--- title: "What is an element?" subtitle: "Year 8 Chemistry · Elements and compounds · Lesson 2" --- # What is an element? **Name:** {{field-24}} **Date:** {{field-15}} **Class:** {{field-15}} > **Key question:** Everything around you, from your phone to your fingernails, is built from fewer than a hundred kinds of atom. Each kind is an element. :::teacher-note **In one sentence:** Students define an element as a pure substance of only one type of atom, tell elements from compounds by their particles, and use chemical symbols (AC9S8U06, node 2). **Students should be able to say:** I can define an element. I can tell an element from a compound by the atoms in its particles. I can use chemical symbols, including two-letter ones. **Width:** the unmarked parts are the floor every student does (Engage, Explore, Explain, Evaluate). Typical and Wider sections add contexts, not difficulty. Students work in the online tutorial alongside this sheet. ::: ## Engage :::keep-together **A jar of pure oxygen and a jar of pure water. Both are pure substances. Which is an element?** ☐ Only the oxygen ☐ Neither: oxygen particles have two atoms ☐ Only the water ☐ Both: they are pure My prediction, and why: {{lines-3}} ::: :::teacher-note **What happens:** Only the oxygen. Every oxygen particle (O₂) is two oxygen atoms: one type of atom, so oxygen is an element. Water is pure too, but each particle has hydrogen and oxygen atoms: two types, so water is a compound. ::: ## Explore Try it on the board in the tutorial. Was your prediction right? What was different? {{lines-2}} ## Explain: elements and symbols :::keep-together ### What an element is **What is an element?** ☐ A pure substance made of only one type of atom ☐ Any substance made of single atoms ☐ Any pure substance ::: :::teacher-note **Answer:** A pure substance made of only one type of atom One type of atom, and nothing else. The atoms may be single (helium) or joined (oxygen, O₂). ::: :::keep-together ### Element or compound **Element or compound?** Groups: Element, Compound - O₂ (oxygen): {{field-15}} - He (helium): {{field-15}} - N₂ (nitrogen): {{field-15}} - O₃ (ozone): {{field-15}} - H₂O (water): {{field-15}} - CO₂ (carbon dioxide): {{field-15}} - NaCl (salt): {{field-15}} ::: :::teacher-note **Answer:** O₂ (oxygen) → Element; He (helium) → Element; N₂ (nitrogen) → Element; O₃ (ozone) → Element; H₂O (water) → Compound; CO₂ (carbon dioxide) → Compound; NaCl (salt) → Compound ::: :::keep-together ### Symbols **Match each symbol to its element.** A. Copper B. Carbon C. Iron D. Oxygen E. Sodium F. Gold {{columns 60% 40%}} {{rows 1}} | Part | Letter | |---|---| | `O` | | | `C` | | | `Fe` | | | `Na` | | | `Cu` | | | `Au` | | ::: :::teacher-note **Answer:** O → D. Oxygen; C → B. Carbon; Fe → C. Iron; Na → E. Sodium; Cu → A. Copper; Au → F. Gold Some symbols come from Latin names: Fe from ferrum (iron), Na from natrium (sodium), Cu from cuprum (copper), Au from aurum (gold). ::: :::keep-together ### Co or CO **Why is the symbol for cobalt written Co, not CO?** ☐ It doesn't matter how you write it ☐ A symbol is one capital letter, or a capital and a small letter; CO means a carbon atom joined to an oxygen atom ☐ CO is the symbol for cobalt gas ::: :::teacher-note **Answer:** A symbol is one capital letter, or a capital and a small letter; CO means a carbon atom joined to an oxygen atom Each symbol starts with a capital. Co is one element (cobalt); CO is two symbols, C and O: carbon monoxide. ::: :::keep-together ### The periodic table **What does the periodic table show?** ☐ Only the elements found in living things ☐ Every substance, including compounds ☐ Every element, one box each, in order of atomic number, with similar elements in columns ::: :::teacher-note **Answer:** Every element, one box each, in order of atomic number, with similar elements in columns About 118 elements are known; around 90 occur naturally. Everything else is made from them. ::: :::keep-together ### Reading a box **A box on the periodic table shows 26, Fe and Iron. What does each part tell you?** ☐ 26 is the temperature iron melts at ☐ 26 is the atomic number (its place in the table), Fe is the symbol and Iron is the name ☐ 26 is the number of atoms in one particle of iron ☐ Fe is the formula of a compound of iron ::: :::teacher-note **Answer:** 26 is the atomic number (its place in the table), Fe is the symbol and Iron is the name Each box gives an element's atomic number, symbol and name (often its atomic mass too). Find the name and you have the symbol, or the other way round. ::: :::typical ## Elaborate: Co or CO? :::keep-together ### Two capitals **CO is a poisonous gas from faulty heaters. Cobalt (Co) is a hard, shiny metal. How does the board show CO is not an element?** ☐ The particles are moving ☐ It has two letters ☐ Each particle has a carbon atom and an oxygen atom: two types ::: :::teacher-note **Answer:** Each particle has a carbon atom and an oxygen atom: two types Two types of atom joined: a compound. Two capital letters in a formula mean two elements. ::: ::: :::wider ## Elaborate: two forms of oxygen :::keep-together ### One element, two substances **Ozone (O₃) and oxygen (O₂) are both made of only oxygen atoms, but ozone protects us from ultraviolet light high in the atmosphere and is harmful to breathe. What does this show?** ☐ Ozone must be a compound ☐ One element can form different substances when its atoms join in different ways ☐ O₃ contains a different type of oxygen atom ::: :::teacher-note **Answer:** One element can form different substances when its atoms join in different ways Carbon does the same: diamond and graphite are both pure carbon. How the atoms are joined changes the properties. ::: ::: ## Evaluate: look back Write one sentence: "At first I thought… Now I think… because…" **Revision summary** - An **element** is a pure substance made of only one type of atom. Its particles can be single atoms (He) or atoms joined together (O₂, O₃). - A **compound** has two or more types of atom joined (H₂O, CO₂). - Each element has a **symbol**: one capital letter, or a capital and a small letter (C, Co, Na). Some come from Latin names (Fe, Na, Cu, Au). - The **periodic table** lists all the elements, about 118. ## Exit ticket :::keep-together **Nitrogen gas is N₂ and ammonia is NH₃. Which is an element, and how can you tell from the formula?** {{lines-3}} ::: :::teacher-note **Answer:** Nitrogen: only one symbol (N), so one type of atom. Ammonia has two symbols (N and H): a compound. :::