giant structures Flashcards
What are the physical properties of metals?
- High melting temperatures
- Good electrical conductivity
- Good thermal conductivity
- Malleability
- Ductility
Describe the structure of metals
An array of atoms with at least some of their outer shell electrons removed and free to move through the structure. Free electrons are called delocalised electrons and they are free to move throughout the structure. There are electrostatic forces of attraction between the nuclei of the metal cations and the delocalised electrons (metallic bonding)
Is the melting temperature of metals high or low? Why?
Metals have a giant lattice structure so there are many forces of attraction between the nuclei of the cations and the delocalised electrons that need to be overcome. The energy required to do this is very large
What are the two features of metals that affect melting temperature?
- Number of delocalised electrons per cation
- Size of the cation
How does the number of delocalised electrons per cation affect melting temperature?
Group 1 metals have low melting temperatures
Group 2 metals have higher melting temperatures
Metals in the d-block have high melting temperatures since they have more delocalised electrons per ion
How does the size of the cation affect melting temperature?
The smaller the cation, the closer the delocalised electrons to the cation’s nucleus, resulting in an increase in the forces of attraction between the nuclei and the delocalised electrons and so an increase in melting temperature
Do metals conduct electricity? Explain answer
When a potential difference is applied across the ends of a metal, delocalised electrons are attracted to and move towards the positive terminal of the cell. Movement of electrical charge constitutes an electric current
What two factors contribute to metals thermal conductivity?
- Free-moving delocalised electrons pass kinetic energy along the metal
- Cations are closely packed and pass kinetic energy from one cation to another
What is malleability?
The ability of materials to be hammered or pressed into different shapes
What is ductility?
The ability of materials to be drawn into a wire
Are metals malleable and ductile? Why?
Metals can be hammered or pressed into different shapes (malleability) and can be drawn into a wire (ductility). This is because when stress is applied to a metal, the layers of cations slide over one another. As the delocalised electrons are free moving, they move with the cations and prevent strong forces of repulsion between cations in one layer and cations in another layer
Where does ionic bonding occur? Give an example
In solid materials consisting of a regular array of oppositely charged ions extending through a giant lattice network. For example, NaCl consists of a regular array of Na+ and Cl-
Describe the structure and bonding in ionic solids
There are strong electrostatic interactions between ions. Ions are arranged so that the electrostatic attractions between oppositely charged ions are greater than the electrostatic repulsions between ions with the same charge
How do you determine the strength of ionic bonding?
Calculate the amount of energy required in one mole of solid to separate the ions in the gas phase
What factor affects the strength of ionic bonding?
Size of the cations as they determine how closely packed they are in the lattice
What is the trend in ionic radii down a group?
As you go down the groups, the ions have more electron shells; therefore, the ions get larger
What is the trend in ionic radii across a period?
The ionic radius decreases as the number of protons increases. As the positive charge of the nucleus increases, the electrons are attracted more strongly and are therefore pulled closer to the nucleus
What are the physical properties of ionic compounds?
- High melting temperature
- Brittleness
- Poor electrical conductivity when solid but good when molten
- Often soluble in water
Why do ionic solids have high melting temperatures?
Ionic solids are giant lattice networks of oppositely charged ions. There are many ions in the lattice and the combined electrostatic forces of attraction among all the ions is large so lots of energy is required to overcome the forces of attraction
Why are ionic solids brittle?
If stress is applied to a crystal of an ionic solid, the layers of ions slide over one another. Ions of the same charge end up side by side and so repel one another, causing the crystal to break apart
Do ionic compounds conduct electricity when solid?
No, because there are no delocalised electrons and the ions are not free to move under the influence of a potential difference
Do ionic compounds conduct electricity when molten? Why?
Yes, because when molten, the ions are mobile and so will migrate to the electrodes of opposite sign when a potential difference is applied. If direct current is used, the compound undergoes electrolysis as the ions are discharged at the electrodes
What ionic compound can conduct electricity when solid?
Lithium nitride (Li3N) so is used in batteries
Are ionic compounds soluble? Why?
Many are soluble in water - because the energy required to separate ions can be supplied by the hydration of the separated ions produced
What is the equation for the burning of sodium in chlorine?
2Na (s) + Cl2 (g) —> 2NaCl (s)
What is the equation for the formation of magnesium oxide?
2Mg (s) + O2 (g) —> 2MgO (s)
What evidence is there for the existence of ions?
The ability of an ionic compound to conduct electricity and undergo electrolysis when molten or in aqueous solution
What atoms is a covalent bond formed between?
Formed between two atoms when an atomic orbital containing a single electron from one atom overlaps with an atomic orbital, also containing a single electron, of another atom
What causes a sigma bond to form? What does it lead to the formation of?
An end-on overlap. Occurs between 2 s orbitals and 2 p orbitals (when they are horizontal). Leads to the formation of a single covalent bonds between the two atoms
What causes a pi bond to form? What does it lead to the formation of?
A sideways overlap of two p-orbitals (vertical). Pi bonds cannot form until a sigma bond has formed, so pi bonds inly exist between atoms that are joined by double or triple bonds
What happens when two hydrogen atoms bond?
The two s orbitals overlap to form a new molecular orbital. The two electrons then exist in this new orbital. It forms a sigma bond
What happens when two chlorine atoms bond?
The two p orbitals (each containing a single electron) overlap, forming a sigma bond
Give an example of when a pi bond is formed
Ethene - one of the bonds between the carbon atoms is a sigma bond, the other is a pi bond
Nitrogen - the triple bond is made up of one sigma bond and two pi bonds
What is bond length?
The distance between nuclei of the two atoms that are covalently bonded together
How is the strength of a covalent bond measured?
The amount of energy required to break one mole of the bond in the gaseous state
What is the relationship between bond length and bond strength in covalent molecules?
The shorter the bond, the greater the bond strength. This is a result of an increase in electrostatic attraction between the two nuclei and the electrons in the overlapping atomic orbitals
What is electronegativity?
The ability of an atom to attract a bonding pair of electrons
What is the trend in electronegativity down a group?
Decreases
What is the trend in electronegativity across a period?
Increases
What is the distribution of electron density of two atoms of the same element?
Symmetrical because the ability of each atom to attract the bonding pair of electrons is identical
What is the distribution of electron density in a polar covalent bond?
Distribution will not be symmetrical about the two nuclei
What is a polar covalent bond?
A type of covalent bond between two atoms where the bonding electrons are unequally distributed. Because of this, one atom carries a slight negative charge and the other a slight positive charge
