Electron Arrangements of the First Twenty Elements

Introduction

Once you know that electrons fill shells in order, you can write down the electron arrangement of an atom in a few seconds. A short code such as 2,8,1 tells you how many electrons sit in each shell, which period the element is in and, for main-group elements, which group it belongs to. This page shows how to write arrangements for the first twenty elements, from hydrogen to calcium, and how to read information back out of them.

Core explanation

Start with the atomic number. A neutral atom has the same number of electrons as protons. The atomic number is the number of protons, so it also tells you how many electrons you need to place. Sodium has atomic number 11, so a sodium atom has 11 electrons.

Fill the shells in order. For the first twenty elements, use the capacities:

- 1st shell: up to 2 electrons - 2nd shell: up to 8 electrons - 3rd shell: up to 8 electrons - 4th shell: the remaining electrons (1 or 2 for potassium and calcium)

Fill each shell completely before starting the next. Write the numbers separated by commas, innermost shell first.

The first twenty arrangements:

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

Patterns to notice. Some elements share the same final number — lithium (2,1), sodium (2,8,1) and potassium (2,8,8,1) — all end in 1. They are all Group 1. Fluorine (2,7) and chlorine (2,8,7) both end in 7 and are both in Group 7. Helium, neon and argon end with a full shell and are noble gases.

Reading an arrangement. Two pieces of information come straight out of any arrangement: the number of shells gives the period, and the last number (the outer electrons) gives the main-group number for Groups 1 to 7. For the noble gases, the full outer shell places them in Group 0.

Check your answer. Always add up the numbers. The total must equal the atomic number. A common slip is writing 2,8,9 for potassium; this breaks the rule that the third shell holds 8 electrons before the fourth starts.

Step-by-step reasoning

Writing the arrangement of sulfur (atomic number 16):

1. Number of electrons = 16. 2. Put 2 in the first shell; 14 remain. 3. Put 8 in the second shell; 6 remain. 4. Put the remaining 6 in the third shell. 5. Arrangement: 2,8,6. Check: 2 + 8 + 6 = 16.

Visual explanation

Draw a small circle for the nucleus with rings around it. For sulfur, place 2 crosses on the first ring, 8 on the second (often drawn in pairs) and 6 on the third. In the interactive periodic table, select any of the first twenty elements to see its shell diagram and its arrangement written as numbers side by side.

Real-world analogy

Filling shells is like seating guests at a wedding with a small top table for 2, then larger tables for 8. You fill the top table first, then the next table, and only start a new table when the one before is full. Counting the guests at the last table tells you something special about that group.

Real-world example

Chemists designing new materials, such as silicon chips, start from electron arrangements. Silicon (2,8,4) has four outer electrons, allowing each atom to bond to four neighbours in a strong network. Adding a small amount of phosphorus (2,8,5), with one extra outer electron, changes how well the silicon conducts electricity.

Why?

Why fill the inner shells first? Electrons are attracted to the positive nucleus. Shells closer to the nucleus have lower energy, so electrons occupy them first, just as a ball rolls to the lowest point it can reach. An atom is most stable when its electrons are in the lowest available energy levels.

Common misconception

"Each shell holds 8 electrons." The first shell holds only 2. Writing lithium as 3 or sodium as 8,3 ignores this. Every atom beyond helium starts with 2 in the first shell.

Worked example

Question: Write the electron arrangement of an atom with atomic number 19, and state its group and period.

Reasoning: 19 electrons: 2 in the first shell (17 left), 8 in the second (9 left), 8 in the third (1 left), 1 in the fourth. Arrangement 2,8,8,1. Four shells means period 4; one outer electron means Group 1.

Answer: 2,8,8,1; Group 1, period 4 (potassium).

Quick check

1. Write the electron arrangement of nitrogen, atomic number 7. Answer: 2,5

Exam focus

You must be able to write the arrangement of any element with atomic number 1 to 20 without a data sheet. Show your working by checking the total. Questions often ask you to "deduce the group and period" from an arrangement: last number for the group, number of shells for the period.

Advanced insight

Beyond calcium, this simple 2,8,8 pattern breaks down. In the transition metals, electrons begin filling the third shell again until it holds 18, so iron's arrangement is written 2,8,14,2. More advanced chemistry replaces shells with sub-shells labelled s, p, d and f, which explain these irregularities in detail.

Summary

To write an electron arrangement, take the atomic number as the number of electrons and fill shells in order with 2, 8, 8 and then the rest. Write the numbers separated by commas and check that they add up to the atomic number. The number of shells gives the period, and the outer-shell number gives the main-group number.

Practice questions

1. Write the electron arrangement of magnesium (atomic number 12). Answer: 2,8,2 2. Write the electron arrangement of argon (atomic number 18) and explain what is special about it. Answer: 2,8,8; its outer shell is full, which is why argon is an unreactive noble gas. 3. An atom has the arrangement 2,8,5. Give its atomic number, group and period. Answer: Atomic number 15 (phosphorus); Group 5; period 3. 4. A student writes the arrangement of calcium as 2,8,10. Explain the error and give the correct arrangement. Answer: For the first twenty elements the third shell holds only 8 before the fourth shell starts, so calcium is 2,8,8,2. 5. Which two elements among the first twenty have arrangements ending in 6? Answer: Oxygen (2,6) and sulfur (2,8,6).