Why and how atoms combine - Lewis diagrams and the Octet Rule - Ionic and covalent bonding

Oct 26, 2026

Atoms combine through valence electrons in the outer shell, which can be represented using Lewis diagrams. The document explains how to draw Lewis diagrams for elements and how valence electrons relate to group numbers, including exceptions such as helium. It then introduces the octet rule, ion formation by gaining or losing electrons to reach noble-gas configurations, and compares ionic versus covalent bonding, emphasizing electron transfer versus sharing. Practice questions include identifying ionic/covalent compounds and completing bond diagrams using multiple methods.

LanguageEnglish

CategoryHealthcare

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Reading Time10 min

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Why and how atoms combine

Lewis diagrams

The Octet Rule

The Octet rule and ion formation

Types of bonds

Formation of ions

Ionic bonding continued

Covalent bonding

Why and how atoms combine Lewis diagrams When atoms combine, only electrons in the outer (valence) shell are involved. We can represent these valence electrons with Lewis diagrams. Lewis diagrams for the first 20 elements are shown below. | IA | IIA | IIIA | IVA | VA | VIA | VIIA | 0 | | --- | --- | --- | --- | --- | --- | --- | --- | | H | | | | | | | He | | Li |
Be |
B |
C
|
N
|
O
|
F
|
Ne
| | Na |
Mg |
Al |
Si
|
P
|
S
|
Cl
|
Ar
| | K |
Ca | | | | | | | To write Lewis diagrams: 1. Write the element symbol. Around this draw dots – one for each valence electron. 2. The dots should be spread over four sides. Dots are not paired until all sides have at least one dot. 3. It does not matter on which side dots are placed. For example, hydrogen can be drawn four ways: H = H = H = H 4. The number of valence electrons is equal to the group number. For example, hydrogen is in group IA (group 1) and it has one valence electron. Neon is in 0 (group 8) and it has 8 valence electrons. The only exception is He which is in group 8 but has 2 valence electrons. Q1 – Write Lewis diagrams for a) Ne, b) Sb (Z=51), c) Rb, d) a neutral atom with 9 total electrons Q2 – Write all possible variations of the Lewis diagram for phosphorus. The Octet Rule The noble gasses do not react with other elements. In other words, noble gasses are very stable. Let’s look at the electron configuration of noble gasses (note: K is the name given to shell 1, L to 2, etc.) | Element | Shell | | | | | | | --- | --- | --- | --- | --- | --- | --- | | | K | L | M | N | O | P | | He | 2
| | | | | | | Ne | 2 | 8
| | | | | | Ar | 2 | 8 | 8
| | | | | Kr | 2 | 8 | 18 | 8
| | | | Xe | 2 | 8 | 18 | 18 | 8
| | | Rn | 2 | 8 | 18 | 32 | 18 | 8
| Most Noble gasses have 8 electrons in their outer shell. This is a stable conformation. The Octet Rule When atoms form ions or combine in compounds they obtain electron configurations of the nearest noble gas (usually this means that there will be 8 outer electrons) . The Octet rule and ion formation As stated, when atoms form ions they seek an electron configuration like that of the nearest noble gas. Nearest refers to the number of representative elements between an element and a noble gas. For example Br is 1 space removed from Kr, but 7 spaces removed from Ar (see your periodic table) . So, Br would gain 1 electron to form Br–. Similarly, Mg would lose 2 electrons to become Mg2+ . | Q3–Complete the chart: | Br | P | Ne | Al | Ca | | --- | --- | --- | --- | --- | --- | | Nearest Noble gas (spaces removed) | Kr (1) | | | | | | Resulting ion | Br– | | | | | Group IVA elements are equal distances from two noble gasses. These elements tend not to form ions. Types of bonds There are two types of bonds: ionic and covalent. Ionic bonding occurs between metals and non-metals. Covalent bonding occurs between two non-metals. (When two metals combine an alloy is formed. Alloys are unusual substances; we will not concern ourselves with bonding in alloys) Na + Cl = Ionic (metal) (non metal) H + O = Covalent (non metal) (non metal) Q4 – Identify each compound as ionic or covalent: MgO, CaCl2, SO2, PbCl2, CCl4, CH4 Formation of ions Ionic bonding involves the formation of ions: metals lose electrons to become positive ions, non-metals gain electrons to become negative ions. This can be shown with Bohr-Rutherford diagrams and Lewis diagrams: 23 11p+ 12n0 2e- 8e- 1e- Na loses an electron to become Na+ . Notice that its electron configuration resembles Ne (the nearest noble gas) . This follows the octet rule. Na [ Na ]+ Cl gains an electron to become Cl- . Notice that its electron configuration resembles Ar (the nearest noble gas) . This follows the octet rule. [ Cl ] - or, simply [ Cl ] - Ionic bonding

FAQs

What part of an atom is involved when atoms combine?

Only electrons in the valence (outer) shell are involved. Lewis diagrams are used to represent these valence electrons.

How do you determine the number of valence electrons for a main-group element?

The number of valence electrons equals the group number. The exception noted is helium, which is in group 8 but has 2 valence electrons.

What is the difference between ionic and covalent bonding?

Ionic bonding occurs between metals and non-metals, with electrons transferred to form ions. Covalent bonding occurs between two non-metals, with electrons shared while still following the octet rule.

Oct 26, 2026

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