Ch 2 · Atomic Structure and the Periodic Table · O Level Chemistry 5070
Topic 2 of 7 in Atoms, Elements and Compounds — O Level Chemistry 5070 — 48 lessons.
Introduction
Everything in chemistry follows from the structure of the atom. The number of protons decides which element an atom is, and the arrangement of its electrons decides how it bonds and reacts. Once you can write the electronic configuration of any of the first twenty elements you can read the Periodic Table like a map: the Group tells you the outer electrons and the Period tells you the number of shells.
Key ideas — The structure of the atom
An atom has a central nucleus containing protons and neutrons, surrounded by electrons arranged in shells (energy levels). The nucleus is tiny compared with the whole atom but contains almost all of its mass. Atoms are electrically neutral because the number of electrons equals the number of protons.
🧑🏫 Teacher's explanation
Think of an atom like a tiny solar system, but much more crowded. In the center, you have the 'sun'—the nucleus—which is incredibly dense and contains the heavy hitters: protons and neutrons. Orbiting this center are the electrons, which are like tiny, fast-moving gnats. Even though there are many electrons, they are so light that they barely contribute to the atom's weight.
📖 Definitions
🔍 Worked example — Easy
An atom has a proton number of 13 and a mass number of 27. Determine the number of protons, neutrons, and electrons in this neutral atom.
🔍 Worked example — Standard
Write the electronic configuration for a Magnesium ion (Mg^2+). (Atomic number of Mg = 12, Mass number = 24).
🔍 Worked example — Exam-style
An element X has the electronic configuration 2, 8, 5. Determine its proton number and mass number if it contains 16 neutrons.
⚠️ Common errors
🧠 Quick check 1
🧠 Quick check 2
🎯 Exam question
Key ideas — Proton number and mass number
So the number of neutrons is A - Z. In the symbol ^23_11Na the top number is the mass number and the bottom number is the proton number: sodium has 11 protons, 11 electrons and 23 - 11 = 12 neutrons.
🧑🏫 Teacher's explanation
Think of an atom like a busy airport terminal. The nucleus is the secure, central control tower where the heavy, essential staff (protons and neutrons) stay. The electrons are like the passengers rushing through the terminal in organized rows (shells). The protons are the 'ID cards' of the atom; if you change the number of protons, you change the identity of the person entirely.
📖 Definitions
🔍 Worked example — Easy
An atom has a proton number of 6 and a mass number of 12. Determine the number of protons, neutrons, and electrons in this neutral atom.
🔍 Worked example — Standard
Calculate the number of neutrons in an isotope of Magnesium with the symbol ^26_12Mg.
🔍 Worked example — Exam-style
An ion has a charge of 2- and a mass number of 32. The element is Sulfur (S) with a proton number of 16. Calculate the number of electrons in this ion.
⚠️ Common errors
🧠 Quick check 1
🧠 Quick check 2
🎯 Exam question
(a) State the mass number of this isotope. [1] (b) Determine the number of electrons in a neutral atom of this element. [1] [2 marks]
Key ideas — Electronic configuration
Electrons fill the shells nearest the nucleus first. For the first twenty elements the shells hold a maximum of 2, 8, 8 electrons in turn. The configuration is written as the number of electrons in each shell separated by commas.
🧑🏫 Teacher's explanation
Think of an atom like a multi-story hotel. The nucleus is the ground floor lobby where the heavy, permanent residents (protons and neutrons) live. The electrons are the guests. To keep things organized, guests always fill the lowest floors first before moving to the next level.
📖 Definitions
🔍 Worked example — Easy
Determine the electronic configuration of a neutral Phosphorus atom (P) given its proton number is 15.
🔍 Worked example — Standard
An ion has a charge of 2+ and a proton number of 12. Write its electronic configuration.
🔍 Worked example — Exam-style
An element X has the electronic configuration 2, 8, 7. Identify its Group number and Period number.
⚠️ Common errors
🧠 Quick check 1
🧠 Quick check 2
🎯 Exam question
(a) Write the electronic configuration of element Y. (b) Write the electronic configuration of the ion Y^2-. (c) State the Group number of element Y. [4 marks]
Key ideas — Reading the Periodic Table
The Periodic Table is arranged in order of proton number, and the electronic configuration explains its shape.
🧑🏫 Teacher's explanation
Think of the Periodic Table as a highly organized apartment building. The 'Period' is the floor number—it tells you how many levels (shells) the building has. The 'Group' is like the room type—everyone in the same room type has the same number of balconies (outer electrons), which means they behave in very similar ways.
📖 Definitions
🔍 Worked example — Easy
An element has the electronic configuration 2, 8, 3. State its Group number and Period number.
🔍 Worked example — Standard
Magnesium has an atomic number of 12. Write its electronic configuration and state its Group and Period.
🔍 Worked example — Exam-style
An ion has the electronic configuration 2, 8. Identify the element and its position in the periodic table.
⚠️ Common errors
🧠 Quick check 1
🧠 Quick check 2
🎯 Exam question
(a) Write the electronic configuration of element X. [1] (b) State the Group number and Period number of element X. [2] [3 marks]
Worked example
Question. An atom of element X is represented as ^35_17X.
🔍 Worked example — Easy
An atom of Magnesium is represented as ^24_12Mg. Determine the number of protons, neutrons, and electrons.
🔍 Worked example — Standard
An element has an electronic configuration of 2, 8, 5. Identify its Group and Period in the Periodic Table.
🔍 Worked example — Exam-style
An ion has a charge of -2 and a proton number of 12. Write its electronic configuration.
⚠️ Common errors
Common mistakes
Practice
Answers Proton number: the number of protons in the nucleus of an atom [B1]; mass number: the total number of protons and neutrons in the nucleus [B1]. Neutron: relative mass 1, charge 0 [B1]; electron: relative mass 1/1840, charge –1 [B1]. Aluminium 2,8,3 [B1]; sulfur 2,8,6 [B1]; calcium 2,8,8,2 [B1]. mathrmAl^3+: 2,8 [B1]; mathrmS^2-: 2,8,8 [B1]. Group II [B1]; Period 4 [B1]; calcium [B1]. They have a full outer shell of electrons, so they do not need to lose, gain or share electrons [B1].