4. Electron Arrangements and Group Number

Electron Arrangements and Group Number

The periodic table provides much more information than simply the names and symbols of elements. An element's position can help us predict how its electrons are arranged, how many valence electrons it has, and how it is likely to behave during chemical reactions.

For the first 20 elements, these relationships are especially useful because their electron arrangements can be represented using simple Bohr diagrams.

---

Electron Shells

Electrons occupy regions around the nucleus called electron shells or energy levels.

For the first 20 elements, we can use a simplified model:

• first shell holds up to 2 electrons
• second shell holds up to 8 electrons
• third shell holds up to 8 electrons in this introductory model
• electrons then begin entering the fourth shell

For example, a sodium atom has 11 electrons.

Its electron arrangement is:

2, 8, 1

This means:

• 2 electrons in the first shell
• 8 electrons in the second shell
• 1 electron in the third shell

Suggested placement: A Bohr model of sodium works well here because students can clearly see the relationship between the notation 2,8,1 and the three occupied shells.

---

Atomic Number and Electron Arrangement

The atomic number tells us the number of protons in an atom.

In a neutral atom:

Number of electrons = Number of protons = Atomic number

Therefore, we can use the atomic number to determine how many electrons need to be placed into the shells.

Example: Oxygen

Oxygen has atomic number 8.

Therefore, a neutral oxygen atom has:

• 8 protons
• 8 electrons

Fill the shells:

• first shell → 2 electrons
• second shell → remaining 6 electrons

Electron arrangement:

2, 6

Example: Calcium

Calcium has atomic number 20, so it has 20 electrons.

Its electron arrangement is:

2, 8, 8, 2

---

Period Number and Electron Shells

The period number tells us the number of occupied electron shells.

For example:

Notice the pattern.

Sodium has:

2,8,1 → 3 occupied shells → Period 3

Potassium has:

2,8,8,1 → 4 occupied shells → Period 4

So we can remember:

Period number → Number of occupied electron shells

---

Group Number and Valence Electrons

The electrons in the outermost occupied shell are called valence electrons.

These electrons are particularly important because they are involved in chemical bonding and reactions.

For the main-group elements, the position of an element provides information about its valence electrons.

For Groups 13–18, a useful introductory rule is:

Valence electrons = Group number − 10

For example:

Group 16 → 16 − 10 = 6 valence electrons

Group 17 → 17 − 10 = 7 valence electrons

This simple relationship applies to the main-group elements, not the transition metals in Groups 3–12.

---

Why Do Elements in the Same Group Behave Similarly?

Elements in the same group have the same number of valence electrons.

Consider Group 1:

All three atoms have one valence electron.

Because chemical reactions mainly involve valence electrons, lithium, sodium, and potassium tend to behave in similar ways.

The same pattern occurs in Group 17:

• fluorine → 2,7
• chlorine → 2,8,7

Both have 7 valence electrons, so they have similar chemical properties.

---

Stable Electron Arrangements

Atoms often become more stable by achieving a full outer electron shell.

For many elements, this means having 8 electrons in the outer shell.

This is sometimes called the octet rule.

The noble gases in Group 18 already have full outer shells.

For example:

• helium → 2
• neon → 2,8
• argon → 2,8,8

Because their outer shells are already full, noble gases are generally very unreactive.

---

Electron Arrangements and Ion Formation

Atoms can become more stable by losing or gaining electrons.

When this happens, the atom becomes an electrically charged particle called an ion.

Metals Usually Lose Electrons

Sodium has the electron arrangement:

2,8,1

It has one valence electron.

Sodium can lose this electron:

2,8,1 → 2,8

It now has 10 electrons but still has 11 protons.

The result is a +1 ion:

Na⁺

---

Non-Metals Usually Gain Electrons

Chlorine has the electron arrangement:

2,8,7

It needs one additional electron to complete its outer shell.

It can gain one electron:

2,8,7 → 2,8,8

The resulting ion has a −1 charge:

Cl⁻

Suggested placement: A sodium-to-chlorine electron-transfer diagram is useful here because students can visually see why Na forms Na⁺ and Cl forms Cl⁻.

---

Predicting Ion Charges from Groups

For many main-group elements, the group can help us predict the charge of the ion they commonly form.

Group 14 is less straightforward because losing or gaining four electrons is generally energetically difficult, so these elements commonly form covalent bonds instead of simple ions.

---

Using Position to Predict Electron Arrangement

Suppose an unfamiliar element is located in:

Period 3, Group 2

We can predict:

• Period 3 → 3 occupied electron shells
• Group 2 → 2 valence electrons

Its electron arrangement must therefore end with 2 electrons in its third shell.

For the Period 3 element in Group 2:

2,8,2

This element is magnesium (Mg).

We can also predict that magnesium is likely to lose its two valence electrons and form:

Mg²⁺

---

Drawing a Bohr Diagram

A Bohr diagram is a simplified model showing electrons arranged in shells around the nucleus.

To draw one using the periodic table:

Step 1 – Find the atomic number

For example, aluminium has atomic number 13.

Therefore, a neutral aluminium atom has 13 electrons.

Step 2 – Fill the first shell

Place 2 electrons in the first shell.

Remaining electrons:

13 − 2 = 11

Step 3 – Fill the second shell

Place 8 electrons in the second shell.

Remaining electrons:

11 − 8 = 3

Step 4 – Place the remaining electrons

Place the final 3 electrons in the third shell.

Electron arrangement:

2,8,3

Aluminium is therefore in:

• Period 3 because it has 3 occupied shells
• Group 13 because it has 3 valence electrons

---

Interpreting a Bohr Diagram

We can also work backwards.

Imagine a Bohr diagram shows:

2,8,6

Add the electrons:

2 + 8 + 6 = 16 electrons

For a neutral atom:

16 electrons → 16 protons → atomic number 16

Atomic number 16 is sulfur (S).

We can also determine:

• 3 occupied shells → Period 3
• 6 valence electrons → Group 16
• needs 2 electrons for a full outer shell
• likely ion → S²⁻

A simple electron diagram can therefore provide a surprising amount of information about an element.

---

Putting It All Together

Consider chlorine:

Atomic number: 17

From this one element we can connect several ideas:

Atomic number 17

↓

17 protons and 17 electrons

↓

Electron arrangement: 2,8,7

↓

3 occupied shells → Period 3

↓

7 valence electrons → Group 17

↓

Needs 1 electron for a full outer shell

↓

Usually forms Cl⁻

↓

Chemically reactive non-metal

This demonstrates why the periodic table is such a powerful tool for predicting atomic structure and chemical behaviour.

---

Did You Know?

The Bohr model is a simplified model of electron arrangement.

Electrons do not actually travel around the nucleus in fixed circular paths like planets orbiting the Sun. Modern atomic theory describes electrons using orbitals and probability distributions.

However, Bohr diagrams remain extremely useful for learning about electron shells, valence electrons, bonding, and periodic-table patterns.

---

Key Terms

Electron arrangement – The distribution of electrons among an atom's electron shells.

Electron shell – An energy level occupied by electrons around the nucleus.

Valence electron – An electron in the outermost occupied shell.

Bohr diagram – A simplified representation showing electrons arranged in shells around a nucleus.

Ion – An atom or group of atoms with an electrical charge caused by gaining or losing electrons.

Cation – A positively charged ion.

Anion – A negatively charged ion.

Octet rule – The tendency of many atoms to achieve eight electrons in their outer shell.

---

Key Takeaways

• The atomic number tells us how many electrons a neutral atom contains.
• Electrons occupy different electron shells.
• For the first 20 elements, simple arrangements such as 2,8,1 can be used.
• The period number tells us the number of occupied electron shells.
• The group number provides information about the number of valence electrons for main-group elements.
• Elements in the same group have similar chemical properties because they have similar valence-electron arrangements.
• Metals commonly lose electrons and form positive ions.
• Non-metals commonly gain electrons and form negative ions.
• Electron arrangements can help predict ion formation and chemical behaviour.
• Periodic-table information can be used to draw and interpret simple Bohr diagrams.