Lewis diagrams — valence electrons & bonding
1.5b · The Periodic Table We are here
Lewis diagrams — valence electrons & bonding
Extension: not in the workbook
By the end of this section:
- I can give the number of valence electrons of any of the first 20 elements from its group number
- I can draw the Lewis dot diagram of an atom: the symbol with one dot for each valence electron
- I can explain why atoms with a full outer shell (the noble gases) do not form bonds
- I can work out how many electrons an atom loses or gains to become stable, and draw the Lewis diagram of the ion it forms
- I can show an ionic bond as a transfer of electrons from a metal to a non-metal, and write the formula of the compound formed
- I can recognise that two non-metals bond by sharing electrons (a covalent bond)
1 · Read
The notes, the key idea and the words to knowThe electrons in the outer shell are the valence electrons. They are the only electrons that take part in bonding, so a Lewis dot diagram shows just those: the element’s symbol with one dot for each valence electron around it.
For the first 20 elements the group number gives the count. Groups 1 and 2 have 1 and 2; groups 13 to 18 (3 to 8 in the short numbering) have 3 to 8. Helium is the exception: its outer shell is full with just 2.
| Group (short) | Valence electrons | Period 2 | Period 3 |
|---|---|---|---|
| 1 | 1 | Li | Na |
| 2 | 2 | Be | Mg |
| 13 (3) | 3 | B | Al |
| 14 (4) | 4 | C | Si |
| 15 (5) | 5 | N | P |
| 16 (6) | 6 | O | S |
| 17 (7) | 7 | F | Cl |
| 18 (8) | 8 | Ne | Ar |
How to draw one. Write the symbol. Put the first four dots one per side — right, top, left, bottom. Only then start pairing, in the same order. Nitrogen (5) is a pair on the right and a single on the other three sides; chlorine (7) is three pairs and one single.
- Find the outer electrons. Write the electron arrangement. The last number is the number of dots.
- Place the dots. Put the first four dots on the right, top, left and bottom, then pair them up in the same order.
- Make the ion. Metals lose their outer electrons, so the ion has no dots. Non-metals gain electrons up to eight. Draw square brackets and the charge.
- Helium, beryllium, magnesium and calcium: two single dots (right and top), not a pair.
- The dots say nothing about the inner shells. Sodium (2, 8, 1) and lithium (2, 1) both get one dot.
- Eight dots (two for helium) is a full outer shell. These atoms are stable and form no bonds: the noble gases.
Why the dots predict what an atom doesTap to open
An atom with a full outer shell is stable. Every other atom loses, gains or shares electrons until its outer shell is full like the nearest noble gas. The number it must lose or gain is its oxidation number.
- Metals (left of the table, 1 to 3 dots) lose their dots and become positive ions (cations): Na (1 dot) → Na+, Mg (2 dots) → Mg2+.
- Non-metals (right of the table, 5 to 7 dots) gain dots to reach 8 and become negative ions (anions): Cl (7 dots) → Cl− (8), O (6 dots) → O2− (8).
Ionic bond = a transfer. A metal gives its dots to a non-metal. Sodium’s one dot moves into chlorine’s empty slot: Na+ and Cl− form, opposite charges attract, and that attraction is the ionic bond. An ion is written in square brackets with its charge at the top right.
Formula of the compound. The charges must cancel. Mg2+ gives two electrons but each Cl takes only one, so one magnesium needs two chlorines: MgCl2. Short cut: cross the charges over. Al3+ and O2− give Al2O3; Ca2+ and O2− give Ca2O2, which simplifies to CaO.
| Metal | Non-metal | Ions | Formula |
|---|---|---|---|
| Na (1 dot) | Cl (7 dots) | Na+ and Cl− | NaCl |
| Mg (2) | Cl (7) | Mg2+ and 2 Cl− | MgCl2 |
| K (1) | O (6) | 2 K+ and O2− | K2O |
| Al (3) | O (6) | 2 Al3+ and 3 O2− | Al2O3 |
Sharing instead of giving: covalent bondsTap to open
Two non-metals both need electrons, so neither gives any away. They share: each atom puts one electron into a shared pair that counts for both. A bond made by sharing is a covalent bond, and the result is a molecule, not a pair of ions.
Chlorine (7 dots) shares one pair with another chlorine: Cl2, each atom now with 8. Hydrogen (1 dot) shares one pair: H2, each with 2, which is full for hydrogen. Two shared pairs make a double bond (O2), three a triple bond (N2).
In a dot-and-cross diagram one atom’s electrons are drawn as dots and the other’s as crosses, so you can see where each shared electron came from. The booklet’s extension page (p. 57) asks for these for H2O, CH4, NH3 and HI.
- Metal + non-metal → transfer → ionic (NaCl, MgCl2).
- Non-metal + non-metal → sharing → covalent (H2, Cl2, HCl, H2O).
-
Not thisThe dots show all the atom’s electrons.
ThisOnly the outer-shell (valence) electrons. Sodium has 11 electrons and one dot.
-
Not thisPair the dots up as you go.
ThisOne dot per side first (right, top, left, bottom), then pair. Carbon is four singles; oxygen is two pairs and two singles.
-
Not thisA positive ion has gained something.
ThisA positive ion has lost electrons. Na loses its one dot and becomes Na+, with no dots left to draw.
-
Not thisSodium and chlorine make a molecule, NaCl.
ThisThey make ions, Na+ and Cl−, held together in a giant lattice. Molecules come from sharing, between two non-metals.
-
Not thisA group 1 metal has one dot, so it gains seven more.
ThisLosing one is far easier than gaining seven. Metals lose their few dots; non-metals gain the few they are short of.
- valence electron
- an electron in the outer shell; the ones that take part in bondingFor example : chlorine (2, 8, 7) has 7Not : the 2 and the 8 underneath — inner shells, never drawn
- Lewis dot diagram
- the symbol with one dot per valence electron around itFor example : carbon: C with four single dotsNot : a Bohr diagram — that draws every shell
- full outer shell (stable)
- 8 valence electrons (2 for helium); no reason to lose, gain or shareFor example : neon and argon, 8 dots eachNot : sodium’s 2, 8, 1 — the 8 is an inner shell, the outer has 1
- ion (cation / anion)
- an atom that has lost electrons (positive, cation) or gained them (negative, anion)For example : Na+, Mg2+, Cl−, O2−Not : an atom that lost a proton — that would be a different element
- ionic bond (electron transfer)
- a metal gives electrons to a non-metal; the opposite charges attractFor example : NaCl, MgCl2, CaONot : H2O — two non-metals, nothing is given away
- covalent bond (electron sharing)
- two non-metals share a pair of electrons that counts for bothFor example : H2, Cl2, HCl, H2O, CH4Not : NaCl — a metal is involved, so it is a transfer
- oxidation number
- how many electrons an atom must lose (+) or gain (−) to reach a full outer shellFor example : Mg +2, Al +3, Cl −1, O −2Not : the group number itself — chlorine is in group 17 but its oxidation number is −1
2 · Watch
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- In your class workbook: Chapter 1.4 The Periodic Table of the elements (pp. 27–31) · Revise this section
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