Periodic table position and patterns

An element’s place in the Periodic Table tells you a lot before you have seen it react. In this lesson you will place an element from its proton number or electron arrangement, explain why elements in one group behave alike, and use the number of outer electrons to predict whether an element is a metal or a non-metal.

You need two ideas from earlier topics. The proton number is the number of protons in an atom, and a neutral atom has the same number of electrons. Electrons fill shells in order: at most 2 in the first shell and 8 in the second; for the first 20 elements the third shell also takes 8 before the fourth shell starts. So calcium, with 20 electrons, is 2,8,8,2.

The table is ordered by proton number

The Periodic Table arranges the elements in order of increasing proton number. Each element has one more proton than the element before it. A horizontal row is a period; a vertical column is a group. In this course the groups are numbered 1 to 18. The block of elements between Group 2 and Group 13 (Groups 3 to 12) is not needed here.

Why proton number and not mass? Look at argon and potassium. Argon (proton number 18) has a relative atomic mass of 39.9, while potassium (proton number 19) has 39.1. Ordered by mass, potassium would come first. It would land among the unreactive gases, and argon would sit below sodium among the reactive metals. Ordered by proton number, each element lands in the group whose members it resembles.

This extract shows the first 20 elements with their proton numbers and electron arrangements. Hydrogen sits on its own at the top: it has one electron but is a non-metal, not a Group 1 metal.

Each cell gives the symbol, the proton number and, in brackets, the electron arrangement.

Position follows the electron arrangement

Read down the table and across it, and two rules appear:

• Period number = number of occupied shells. Every element in Period 3 has electrons in three shells.
• Group number follows the number of outer electrons. Groups 1 and 2 have 1 and 2 outer electrons. Groups 13 to 17 have 3 to 7: take 10 from the group number. Group 18 elements have 8 outer electrons, except helium, whose only shell is full with 2.

So the position and the electron arrangement carry the same information. Given one, you can work out the other.

Worked example 1

Locate an element from its electron arrangement

Problem

An element has the electron arrangement 2,8,7. State its period and group, and predict the charge on its ion.

Show full solution

1. Count occupied shells

   Method

   Three numbers are listed, so three shells are occupied.

   Reason

   The number of occupied shells gives the period.

   Working

   Answer

   Period 3.

2. Count outer electrons

   Method

   The last number is 7, so there are seven outer electrons.

   Reason

   Groups 13 to 17 have the group number minus 10 outer electrons, and 7 + 10 = 17.

   Working

   Answer

   Group 17.

3. Predict the ion

   Method

   Gaining one electron gives a full outer shell of eight.

   Reason

   Gaining one electron is far easier than losing seven.

   Working

   Answer

   Ion charge 1− (the element is chlorine, forming Cl⁻).

Quick check

Shells give the period; outer electrons give the group. The total number of electrons (17) is the proton number, which matches chlorine in the extract.

Spot the mistake 2

Period or group?

About 4 min

Learner claim

Element X has the electron arrangement 2,8,6. A learner writes: “X is in Period 6 because it has six outer electrons.” Correct the position and predict X’s ion.

Use each number for its own job

Period of X

Period 6CorrectIncorrectPeriod 16CorrectIncorrectPeriod 3CorrectIncorrect

Group of X

Group 6CorrectIncorrectGroup 3CorrectIncorrectGroup 16CorrectIncorrect

Ion formed by X

X⁶⁺CorrectIncorrectX²⁺CorrectIncorrectX²⁻CorrectIncorrect

Check answer

Show solution step by step

1. Separate the two counts

   Method

   Count the shells for the period and the outer electrons for the group.

   Reason

   The learner used the outer-electron count for the period.

   Working

   Answer

   3 shells → Period 3; 6 outer electrons → Group 16.

2. Predict the ion

   Method

   X gains two electrons.

   Reason

   Two more electrons complete an outer shell of eight.

   Working

   Answer

   X²⁻ (X is sulfur).

Common mistake

Only the first 20 elements are used here, and none of them is in Period 6. If a period number is larger than the number of shells in the arrangement, the reading is wrong.

Now reverse the reasoning: start from a position and work out the proton number and electron arrangement.

In the extract below, the letters Q, R and T replace some symbols. They are not chemical symbols.

Guided practice 3

Fill a gap in a lettered extract

About 5 min

Problem

Use only the lettered extract above, not the full table. Work out the proton number and electron arrangement of R, and say which element in the extract R should resemble.

Count along the period, then down the group

Proton number of R

Electrons in the first shell

Electrons in the second shell

Electrons in the third shell

R resembles

Nitrogen (N)CorrectIncorrectTCorrectIncorrectSilicon (Si)CorrectIncorrect

Check answer

Hints

Hint 1

Order by proton number

Each step to the right along a period adds one proton. Groups 3 to 12 are not in Period 3.

Hint 2

Same group, same pattern

Elements in the same column have the same number of outer electrons.

Show solution step by step

1. Count along Period 3

   Method

   Step one place to the right of silicon.

   Reason

   The table is ordered by increasing proton number.

   Working

   Answer

   Si = 14, so R = 15.

2. Build the arrangement

   Method

   Fill the shells in order.

   Reason

   Period 3 means three occupied shells; Group 15 means five outer electrons.

   Working

   Answer

   15 = 2 + 8 + 5, so 2,8,5.

3. Find the group partner

   Method

   R should resemble nitrogen (2,5).

   Reason

   Both have five outer electrons, so they react in similar ways.

   Working

   Answer

   R is phosphorus, P.

Quick check

Check the answer both ways: 2,8,5 has three shells (Period 3) and five outer electrons (Group 15), which is exactly where R sits.

Why a group behaves alike

Chemical reactions involve outer electrons. In Groups 1, 2 and 17, the members have the same number of outer electrons and tend to react in similar ways. Group 18 members share a full outer shell; helium’s full shell has 2 electrons, while neon and argon have 8.

The formulae match within each group because the ions have the same charge. Lithium, sodium and potassium also all react with water in the same way, forming an alkaline solution and hydrogen.

“Similar” does not mean “identical”. Members of a group differ in how readily they react. The next two lessons look at those trends in Groups 1 and 17.

From metal to non-metal across a period

Now read across Period 3. Each element has one more outer electron than the one before.

Sodium, magnesium and aluminium are shiny, conduct electricity and form positive ions: they are metals. Phosphorus, sulfur and chlorine do not conduct, and they gain or share electrons: they are non-metals. Silicon looks shiny but conducts poorly, so it sits at the changeover. Across a period, the elements change from metallic to non-metallic character, and the change is gradual rather than a sudden jump.

The outer electrons explain the change:

• The metals in this extract have few outer electrons (1 to 3). In reactions they tend to lose those electrons to form positive ions.
• An atom with more outer electrons (5 to 7) can reach a full outer shell most easily by gaining or sharing electrons. That is typical of non-metals.

These are useful patterns, not a classification test based on electron count alone. Hydrogen (1 outer electron) and helium (2) are non-metals. Boron has 3 outer electrons but is not a metal: like silicon, it is usually described as a metalloid, with some properties of metals and some of non-metals. Use physical and chemical evidence alongside the electron arrangement.

Exam-style question 1

Describe and explain the change across Period 3

3 marks

Examination question

Use the Period 3 table above. Describe how the character of the elements changes from sodium to chlorine, and explain the change in terms of outer electrons. [3 marks] Work on paper or in the optional notes and compare all three criteria.

Describe, give evidence, then explain

Optional answer or comparison notes

Show solution step by step

1. Describe the change

   1 mark

   Method

   State the direction of change.

   Reason

   The question asks how the character changes, from left to right.

   Working

   Answer

   The elements change from metals (Na, Mg, Al) to non-metals (P, S, Cl).

2. Quote the evidence

   1 mark

   Method

   Use a property from the table.

   Reason

   A described trend needs data to support it.

   Working

   Answer

   Na, Mg and Al conduct electricity well; P, S and Cl do not conduct.

3. Explain with outer electrons

   1 mark

   Method

   Link the number of outer electrons to electron loss or gain.

   Reason

   This is the reason behind the pattern.

   Working

   Answer

   Outer electrons increase from 1 to 7. Atoms with few outer electrons lose them to form positive ions (metallic); atoms with many gain or share electrons (non-metallic).

Self-mark with the mark scheme

Tick each mark point that your answer includes.

Award the direction of change, one supporting property from the data and the outer-electron explanation separately.

Mark points

1. 1 mark: States that the elements change from metallic to non-metallic from left to right.
2. 1 mark: Quotes a property from the data, such as electrical conductivity, that supports the change.
3. 1 mark: Explains that few outer electrons are lost (metallic) while many outer electrons are gained or shared (non-metallic).

Calculate your mark

Common mistakes

Try these

Check your understanding 1 Locate the unknown metal

Element E is one of the first 20 elements. It is a conducting, shiny solid; its neutral atoms have three occupied shells and form E²⁺ ions by losing their two outer electrons. Find its group, period and proton number.

Group of E

Period of E

Proton number of E

Check answer

Show answer

Two outer electrons place E in Group 2; three occupied shells place it in Period 3. The arrangement is 2,8,2, so a neutral atom has 12 electrons and 12 protons. E is magnesium.

Check your understanding 2 Explain the shared chloride formula

Explain why lithium, sodium and potassium all form chlorides with formula MCl, where M stands for the metal.

Write on paper or in the optional notes. Compare the complete explanation with the model.

Optional working or comparison notes

Show answer

Each Group 1 atom loses its one outer electron to form a 1 + ion. One metal ion balances one chloride ion of charge 1-, giving a 1:1 ratio and formula MCl.

Check your understanding 3 Build the position from proton number

A neutral atom has proton number 9. Give its electron arrangement, period and group, and classify its element as metal or non-metal.

Electrons in the first shell

Electrons in the second shell

Period

Group

Element class

Non-metalCorrectIncorrectMetalCorrectIncorrect

Check answer

Show answer

Nine electrons give arrangement 2,7. Two occupied shells mean Period 2; seven outer electrons mean Group 17. The element is fluorine, a non-metal whose atoms gain one electron when forming fluoride ions.

Check your understanding 4 Use the mixed silicon evidence

Silicon is shiny but conducts electricity poorly. Explain what these mixed properties show about the change from metals to non-metals across Period 3.

Write on paper or in the optional notes. Compare the complete explanation with the model.

Optional working or comparison notes

Show answer

The change is gradual. Silicon has metallic lustre but lacks the good electrical conductivity of the metals before it. It has some properties of both classes and is described as a metalloid; shininess alone cannot identify an element as a metal.

Check your understanding 5 Correct the metallic-character trend

A learner says: “Across Period 3 from sodium to chlorine, elements become more metallic because the number of electrons increases.” Correct the trend and explain it using outer electrons.

Write on paper or in the optional notes. Compare the complete explanation with the model.

Optional working or comparison notes

Show answer

The elements become less metallic and more non-metallic. The outer-electron count rises from 1 to 7. Atoms with few outer electrons tend to lose them to form positive ions; those with many tend to gain or share electrons. Total electron count alone does not explain metallic character.