Electron Shell Diagrams Practice (First 20 Elements)

Electron shell diagrams, the rings-and-dots pictures of atoms, are one of the first things students learn in chemistry and a guaranteed exam topic. These questions cover writing arrangements such as 2,8,1, drawing diagrams for atoms and ions, and linking shells to groups and periods. All answers are at the end.

If you need to review the rules first, read electron shells and the 2, 8, 8 rule.

The rules for the first 20 elements

• The first shell holds up to 2 electrons.
• The second shell holds up to 8.
• The third shell holds up to 8 (for elements 1 to 20).
• The fourth shell starts filling at potassium (element 19).
• Fill shells from the inside out.
• Electrons = atomic number, for a neutral atom.

Section A: writing electron arrangements

Q1. Write the electron arrangement for each of these atoms: (a) Carbon (Z = 6) (b) Sodium (Z = 11) (c) Sulfur (Z = 16) (d) Argon (Z = 18) (e) Calcium (Z = 20)

Q2. An atom has the electron arrangement 2,8,5. (a) What is its atomic number? (b) Name the element.

Q3. Which element has the arrangement 2,8,8,1?

Q4. Which element in the first 20 has exactly one electron in its outer shell and exactly two shells? Explain your answer.

Section B: drawing diagrams

Q5. Describe how you would draw a shell diagram for magnesium (Z = 12): how many rings, and how many electrons on each?

Q6. A student drew chlorine (Z = 17) with 2 electrons on the first shell, 8 on the second and 8 on the third. What mistake did they make?

Q7. Draw (or describe) the shell diagram for fluorine, and state how many unpaired outer electrons it has when electrons are drawn in pairs where possible.

Section C: groups and periods

Q8. Explain how the electron arrangement of an element tells you: (a) its period (b) its group (for main-group elements)

Q9. Using only electron arrangements, identify the group and period of: (a) 2,6 (b) 2,8,2 (c) 2,8,8

Q10. Why do lithium (2,1), sodium (2,8,1) and potassium (2,8,8,1) have similar chemical properties?

Section D: ions

Q11. Write the electron arrangement of each ion: (a) Na⁺ (b) Mg²⁺ (c) Cl⁻ (d) O²⁻

Q12. Which noble gas has the same electron arrangement as each ion in Q11?

Q13. Aluminium (Z = 13) forms an ion with the arrangement 2,8. What is the charge on the ion? Explain.

Q14. Sulfur forms the ion S²⁻. Describe its shell diagram and explain why sulfur forms a 2− ion rather than a 6+ ion.

Section E: challenge

Q15. Two elements X and Y have arrangements 2,8,3 and 2,7. (a) Identify X and Y. (b) Predict the formula of the compound they form. (c) Explain your answer in terms of electrons.

---

Answer key

A1.

(a) Carbon: 2,4 (b) Sodium: 2,8,1 (c) Sulfur: 2,8,6 (d) Argon: 2,8,8 (e) Calcium: 2,8,8,2

A2.

(a) 2 + 8 + 5 = 15 (b) Phosphorus

A3.

2 + 8 + 8 + 1 = 19 → potassium.

A4.

Lithium (2,1). Two shells means period 2, and one outer electron means group 1; lithium is the only element in both. Hydrogen (1) has one electron but only one shell, and sodium (2,8,1) has three shells.

A5.

Draw the nucleus in the centre, then three rings: 2 electrons on the first, 8 on the second and 2 on the third (2 + 8 + 2 = 12). The arrangement is 2,8,2.

A6.

Chlorine has only 17 electrons, so the arrangement is 2,8,7, not 2,8,8. The student drew 18 electrons, which would be argon (or a chloride ion, Cl⁻).

A7.

Fluorine (Z = 9): nucleus, then 2 electrons on the first ring and 7 on the second. Drawn in pairs where possible, the outer shell has three pairs and one unpaired electron, which is why fluorine forms one covalent bond (compare Lewis symbols).

A8.

(a) The number of shells containing electrons equals the period number. (b) The number of electrons in the outer shell gives the group: 1 or 2 outer electrons → group 1 or 2; 3 to 8 outer electrons → group 13 to 18 (or groups 3 to 0/8 in older numbering).

A9.

(a) 2,6: two shells → period 2; 6 outer electrons → group 16. (Oxygen.) (b) 2,8,2: three shells → period 3; 2 outer → group 2. (Magnesium.) (c) 2,8,8: three shells → period 3; 8 outer (full) → group 18. (Argon.)

A10.

They all have one electron in their outer shell. Chemical reactions involve outer electrons, so elements with the same number of outer electrons react in similar ways: each loses its single outer electron to form a 1+ ion. See the alkali metals.

A11.

(a) Na⁺: sodium (2,8,1) loses one electron → 2,8 (b) Mg²⁺: magnesium (2,8,2) loses two → 2,8 (c) Cl⁻: chlorine (2,8,7) gains one → 2,8,8 (d) O²⁻: oxygen (2,6) gains two → 2,8

A12.

Na⁺, Mg²⁺ and O²⁻ (2,8) match neon. Cl⁻ (2,8,8) matches argon. Ions with the same electron arrangement are called isoelectronic.

A13.

Aluminium atom: 2,8,3. To reach 2,8 it loses 3 electrons. It still has 13 protons but only 10 electrons, so the charge is +3: Al³⁺.

A14.

S²⁻ has 16 + 2 = 18 electrons: 2,8,8, drawn as three full shells with square brackets and a 2− charge. Sulfur gains 2 electrons rather than losing 6 because gaining two needs far less energy than removing six. Removing electrons gets harder with each one, as the ionization energy graphs show, while gaining two electrons completes the shell.

A15.

(a) X = 2 + 8 + 3 = 13 → aluminium. Y = 2 + 7 = 9 → fluorine. (b) AlF₃. (c) Aluminium loses its 3 outer electrons (forming Al³⁺, 2,8). Each fluorine atom can accept only 1 electron (forming F⁻, 2,8), so three fluorine atoms are needed to take all three electrons.

---

Reference: the first 20 elements

Z	Element	Arrangement		Z	Element	Arrangement
1	H	1		11	Na	2,8,1
2	He	2		12	Mg	2,8,2
3	Li	2,1		13	Al	2,8,3
4	Be	2,2		14	Si	2,8,4
5	B	2,3		15	P	2,8,5
6	C	2,4		16	S	2,8,6
7	N	2,5		17	Cl	2,8,7
8	O	2,6		18	Ar	2,8,8
9	F	2,7		19	K	2,8,8,1
10	Ne	2,8		20	Ca	2,8,8,2

Why does the fourth shell start before the third holds 18? The 4s subshell is slightly lower in energy than 3d, so it fills first. This is explained by the Aufbau principle and the fuller s, p, d and f notation.

Common mistakes

• Putting more than 2 electrons in the first shell.
• Miscounting total electrons. Always check that the numbers add up to the atomic number (for atoms).
• Forgetting brackets and charges on ion diagrams.
• Adding electrons to positive ions. Positive ions have lost electrons.
• Using 2,8,8 beyond calcium. From scandium onwards, the third shell fills up to 18.

Key takeaways

• For the first 20 elements, shells fill in the pattern 2, 8, 8, 2.
• Number of shells = period; outer electrons = group (main-group elements).
• Elements with the same number of outer electrons have similar chemistry.
• Ions form by losing or gaining electrons to reach a noble-gas arrangement.
• Check any element’s shell diagram on its page, for example calcium.