1.5 Solid structures Flashcards
What is an ionic solid?
Ionic solids are made up of oppositely charged ions packed around each other. This maximises electrostatic attraction between the oppositely charged ions and minimises repulsion between ions with the same charge, ensuring maximum bond energy.
What is a coordination number?
The coordination number of an ion gives the number of its nearest neighbours.
What is the coordination number of sodium chloride for Na+ and Cl- and their ratio?
Both have a coordination number of 6
thus ratio of 6:6
What is the coordination number of caesium chloride for Cs+ and Cl- and their ratio?
Both have a coordination number of 8
ratio is 8:8
What is the reason that the coordination number of caesium chloride is larger than sodium chloride?
Because caesium cation is larger than the sodium cation and can can thus fit more chlorine anions around it.
Discuss the melting point of ionic solids(crystals) :
They have high melting points and boiling points due to the strong electrostatic attraction between the oppositely charged ions, which requires a lot of energy to overcome.
Discuss the solubility of an ionic solid:
They are often soluble in water. Water molecules are polar as the oxygen has a partially negative charge and the hydrogen a partial positive charge. In solution the partial negative charge on the oxygen is attracted to the positive ions, and the partial positive charge on the hydrogens are attracted to the negative ions.
Discuss the strength of an ionic solid (crystal) :
Hard but brittle. When enough force is applied, layers of ions slide over eachother causing ions of the same charge to be next to eachother. The ions repel eachother and the crystal shatters.
Discuss the electrical conductivity of an ionic solid:
They are poor electrical conductors when solid, but good when molten or dissolved. This is because an electrical current can flow through if the particles are able to move when a potential difference is applied
What are giant covalent structures?
Giant covalent structures are made up of atoms that form multiple covalent bonds to other atoms, forming a giant lattice structure.
Diamond and graphite are two forms of elemental carbon (known as allotropes).
How is diamond bonded?
In diamond, each carbon bonds strongly to four other carbon atoms in a tetrahedral arrangement to form a giant 3D structure.
How is graphite bonded?
Although carbon can form four covalent bonds, in graphite only three bonds are made by each carbon atom. Hexagonal layers (only one atom thick) are formed, which are held together by weak van der Waals forces.
name properties that diamond and graphite both have:
- They have high melting points and boiling points as each carbon atom has three or four strong covalent bonds, which require a lot of heat energy to overcome.
- They are insoluble in water as there are no charged particles capable of interacting with the permanent dipole of water molecules.
Name some of diamonds properties:
- It is very hard (i.e., difficult to scratch) due to each carbon atom being bonded to four others, with strong covalent bonds. This holds the atoms together in a rigid 3-dimensional structure.
- It is an electrical insulator, due to there being no delocalised electrons within the structure.
what are some properties of graphite?
- It is soft and slippery due to the layers of carbon atoms (which are only attracted to each other with weak intermolecular forces) being able to slide over each other easily.
- It is an electrical conductor as there is one non-bonding electron in the valence shell of each carbon atom and these become delocalised between the layers.
what are simple covalent structures?
Simple covalent structures consist of simple covalent molecules held together in a lattice structure by weak intermolecular forces.
What are simple molecular solids?
Simple molecular solids have covalent bonds within molecules held together by weak intermolecular forces.
What are some physical properties of simple molecular solids?
- Low melting and boiling points. Although the covalent bonds within the molecules are strong, the intermolecular forces holding the molecules together are weak and do not need much energy to break.
- Soft. The weak intermolecular forces between the molecules are easily broken.
- Normally insoluble in water. There are ions to attract the polar water molecules. However compounds that can form hydrogen bonds with water are soluble.
- poor conductors of electricity as they do not contain delocalised electrons or ions.
Give two examples of simple molecular solids:
Iodine- In iodine, the iodine molecules (I2) are held together by van der Waals forces. The van der Waals forces between I2 molecules are relatively strong compared to those between Cl2 molecules. This is because the strength of the van der Waals force increases with molecular size.
Ice- In ice, the H2O molecules are held together by hydrogen bonding between the partially positive hydrogen (Hẟ+) atoms and the lone electron pairs on the oxygen atoms. These strong intermolecular forces mean that the H2O molecules in ice form a tetrahedral structure that is both rigid and spaced out. Due to this unusual solid structure, ice is less dense and takes up a greater volume than liquid water at 0 degrees celcius.
Describe a metallic structure:
Metallic structures have a regular arrangement of metal cations (positively charged ions) closely packed together and surrounded by a ‘sea’ of delocalised electrons.
The ‘sea’ of electrons is formed because the valence electrons (i.e., outer electrons) are so weakly bound to their atoms that these electrons can move freely throughout the lattice of metal ions.
The metal structure is held together by the strong electrostatic attraction between the metal cations and the delocalised electrons.
All metals have similar properties. These properties include what?
- They are good conductors of heat and electricity, as the delocalised electrons can carry energy (either thermal or electrical) through the lattice of metal ions.
- They are malleable (able to change shape permanently without breaking), as the layers of metal cations can easily slide over each other and the delocalised electrons move with the ions to maintain the metallic bonding.
- The melting and boiling points of metals depend on the number of delocalised electrons per atom. Generally, the more delocalised electrons per atom, the higher the melting and boiling points.
This also applies to the hardness of the metal. Generally, the more delocalised electrons per atom, the harder the metal.
