Introduction to Chemistry 101Chemistry is the study of matter and the changes it undergoes. In this ebook you will learn about the building blocks of matter: atoms, molecules and ions, how elements are arranged in the periodic table, and how pure substances (compounds) differ from mixtures. You will also explore simple methods for separating mixtures and practice questions that mirror WAEC/NECO/UTME formats.Learning ObjectivesIdentify the main components of the periodic table and classify elements as metals, non‑metals or metalloids.Define element, compound and mixture and give examples of each.Describe the structure of an atom, including protons, neutrons and electrons.Explain what a molecule and an ion are, and write simple chemical formulas.Outline basic techniques for separating mixtures.Solve short WAEC/NECO/UTME‑style practice questions.

The Periodic TableThe periodic table is a systematic arrangement of all known chemical elements, ordered by increasing atomic number (the number of protons in an atom’s nucleus). Elements in the same group (vertical column) have similar chemical properties, while elements in the same period (horizontal row) have the same number of electron shells.Key pointsAtomic number = number of protons = number of electrons in a neutral atom.Atomic mass ≈ protons + neutrons.There are 18 groups and 7 periods (including the lanthanides and actinides as a separate block).Metals are found on the left and centre; non‑metals on the right.Examples: Sodium (Na, group 1), Chlorine (Cl, group 17), Carbon (C, group 14).

Elements and Their PropertiesAn element is a pure substance that cannot be broken down into simpler substances by chemical means. Each element is represented by a unique symbol (one or two letters) and has an atomic number.Atomic number (Z) – number of protons (and electrons in a neutral atom).Atomic mass (A) – approximate number of protons + neutrons.Examples of elements and their symbols:Hydrogen – H (Z=1)Oxygen – O (Z=8)Iron – Fe (Z=26)Gold – Au (Z=79)Elements show characteristic physical and chemical properties that repeat in patterns across the periodic table.

Classification of ElementsElements are grouped according to their properties:Metals – good conductors of heat and electricity, shiny, malleable, ductile. Examples: iron (Fe), copper (Cu), gold (Au).Non‑metals – poor conductors, brittle, dull. Examples: oxygen (O), carbon (C), sulfur (S).Metalloids – properties between metals and non‑metals. Examples: silicon (Si), germanium (Ge).Knowing whether an element is a metal or non‑metal helps predict how it will react in chemical reactions.

Compounds and MixturesA compound is a substance formed when two or more elements chemically combine in fixed ratios. The atoms are chemically bonded, and the compound has properties different from its constituent elements.A mixture is a physical combination of two or more substances where each retains its own properties. No chemical bonding occurs.ExamplesCompound: Water (H₂O) – made of hydrogen and oxygen.Mixture: Air – contains nitrogen, oxygen, carbon dioxide, etc.Compounds can be broken down by chemical reactions (e.g., electrolysis of water), while mixtures can be separated by physical methods.

Physical and Chemical PropertiesPhysical properties can be observed or measured without changing the substance’s chemical identity. Examples: colour, density, melting point, solubility.Chemical properties describe how a substance reacts with other substances. Examples: flammability, reactivity with acid, ability to oxidize.Understanding these properties helps in identifying substances and predicting their behaviour in reactions.

AtomsAn atom is the smallest particle of an element that retains the properties of that element. It consists of three subatomic particles:Protons (+) – located in the nucleus, give the atom its positive charge.Neutrons (0) – also in the nucleus, add mass but no charge.Electrons (−) – orbit the nucleus in energy levels (shells).The atomic number (Z) equals the number of protons (and electrons in a neutral atom). The mass number (A) equals protons + neutrons.Example: Carbon‑12 has 6 protons, 6 neutrons, 6 electrons.

MoleculesA molecule is a group of two or more atoms chemically bonded together. The atoms may be of the same element (e.g., O₂) or different elements (e.g., H₂O).Molecules are represented by chemical formulas that show the types and numbers of atoms.ExamplesHydrogen gas: H₂ (two hydrogen atoms)Water: H₂O (two hydrogen, one oxygen)Carbon dioxide: CO₂ (one carbon, two oxygen)Understanding molecules is essential for writing and balancing chemical equations.

IonsAn ion is an atom or molecule that has gained or lost electrons, giving it a net electric charge.Cations are positively charged ions (loss of electrons). Example: Na⁺ (sodium ion).Anions are negatively charged ions (gain of electrons). Example: Cl⁻ (chloride ion).The charge is written as a superscript after the symbol. Ions are important in forming compounds such as sodium chloride (NaCl) and in electrical conductivity in solutions.

Chemical Formulas and Simple EquationsA chemical formula tells us which elements are present in a compound and how many of each. Subscripts indicate the number of atoms.To write a formula, balance charges for ions. For neutral compounds, the total positive charge equals the total negative charge.Example: Magnesium oxide – Mg²⁺ and O²⁻ combine in a 1:1 ratio → MgO.A simple chemical equation shows reactants on the left and products on the right, with coefficients to balance atoms.Equation: 2H₂ + O₂ → 2H₂O

Separation Techniques for MixturesMixtures can be separated using physical methods that exploit differences in properties:Filtation – separates solid particles from a liquid (e.g., sand and water).Distillation – separates liquids with different boiling points (e.g., water and ethanol).Chromatography – separates coloured components based on solubility (e.g., ink on paper).Magnetism – separates magnetic from non‑magnetic substances (e.g., iron filings from sand).Choosing the right technique depends on the nature of the components and their physical properties.

Practice Questions (WAEC/NECO/UTME‑style)Which of the following is a characteristic of a metal?A) BrittleB) Good conductor of electricityC) Dull appearanceD) Non‑malleableWhat is the atomic number of an element?A) Number of neutronsB) Number of protonsC) Number of electrons in an ionD) Mass of the atomWhich of the following is a compound?A) AirB) Salt waterC) Carbon dioxideD) MilkWrite the chemical formula for calcium chloride (calcium is Ca²⁺, chloride is Cl⁻).Explain the difference between a physical property and a chemical property with one example each.

Answers and Review1. B) Good conductor of electricity2. B) Number of protons3. C) Carbon dioxide4. CaCl₂5. Physical property – colour (e.g., copper is reddish). Chemical property – reactivity with acid (e.g., zinc reacts with HCl to release hydrogen gas).ReviewRemember that the periodic table is ordered by atomic number.Elements are classified as metals, non‑metals or metalloids based on their properties.Compounds have fixed ratios; mixtures do not.Atoms consist of protons, neutrons and electrons; ions are charged atoms.Molecules are bonded atoms; formulas show the composition.These concepts form the foundation for further study in chemistry.

Conclusion and Further ReadingYou have completed a journey through the basic building blocks of chemistry: the periodic table, elements, compounds and mixtures, atoms, molecules and ions. Understanding these topics will help you succeed in more advanced chemistry courses and in Nigerian secondary‑school examinations.Suggested further reading"Chemistry for SS1‑SS2" by O. A. AdegokeOnline resources: Khan Academy – Chemistry basicsWAEC Past Questions – Chemistry (SS1 section)Keep practicing the problems, review the definitions, and you will develop a strong intuition for chemical concepts.