atomic number).
Neutron
An uncharged subatomic particle found in the nucleus. Neutrons contribute to mass but not charge.
Electron
A negatively charged subatomic particle that orbits the nucleus in electron clouds. Electrons drive chemical bonding.
Isotope
Atoms of the same element with different numbers of neutrons, giving them different mass numbers.
Think of it as: same element, different weight. Carbon-12 and Carbon-14 are both carbon, but Carbon-14 has two extra neutrons.
Atomic number (Z)
The number of protons in the nucleus. This defines the element.
Mass number (A)
The total number of protons plus neutrons in the nucleus.
Physical property
A characteristic that can be observed or measured without changing the substance's chemical identity. Examples: colour, density, melting point, boiling point.
Physical change
A change in state or appearance that does not alter the substance's chemical composition. Melting ice is a physical change; the water molecules remain H2O.
Extensive property
A property that depends on the amount of substance present (e.g. mass, volume).
Intensive property
A property that does not depend on the amount of substance (e.g. density, temperature, boiling point).
All matter falls into one of two categories: pure substances or mixtures.
Elements: contain only one type of atom. There are about 118 known elements, each represented by a symbol (H for hydrogen, Fe for iron, Au for gold). Know the first 20 by heart.
Compounds: two or more elements chemically bonded in a fixed ratio. H2O is always two hydrogens and one oxygen. You cannot separate a compound into its elements by physical means; it requires a chemical reaction.
Homogeneous (solutions): uniform composition. Saltwater looks and tests the same at every point. Air is a homogeneous mixture of nitrogen, oxygen, and trace gases.
Heterogeneous: non-uniform composition. The components are visibly distinguishable or can be separated by simple physical methods. Examples: granite, oil and water, cereal in milk.
Atoms are the fundamental units. When atoms bond, they form molecules. A molecule can be made of atoms of the same element (O2, diatomic oxygen) or different elements (CO2, carbon dioxide).
Element symbols are the shorthand: one capital letter, sometimes followed by a lowercase letter. Na is sodium, not nitrogen and arsenic.
Solid (s): fixed shape and fixed volume. Particles are tightly packed in an ordered arrangement and vibrate in place.
Liquid (l): fixed volume but takes the shape of its container. Particles are close together but can slide past one another.
Gas (g): no fixed shape or volume. Particles are far apart and move freely, filling whatever container they occupy.
Aqueous (aq): a substance dissolved in water. This is a label for phase in chemical equations, not a distinct state of matter in the same sense as solid, liquid, or gas.
Physical properties can be observed or measured without altering the substance's identity: colour, melting point, boiling point, density, solubility.
Melting, boiling, dissolving, and freezing are physical changes. The substance changes state but its chemical composition stays the same. Ice, liquid water, and steam are all H2O.
Extensive: depend on how much substance you have. Mass and volume are extensive. Double the sample, double the mass.
Intensive: independent of amount. Density and temperature are intensive. A cup of boiling water and a pot of boiling water are both at 100 degrees C.
The atom has a dense nucleus containing protons and neutrons, with electrons orbiting in electron clouds (also called orbitals at higher levels).
Protons: positive charge (+1), located in the nucleus. The number of protons is the atomic number (Z) and defines the element.
Neutrons: no charge, located in the nucleus. They add mass but do not affect the element's identity.
Electrons: negative charge (-1), orbit the nucleus. In a neutral atom, the number of electrons equals the number of protons.
Number of protons = atomic number (Z)
Number of neutrons = mass number (A) - atomic number (Z)
Number of electrons in a neutral atom = number of protons
Isotopes are atoms of the same element (same number of protons) with different numbers of neutrons. They have different mass numbers but identical chemical behaviour.
Isotopic notation:
Hyphen notation: Carbon-14 (element name, then mass number)
Nuclear notation: the mass number is written as a superscript and the atomic number as a subscript to the left of the element symbol
Example: Carbon-14 has 6 protons (it is carbon, Z = 6) and 14 - 6 = 8 neutrons.
The periodic table arranges elements by increasing atomic number. Elements in the same column (group) share similar chemical properties.
Group 1, Alkali metals: highly reactive metals (Li, Na, K, etc.). They lose one electron easily to form +1 ions.
Group 2, Alkaline earth metals: reactive metals (Be, Mg, Ca, etc.). They form +2 ions.
Group 17, Halogens: reactive nonmetals (F, Cl, Br, I, etc.). They gain one electron to form -1 ions.
Group 18, Noble gases: virtually inert (He, Ne, Ar, etc.). Full valence shells make them stable.
Main group elements: Groups 1, 2, and 13 through 18. Their chemistry is largely predictable from their group number.
Transition metals: the central block (Groups 3 through 12). They can have variable oxidation states and often form coloured compounds.
The table gives you the atomic number, element symbol, element name, and atomic mass for each element. An element's position tells you its group (and therefore its typical ion charge and number of valence electrons) and its period (and therefore how many electron shells it has).
The classification of matter is the basis of materials science. Whether you are designing a new alloy (a homogeneous mixture of metals) or purifying a drug compound, you need to know whether you are working with an element, a compound, or a mixture, because the separation and purification techniques differ completely.
Isotopes have practical uses everywhere: Carbon-14 dating in archaeology, medical imaging with radioactive tracers (e.g. Technetium-99m), and nuclear power from Uranium-235 fission.
Students often think compounds and mixtures are the same thing. A compound has a fixed chemical formula and can only be separated by chemical reactions. A mixture can be separated by physical methods (filtration, distillation, evaporation).
"Aqueous" is not a fifth state of matter. It is a phase label used in chemical equations to indicate a substance is dissolved in water.
Students sometimes assume isotopes of an element behave differently in chemical reactions. They do not: Carbon-12 and Carbon-14 form the same compounds. The difference is in nuclear properties (stability, radioactivity), not chemistry.
"Atom" and "molecule" are not interchangeable. An atom is a single unit; a molecule is a bonded group of atoms. O is an atom; O2 is a molecule.
⚠️ Be able to classify any substance as an element, compound, homogeneous mixture, or heterogeneous mixture. This is a staple multiple-choice question.
⚠️ Know the difference between extensive and intensive properties and be able to categorise examples of each.
⚠️ Calculate the number of protons, neutrons, and electrons for any isotope given its atomic number and mass number. This appears on virtually every CHEM101 exam.
⚠️ Know the major periodic table groups by number and name: alkali metals (1), alkaline earth metals (2), halogens (17), noble gases (18).
True or false: saltwater is a compound. (False; it is a homogeneous mixture.)
Fill in the blank: the number of protons in an atom equals its ______ number. (atomic)
True or false: density is an extensive property. (False; density is intensive.)
Fill in the blank: atoms of the same element with different numbers of neutrons are called ______. (isotopes)
True or false: noble gases are in Group 17. (False; Group 18.)
Q: Classify each of the following as an element, compound, homogeneous mixture, or heterogeneous mixture: (a) pure iron, (b) table salt dissolved in water, (c) carbon dioxide, (d) a bowl of cereal with milk.
A: (a) Element. (b) Homogeneous mixture (solution). (c) Compound (CO2, two elements chemically bonded). (d) Heterogeneous mixture.
Q: An atom has 17 protons, 18 neutrons, and 17 electrons. What element is it? What is its mass number? Is it charged?
A: Chlorine (Z = 17). Mass number = 17 + 18 = 35. It has equal protons and electrons, so it is neutral (Cl-35).
Q: How does a physical change differ from a chemical change?
A: A physical change alters the state or appearance of a substance without changing its chemical composition (e.g. melting ice). A chemical change produces a new substance with a different composition (e.g. burning wood).
Q: Why are noble gases unreactive?
A: Noble gases have full valence electron shells, which makes them energetically stable. They have little tendency to gain, lose, or share electrons.
Q: Potassium-40 has 19 protons. How many neutrons does it have?
A: Mass number (40) minus atomic number (19) = 21 neutrons.
The classification of matter connects directly to chemical bonding (covered in Doc 3): the distinction between ionic and molecular compounds depends on whether you are combining metals with nonmetals or nonmetals with nonmetals.
Periodic trends (atomic radius, ion size, effective nuclear charge) build on the atomic structure covered here and are explored in detail in the electron configuration and bonding notes.
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