Structure and bonding (II)
YOU SHOULD BE ABLE TO:
■ Explain metallic bonding using the terms ‘cation’ and ‘mobile electrons’.
■ Define and give examples of ionic crystals, simple molecular and giant molecular crystals.
■ Distinguish between ionic and molecular solids.
QUICK POINT
Ionic solids can only conduct electricity when molten, as only then are the ions free enough to move.
The type of bonding present in substances influences physical properties, such as melting and boiling points, thermal and electrical conductivity, physical state at room temperature and solubility in solvents.
Solids can be divided into the following groups: ionic crystals, molecular (simple and giant) and metallic crystals.
Ionic compounds are crystalline solids able to conduct electricity when molten, due to the movement of ions which can carry an electric current, but not in the solid state. These compounds also have high melting and boiling temperatures due to the strength of the attractive forces between the ions which holds the crystal together in a regular three-dimensional framework.
Simple molecular crystals have low melting and boiling temperatures due to weak forces of attraction between molecules. Iodine is an example of a simple molecular structure.
Giant structures of atoms and molecules, such as silicon dioxide (silica), diamond and graphite, have very high melting and boiling points due to strong covalent bonds throughout their three-dimensional network.
In metallic crystals, the outer electrons of each atom are mobile or delocalised (that is, they do not belong to any particular cation) and come together to form a ‘band’ or ‘sea’ of electrons. These electrons will bind to the cations formed from the electron loss to form a strong bond. In this way, metals are able to conduct heat and electricity since the mobile electrons can move throughout the metal.
The strong bonds between the cations and electrons means that they are hard to break, thus, metals have high melting and boiling points. Metals are also solids (except mercury) and are malleable and ductile.
QUICK CHECK
■ Explain why simple molecules have low melting and boiling points.
■ Why do metals conduct heat and electricity?
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At the end of the day’s event, Company Director of Chippie’s Arty Grant said, “Honestly, I was pleasantly surprised that the current generation appeared as in love with the product as my generations and other past generations. A core value of the company, health or otherwise, is to provide a quality product; that quality is based on consumer needs that will continue to change for the better. Working with Youthlink was a pleasure, as I loved the energy and the willingness to be flexible by the Youthlink team, which made the event a success.”
After the pop-up sales, Youthlink met with some of the sixth-formers to learn a little more of them. These students were Tarjmaar Miller, Jumaane Shakespeare and Fabian Morris. Miller manages to balance sports and academics; he is on the track and field team and manages to keep his average above standard. He is also the school captain. Shakespeare is the first vice-captain. He maintains an average of above 80 per cent and is also the captain of the quiz team. Morris is the current winner of the Rita Marley Public Speaking Competition and the outgoing president of the nation’s premiere advocacy group for the National Secondary Students’ Council. He also received one of the highest awards at the United Nations Association of Jamaica summit at the United Nations.