--- title: "What's inside an atom?" subtitle: "Years 9 and 10 Chemistry · Inside the atom · Lesson 1" --- # What's inside an atom? **Name:** {{field-24}} **Date:** {{field-15}} **Class:** {{field-15}} > **Key question:** If an atom were the size of a football stadium, its nucleus would be a marble on the centre spot. Yet that marble holds nearly all the mass. :::teacher-note **In one sentence:** Students describe the nuclear atom (protons and neutrons in the nucleus, electrons in shells), use atomic and mass numbers to count each particle, and connect the model to the evidence that replaced earlier ones (AC9S9U06, node 1, a hinge node). **Students should be able to say:** I can describe the nuclear atom. I can use the atomic number and mass number to find the protons, neutrons and electrons. I can describe the relative mass and charge of protons, neutrons and electrons. **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 carbon atom has atomic number 6 and mass number 12. How many neutrons does it have?** ☐ 18 ☐ 6 ☐ None: the mass is all protons ☐ 12 My prediction, and why: {{lines-3}} ::: :::teacher-note **What happens:** 6. The mass number counts protons plus neutrons, and the atomic number counts protons, so 12 − 6 = 6 neutrons. A neutral carbon atom also has 6 electrons. ::: ## Explore Try it on the board in the tutorial. Was your prediction right? What was different? {{lines-2}} ## Explain: protons, neutrons, electrons :::keep-together ### Where the mass is **Where is nearly all of an atom's mass?** ☐ Spread evenly through the atom ☐ In the electrons, because they fill most of the space ☐ In the nucleus: protons and neutrons have mass 1 each, an electron about 1/1840 ::: :::teacher-note **Answer:** In the nucleus: protons and neutrons have mass 1 each, an electron about 1/1840 Protons and neutrons each have a relative mass of 1; an electron about 1/1840. So the nucleus, a tiny fraction of the atom's size, holds nearly all its mass. ::: :::keep-together ### Three particles **Proton, neutron or electron?** Groups: Proton, Neutron, Electron - Charge +1: {{field-15}} - No charge: {{field-15}} - Charge −1: {{field-15}} - Mass about 1/1840 of a proton: {{field-15}} - In the nucleus; its number decides the element: {{field-15}} - In the nucleus; changing its number changes the mass, not the element: {{field-15}} - In shells around the nucleus: {{field-15}} ::: :::teacher-note **Answer:** Charge +1 → Proton; No charge → Neutron; Charge −1 → Electron; Mass about 1/1840 of a proton → Electron; In the nucleus; its number decides the element → Proton; In the nucleus; changing its number changes the mass, not the element → Neutron; In shells around the nucleus → Electron ::: :::keep-together ### Neutrons **Sodium has atomic number 11 and mass number 23. How many neutrons does a sodium atom have?** Answer: {{field-15}} ::: :::teacher-note **Answer:** 12 Mass number minus atomic number: 23 − 11 = 12 neutrons. ::: :::keep-together ### Electrons **How many electrons does a neutral sodium atom have?** Answer: {{field-15}} ::: :::teacher-note **Answer:** 11 Neutral means no overall charge: as many electrons (−) as protons (+), so 11. ::: :::keep-together ### The evidence **Rutherford fired alpha particles at thin gold foil. Most went straight through, but a few bounced back. What did that show?** ☐ An atom is mostly empty space, with a tiny, dense, positive nucleus ☐ The positive charge is spread evenly through the atom ☐ Atoms are solid balls ::: :::teacher-note **Answer:** An atom is mostly empty space, with a tiny, dense, positive nucleus Only a tiny, dense, positive centre could turn a few alpha particles right round while letting most pass. The model of the atom changed because of this evidence. ::: :::typical ## Elaborate: chlorine :::keep-together ### One more proton **You add one proton to a chlorine atom. What is it now?** ☐ A chlorine ion ☐ Argon: a different element ☐ Still chlorine, just heavier ::: :::teacher-note **Answer:** Argon: a different element The atomic number (protons) decides the element: 18 protons is argon. ::: ::: :::wider ## Elaborate: an ion :::keep-together ### Where the charge came from **How did you give sodium a charge of +1?** ☐ Added a proton ☐ Added a neutron ☐ Took away one electron, so there is one more proton than electrons ::: :::teacher-note **Answer:** Took away one electron, so there is one more proton than electrons Losing an electron leaves 11 protons (+11) and 10 electrons (−10): a charge of +1. It is still sodium. ::: ::: ## Evaluate: look back Write one sentence: "At first I thought… Now I think… because…" **Revision summary** - An atom has a tiny, dense **nucleus** of **protons** (charge +1, mass 1) and **neutrons** (no charge, mass 1), with **electrons** (charge −1, mass about 1/1840) in **shells** around it. - **Atomic number** = protons; it decides the element. **Mass number** = protons + neutrons. - A neutral atom has as many electrons as protons. An **ion** has gained or lost electrons. - The nuclear model replaced earlier models because of evidence, such as Rutherford's gold foil experiment. ## Exit ticket :::keep-together **Oxygen has atomic number 8 and mass number 16. How many protons, neutrons and electrons does a neutral oxygen atom have?** {{lines-3}} ::: :::teacher-note **Answer:** 8 protons, 8 neutrons (16 − 8), 8 electrons. :::