Key Learning Objectives
By the end of this lesson and revision module, Year 7 students will be able to:
- Describe the structure of an atom including protons, neutrons, and electrons with their relative charges and masses.
- Define atomic number (proton number) and mass number (protons + neutrons).
- Explain how elements are organized in the Periodic Table according to atomic number and electron configuration.
Topic Summary & Essential Notes
All matter is composed of tiny fundamental particles called atoms. An atom contains a dense central nucleus containing positively charged protons and neutral neutrons. Negatively charged electrons orbit the nucleus in specific energy levels or shells. Because the number of positive protons equals the number of negative electrons, an atom has no overall electrical charge. The atomic number identifies the element and indicates the number of protons. The mass number is the total count of protons plus neutrons.
Key Rules, Formulas & Definitions
Subatomic Particle Charges & Masses
Proton (charge +1, mass 1), Neutron (charge 0, mass 1), Electron (charge −1, mass 1/2000 or negligible)
Neutron Count Formula
Number of Neutrons = Mass Number (Top) − Atomic Number (Bottom)
Electron Shell Capacity
Maximum electrons per shell in first 20 elements: Shell 1 = 2, Shell 2 = 8, Shell 3 = 8
2 Step-by-Step Worked Examples
Study these model solutions to understand how examiners award method and accuracy marks:
Worked Example 1
Worked Example 1: Deducing Protons, Neutrons, and Electrons
Question: A Sodium (Na) atom has an atomic number of 11 and a mass number of 23. Determine the number of protons, neutrons, and electrons.
Step-by-Step Solution:
- Step 1: Protons. Protons = Atomic Number = 11.
- Step 2: Electrons. In a neutral atom, Electrons = Protons = 11.
- Step 3: Neutrons. Neutrons = Mass Number − Atomic Number = 23 − 11 = 12.
Final Answer: Protons = 11, Electrons = 11, Neutrons = 12. Electron configuration: 2, 8, 1.
Senior Examiner Insight: Remember that the atomic number (smaller number) is always the number of protons. The mass number is always larger because it counts both protons and neutrons.
Worked Example 2
Worked Example 2: Drawing and Writing Electron Configuration
Question: An Oxygen atom has 8 electrons. State its electronic configuration and predict its group number in the Periodic Table.
Step-by-Step Solution:
- Step 1: Fill the first shell. First shell holds a maximum of 2 electrons (8 − 2 = 6 remaining).
- Step 2: Fill the second shell. Place the remaining 6 electrons in the second shell.
- Step 3: State group. Configuration is (2, 6). Having 6 outer shell electrons places Oxygen in Group 6.
Final Answer: Configuration: 2, 6 | Group 6
Senior Examiner Insight: The number of outer electrons always equals the Group number. The number of occupied shells equals the Period number.
Common Pitfalls & Examiner Warnings
Avoid these common mistakes frequently identified by examiners in Key Stage 3 assessments:
Common Mistake: Assuming electrons have significant mass
Electrons have a tiny mass (1/1840 or 1/2000 of a proton), which is considered negligible when calculating atomic mass.
Common Mistake: Confusing atomic number with mass number
Atomic number is the number of protons only. Mass number is protons PLUS neutrons.
Common Mistake: Overfilling electron shells
Placing more than 2 electrons in the first shell or more than 8 in the second is an immediate mark deduction.
Interactive Self-Check Quiz (3 Questions)
Test your understanding before checking the full worked answer and examiner mark scheme:
Question 1 of 3
Why do neutral atoms carry NO overall electric charge?
Click to Reveal Answer & Mark Scheme
Correct Answer: B) The number of positive protons exactly equals the number of negative electrons.
Detailed Explanation: Each proton has a +1 charge and each electron has a −1 charge. Equal numbers ensure net charge is zero.
Mark Scheme & Scoring Points: 1 mark for stating equal numbers of protons and electrons (+1 and −1 cancel).
Question 2 of 3
A Chlorine atom has mass number 35 and atomic number 17. How many neutrons does it possess?
Click to Reveal Answer & Mark Scheme
Correct Answer: C) 18
Detailed Explanation: Neutrons = Mass Number − Atomic Number = 35 − 17 = 18 neutrons.
Mark Scheme & Scoring Points: 1 mark for 18 neutrons.
Question 3 of 3
What is the electron arrangement of a neutral Magnesium atom (atomic number 12)?
Click to Reveal Answer & Mark Scheme
Correct Answer: A) 2, 8, 2
Detailed Explanation: 2 electrons fill the 1st shell, 8 electrons fill the 2nd shell, and the remaining 2 go into the 3rd shell (2, 8, 2).
Mark Scheme & Scoring Points: 1 mark for correct configuration: 2, 8, 2.
Created by Fiaraz Iqbal
Former Headteacher & Senior Science / Maths Examiner
With over 15 years of leadership and classroom experience, Fiaraz creates high-yield revision resources aligned with National Curriculum standards to help Key Stage 3 and GCSE students secure top grades.
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