2.2 Electrons, bonding and structure Flashcards
How are electrons found in a nucleus?
Electrons orbit the nucleus and exist at certain energy levels, or shells
What is an orbital?
An orbital is a region of space where electrons may be found
How many electrons can an orbital hold?
Each atomic orbital can hold a maximum of two electrons in opposite spins
What is the shape of an S orbital?
Spherical
What is the shape of a P orbital?
Dumb-bell
What are the three main types of chemical bonds?
Ionic, covalent and metallic
What is an ionic bond?
An ionic bond is an electrostatic attraction between two oppositely charged ions
What do ionic bonds generally form between?
A metal and a non-metal
How are ions formed?
When electrons are transferred from one atom to another
The metal ion is positive
The non-metal ion is negative
What is the structure of a giant ionic lattice?
In a giant ionic lattice, each ion is surrounded by oppositely charged ions
These ions attract each other from all directions, forming a three-dimensioned giant ionic lattice
Why can ionic compounds conduct electricity when molten or dissolved but not when solid?
In a solid ionic lattice, the ions are held in fixed positions and they cannot move so the compound does not conduct electricity
However, when the compound is melted or dissolved, the solid lattice breaks down and the ions are free to move. The ionic compound can now conduct electricity
Why do ionic compounds have high melting and boiling points?
A large amount of energy is needed to break the strong electrostatic bonds that hold the oppositely charged ions together in the solid lattice- so ionic compounds have high melting and boiling points
Why are ionic compounds soluble in polar substances like water?
Water is polar so the molecules are attracted to the ions. The ions are pulled away from the lattice and cause it to dissolve
What is a covalent bond?
A covalent bond is an electrostatic force between the shared electrons and positive nucleus
What does a covalent bond form between?
Two non-metals
How does a covalent bond form?
- The negatively charged shared pair of electrons is attracted to the positive charges of both nuclei
- This attraction overcomes the repulsion between the two positively charged nuclei
- The resulting attraction is the covalent bond that holds the two atoms together
- The two electrons are shared
What is a dative covalent bond?
In a dative bond one of the atoms supplies both the shared electrons to the covalent bond
Is the octet rule always possible?
The octet rule is not always possible:
- There may not be enough electrons to reach an octet
- More than four electrons may pair up in bonding
What can the octet rule be modified to say?
- Unpaired electrons pair up
- The maximum number of electrons that can pair up is equivalent to the number of electrons in the outer shell
What are the two types of covalent structures?
- Simple molecular lattice
- Giant covalent lattice
In simple molecular structures, what bonds are present within the atoms in each molecule and in different molecules?
- The atoms within each molecule are held together by strong covalent bonds
- the different molecules are held together by weak intermolecular forces (Van der Waals or London forces)
Why do simple molecular structures have low melting and boiling points?
The intermolecular forces are weak so a small amount of energy is needed to break them
Can simple molecular structures conduct electricity?
They are non-conductors because there are no charged particles free to move
Are simple molecular structures soluble?
They are generally soluble in non-polar solvents such as hexane because the weak London forces are able to form between covalent molecules and these substances.
This helps the lattice to break down and the substance dissolves.
Name three examples of giant covalent structures
Diamond, graphite and silicon dioxide
Why do giant covalent structures have high melting and boiling points?
High temperatures are needed to break the strong covalent bonds within the lattice
Can giant covalent structures conduct electricity?
- Most giant covalent structures are non-conductors of electricity because there are no free charged particles
- Graphite is the exception because each carbon atom is only bonded to three others so delocalised electrons between the layers are able to move freely
Are giant covalent structures soluble?
-Insoluble in both polar and non-polar substances because the covalent bonds in the lattice are too strong to be broken by either polar or non-polar substances
What is a molecular orbital?
A molecular orbital is a region of space where an electron pair can be found.
This occurs when orbitals containing one electron overlap
What is the overlap dependent on?
- The same subshell (s/p/d/f)
- similar sized atomic radius
What is the electron pair repulsion theory?
The shape of a molecule or ion is determined by the number of electron pairs in the outer shell surrounding the central atom.
This is because electrons have a negative charge so each electron pair will repel other electron pairs. Therefore the shape adopted will be the shape that allows all the pairs of electrons to be as far apart as possible
Name the shape name and bond angle for each number of bonded electron pairs: 1- 2- 3- 4- 5- 6-
1-Linear 2-Linear 180 3-Trigonal planar 120 4-Tetrahedral 109.5 5-Trigonal bipyramidal 90 120 6-Octahedral 90
Why does a lone pair repel more than a bonded pair?
A lone pair of electrons is slightly more electron-dense than a bonded pair so it repels more
By how much does a lone pair reduce the bond angle?
2.5 degrees
This is a result of an extra repulsive effect of each lone pair
What is the name of an ammonia molecule and a water molecule?
Ammonia- pyramidal with bond angle 107
Water-non linear/ angular with bond angle 104.5
What is electronegativity?
The electronegativity is the ability of an atom to attract a pair of electrons in a covalent bond to itself
What is a polar molecule?
A polar molecule is one where there is a difference in electronegativity between the two elements so one gains a small negative charge and the other gains a small positive charge
What is the trend of electronegativity?
Electronegativity increases along the period and decreases down the group
What are the three things electronegativity is affected by?
- proton number
- shielding
- atomic radius
Why do some molecules have polar bonds but aren’t polar molecules?
- Non-symmetrical molecules with polar bonds will be polar molecules
- For molecules that are symmetrical, the dipoles may cancel out, making the molecule non-polar
What is a permanent dipole?
A permanent dipole is a small charge difference across a bond that results from a difference in the electronegativities of the bonded atoms
What is a permanent dipole-dipole induced interaction?
When a permanent dipole induces a dipole in a non-polar, nearby molecule. It is a weak attraction.
What is a permanent dipole-permanent dipole interaction?
When a molecule with a permanent dipole is attracted to other molecules with permanent dipoles
What are intermolecular forces?
Intermolecular forces occur due to constant random movements of the electrons within the shells of the atoms in molecules
What are the two types of intermolecular forces?
- hydrogen bonding
- van der Waals’ forces
What is an instantaneous dipole?
At any instant in time, it is possible that more electrons will lie to one side of the atom than the other
An instantaneous dipole is produced because electrons have a negative charge
What does an instantaneous dipole induce?
A weak dipole in nearly atoms/ molecules
What are London dispersion forces?
Attractive forces between induced dipoles in neighbouring molecules
What are London forces caused by?
- The constant random movement of electrons which unbalances the distribution of charge
- This causes an instantaneous dipole
- This induces a dipole in neighbouring molecules
- This causes weak intermolecular forces called London forces
What increases the size of London dispersion forces?
Increasing number of electrons. The greater the number of electrons, the larger the induced dipoles and the greater the attractive forces
Why does the boiling point increase as we move down the noble gas group?
As the number of electrons increases, so does the strength of van der Waals (London dispersion). Therefore more energy is needed to overcome these intermolecular forces.
What is a hydrogen bond?
A strong permanent dipole-permanent dipole attraction between an electron deficient hydrogen atom on one molecule and a lone pair of electrons on a highly electronegative atom (O, F, N) on a different molecule
How does a hydrogen bond form?
The electron-deficient H atom attracts a lone pair of electrons on a O,F or N atom bonded within a different molecule. This is a hydrogen bond.
Why does water have a high boiling and melting point?
The hydrogen bonds are much stronger than other intermolecular forces so that extra strength has to be overcome to melt/boil water
Why does ice float on water?
Water molecules arrange themselves into an orderly pattern and hydrogen bonds form between the molecules. Ice has an open lattice with hydrogen bonds holding the molecules apart
When ice melts the rigid hydrogen bonds collapse so the molecules are closer together
Therefore ice is less dense than water and floats on water
Why does water have high surface tension and viscosity?
Extra intermolecular bonding from the hydrogen bonds cause high surface tension and viscosity so when small insects walk on water, they are walking across a raft of hydrogen bonds
