Atomic structure, shells and the Periodic Table
Use proton number, mass number and electron arrangement to connect atoms with their positions in the table.
Core and Extended · Chemistry 0620, 2026–2028.
Before you begin
- Use subtraction with whole numbers.
- Understand that an ion carries a net electrical charge.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
Which particle has negative relative charge?
What you will learn
- Calculate proton, neutron and electron counts for atoms and ions.
- Write shell arrangements for the first 20 elements and infer group and period.
The nucleus and electron shells
Protons and neutrons occupy the small central nucleus; electrons occupy shells around it. A proton has relative charge +1 and relative mass 1. A neutron has charge 0 and relative mass 1. An electron has charge −1 and a much smaller relative mass, approximately 1/1836; its mass is often treated as negligible in simple atomic mass calculations.
Proton number Z is the number of protons and identifies the element. Mass number A counts protons plus neutrons, so neutrons = A − Z. A neutral atom has as many electrons as protons. For a positive ion, subtract the charge from that electron count; for a negative ion, add the magnitude of the charge. Ion formation does not change the nucleus.
Use the first-20 shell model carefully
For the first 20 elements, use the school-level shell arrangements with up to 2 in the first shell, 8 in the second and 8 in the third before the fourth begins: sodium is 2,8,1; calcium is 2,8,8,2. This is the limited model needed here, not a universal claim that every shell can only hold eight electrons.
For neutral atoms in Groups I to VII, the outer-shell electron count matches the group number. The number of occupied shells gives the period. Noble gases in Group VIII have full outer shells: helium has a full first shell of two, while neon has 2,8. Use a neutral atom's arrangement to locate the element; an ion's changed shell count does not move it to a different element's position.
Pause and explain
A neutral atom has electron arrangement 2,8,7. Which position matches it?
Worked example
For a magnesium atom with Z=12 and A=24, find the three particle counts and shell arrangement. Then describe Mg²⁺.
Show the worked solution
- The atom has 12 protons, 24−12=12 neutrons and 12 electrons.
- The neutral shell arrangement is 2,8,2, placing magnesium in Group II and Period 3.
- Mg²⁺ loses two electrons: 10 electrons arranged 2,8; its nucleus is unchanged.
Answer Mg: 12 p, 12 n, 12 e; 2,8,2. Mg²⁺: 12 p, 12 n, 10 e; 2,8.
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Test this topic →From guided practice to a new situation
Write your answer and reasoning first. Use a hint only if you are stuck. The model solution is for self-checking; your written work is not automatically marked.
For sodium with Z=11 and A=23, find particle counts for Na and Na⁺, and write both shell arrangements.
Give me a hint
Use A−Z for neutrons and remove one electron for the positive ion.
Compare my reasoning
- Both particles have 11 protons and 12 neutrons.
- Na has 11 electrons and arrangement 2,8,1.
- Na⁺ has 10 electrons and arrangement 2,8.
Na: 11 p, 12 n, 11 e; 2,8,1. Na⁺: 11 p, 12 n, 10 e; 2,8.
Look for these in your work
- The nucleus stays unchanged.
- The electron loss matches the positive charge.
Calcium has Z=20 and A=40. Give its neutral shell arrangement, group, period and electron count in Ca²⁺.
Give me a hint
Fill the first-20 shells before relating the neutral arrangement to the table.
Compare my reasoning
- Twenty electrons arrange as 2,8,8,2.
- Two outer electrons give Group II; four shells give Period 4.
- Ca²⁺ has lost two electrons, leaving 18.
2,8,8,2; Group II, Period 4; Ca²⁺ has 18 electrons.
Look for these in your work
- The first-20 arrangement is correct.
- The ion count and neutral table position are kept separate.
An ion has 17 protons, 18 neutrons and 18 electrons. Identify its mass number and charge; is it a different element from a neutral atom with 17 protons?
Give me a hint
Compare protons and electrons to find charge.
Compare my reasoning
- Mass number is 17+18=35.
- One more electron than proton gives charge −1.
- The proton number is unchanged, so it remains chlorine.
Mass number 35, charge −1; it is still chlorine.
Look for these in your work
- Mass number counts nucleons only.
- Charge and element identity use different particle counts.
Common mistakes
- Changing the proton number when an ion forms.
- Using an ion's shell arrangement to assign its element a new group.
What can you explain now?
An aluminium atom has Z=13 and A=27. How many electrons does Al³⁺ have?
Compare with the explanation
10
A neutral atom has 13 electrons; losing three forms Al³⁺ with 10. It still has 13 protons and 14 neutrons.
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Make it stick
Write the counts and shells for one positive ion and one negative ion. Explain which changes and which stay fixed when each forms.
If you needed the solution, close it and solve the problem again from a blank page. A correct answer is stronger when you can explain the reason for each step.