Dot-and-cross diagrams

Alkanes and alkenes are hydrocarbons. They are composed of molecules which contain covalent bonds. For each of the molecules below, use a dot and cross diagram to show the bonding it contains, a Methane, CH4. 

A Draw dot and cross diagrams to represent electrons to show how you would expect atoms for the following elements to combine, what ions would be produced, and what the formulae of the resulting compounds would be. 

This type of diagram is usually called a dot and cross diagram because the electrons from the different atoms are shovm as dots and crosses (although, of course, there is no real difference between the electrons of different atoms). 

Draw dot and cross diagrams to show the electron distribution in the following ... 

It would be rather inconvenient to have to draw a dot and cross diagram every time to describe the bonding in molecules, so a kind of shorthand is used. We simply use a line joining the two atoms to represent a shared pair of electrons or single covalent bond. The line represents one electron from each atom. 

Covalent bonding was previously described (Chapter 4) by means of dot-and-cross diagrams where the electron pairs making the covalent bonds are represented by dots and crosses. A single bond is a shared pair of electrons, a double bond is two shared pairs of electrons and a triple bond is three shared pairs. A simple electrostatic model was used to describe molecules. 

Urey was awarded the 1934 Nobel Prize in Chemistry for his discovery of heavy hydrogen . Lewis - who had been Urey s PhD supervisor - won nothing At this, Lewis stopped work on heavy water. Despite his work on heavy water, not to mention inventing the covalent bond (plus dot-and-cross diagrams), coming up with... 

You will notice that in Figure 4.2 we used dots and crosses to show the electronic configuration of the chloride and sodium ions. This helps us keep track of where the electrons have come from. It does not mean that the electron transferred is any different from the others. Diagrams like this are called dot-and-cross diagrams. 

When drawing a dot-and-cross diagram for an ionic compound it is usually acceptable to draw the outer electron shell of the metal ion without any electrons. This is because it has transferred these electrons to the negative ion. Figure 4.4 shows the outer shell dot-and-cross diagram for sodium chloride. 

Draw dot-and-cross diagrams for the ions in the following ionic compounds. Show only the outer electron shells, a Potassium chloride, KCl b Sodium oxide, Na O c Calcium oxide, CaO d Magnesium chloride, MgClj... 

Some examples of dot-and-cross diagrams for simple covalently bonded molecules are shown in Figure 4.9. 

Figure 4.9 Dot-and-cross diagrams for some covalent compounds a hydrogen, H, b methane, CH jC water, H O, d ammonia, NHj, and e hydrogen chloride, HCl.

Figure 4.10 Dot-and-cross diagrams for a boron trifluoride, BF3, andb sulfur hexafluoride, SFg.

Some atoms can bond together by sharing two pairs of electrons. We call this a double covalent bond. A double covalent bond is represented by a double line between the atoms for example, 0=0. The dot-and-cross diagrams for oxygen, carbon dioxide and ethene, all of which have double covalent bonds, are shown in Figure 4.11. 

Figure 4.11 Dot-and-cross diagrams for a oxygen, Oj, b carbon dioxide, COj, and c ethene, C H. ...

Atoms can also bond together by sharing three pairs of electrons. We call this a triple covalent bond. Figure 4.12 shows a dot-and-cross diagram for the triple-bonded nitrogen molecule. 

Figure 4.14 A dot-and-cross diagram for an aluminium chloride molecule, Al Clg.

Dot-and-cross diagrams can be drawn to show the arrangement of electrons in ionic and covalent compounds. 

Explain why solid sodium conducts electricity but solid sodium chloride does not conduct electricity. [2] Draw a dot-and-cross diagram forcarbon dioxide. [1]... 

Figure 13.4 A dot-and-cross diagram showing the covalent bonding in an ammonia molecule and its pyramidal shape.

DNA (deoxyribonucleic acid) a polymer with a double helical structure containing two sugar-phosphate chains with nitrogenous bases attached to them. The sequence of bases forms a code, which is used to form more DNA by replication or to encode mRNA (transcription), dot-and-cross diagram a diagram showing the... 

Dot-and-cross diagrams, structure and bonding and examples of shapes of molecules using the ideas of electron-pair repulsion. 

---