Structure and Bonding Flashcards
What are some of the physical properties of metals?
- high melting and boiling points
- good electrical conductivity
- good thermal conductivity
- malleability
- ductility
Describe the structure of a metal
Metals consist of a giant metallic lattice of positive ions (which form regular layers), surrounded by a ‘sea’ of delocalised electrons.
What is metallic bonding?
Metallic bonding is the electrostatic force of attraction between positive ions and delocalised electrons within a giant metallic lattice.
Why do metals have high melting points?
In order to melt a metal, you have to overcome many strong forces of attraction between the nuclei of cations and delocalised electrons, so that the cations can move around. Lots of energy is required to break this many metallic bonds, so melting temperatures are very high.
Which factors affect the melting point of a metal?
The number of delocalised electrons - the more delocalised electrons there are, the stronger the electrostatic attraction between positive ions and the electrons, and thus, the stronger the metallic bonds. Also, ionic radius has an effect as smaller ions allow delocalised electrons to get closer to the nucleus, increasing the attraction between them and strengthening the metallic bonds. More energy is needed to break stronger bonds.
Why are metals good conductors of electricity?
The delocalised electrons in the giant metallic lattice can move throughout the structure when a potential difference is applied, which allows an electric current to flow.
Which factors affect the electrical conductivity of a metal?
The greater the number of valence electrons in the outer shell of a metal atom, the more delocalised electrons, and therefore the more charge carriers, there are. The more charge carriers there are, the more current can flow through the metal, increasing the electrical conductivity.
Why are metals thermally conductive?
Metals have high thermal conductivity because the cations are closely packed and can pass kinetic energy from one cation to another by vibrations. Also, the free-moving delocalised electrons can pass kinetic energy along the metal.
Why are metals malleable and ductile?
Metals can be hammered and pressed without shattering (malleability) and drawn into a wire (ductility). This is because metals form regular layers of cations, which are able to slide over one another when a force is applied to one section of the metal disproportionately to another section. The electrostatic forces between the cations and delocalised electrons hold the ions together, and prevent the metal from breaking when hammered.
What is ionic bonding?
Ionic bonding is the strong electrostatic attraction between oppositely charged ions.
What is the structure of an ionic compound?
Ionic compounds form giant ionic lattices, which are regular, repeating patterns of oppositely charged ions (e.g. NaCl consists of rows and columns of alternating Na+ and Cl- ions).
Does the presence of ions in a compound guarantee the presence of ionic bonds?
While the presence of ions often means ionic bonds are dominant, this is not always the case as there can be significant covalent interactions as well, meaning pure ionic bonding is an idealised situation which rarely occurs perfectly due to electronegativity differences between atoms.
How do you work out the strength of an ionic bond?
The strength of an ionic bond is determined by the amount of energy required in one mole of solid to separate the ions to infinity (in the gas phase). When they are at infinite distances from each other, ions cannot interact.
What impact does ionic radius have on the strength of ionic bonding?
The smaller the ionic radii of the bonded ions, the more energy is required to separate them as they are closer together, so experience stronger electrostatic forces between them.
What impact does the ionic charge have on the strength of ionic bonding?
The stronger the charge on the ions involved, the stronger the ionic bond is as the ions have greater electrostatic forces of attraction between them, which require more energy to break.
What are isoelectronic ions?
Ions with the same number of electrons and thus the same electronic configuration (thus will react similarly). e.g. N3-, O2-, F-, Na+, Mg2+ and Al3+ are all isoelectronic.
What is the trend in ionic radii for isoelectronic ions and why?
The ionic radius decreases as as the number of protons increases for isoelectronic ions. This is because the greater the number of protons, the stronger the nuclear charge and thus the stronger the attraction of the outer electrons to the nucleus. This draws the shells inwards, decreasing the ionic radius.
What are some of the physical properties of ionic compounds?
- fairly high melting and boiling temperatures
- brittleness
- poor electrical conductivity when solid but good when molten
- often soluble in water
Why do ionic compounds have high melting and boiling temperatures?
Ionic solids consist of many oppositely charged ions, which form a giant lattice structure. The combined electrostatic forces of attraction between these ions are very strong, and so require lots of energy to break.
Why are ionic compounds brittle?
If a stress is applied to a crystal of an ionic solid, layers of ions may slide over one another, but this will cause ions of the same charge to be near each other, so they will repel, causing the crystals to break apart in layers.
Why do solid ionic compounds not conduct electricity but molten or aqueous ones do?
In solid ionic compounds, there are no delocalised electrons or free ions, so they can’t move under a potential difference. The lack of charge carriers means a current can’t flow through an ionic solid, so electricity can’t be conducted. However, in molten and aqueous compounds, the ions are free to move and carry charge, so electricity can be conducted.
Why are ionic compounds generally soluble in water?
The energy required to break apart the lattice and separate the ions can sometimes be supplied by the hydration of the separated ions produced. Both positive and negative ions are attracted to water molecules because of the polarity that water molecules have.
How can cations and anions be formed?
Cations are formed by the loss of outer electrons to form a full outer shell and anions are formed by the gain of electrons to the outer shell, to fill it up. This can be represented by dot and cross diagrams (remember to use square brackets around ions and to write the charge - also complex ions (e.g. NO32-) are covalently bonded together, but will bond to cations ionically).
What is the evidence for the existence of ions?
Electrolysis can be used to prove the existence of ions. When a direct current is applied to a molten or aqueous ionic compound, the cations migrate to the cathode and the anions migrate to the anode, as opposite charges attract. Here they are oxidised/reduced to atoms. This can be shown with aqueous copper(II)chromate(VI) solution, as aqueous copper(II) ions are blue and aqueous chromate(VI) ions are yellow. The Cu2+ ions migrate to the cathode (-ve electrode) and the solution around this electrode turns blue. The CrO4- ions migrate to the anode (+ve electrode) and the solution around this electrode turns yellow.
What is the trend in ionic radii down a group?
Ionic radii increase down a group as the shell number and shielding increases, meaning the attraction of the electrons to the nucleus is weaker and so the electrons are held further out from the nucleus, increasing the ionic radius. This outweighs the increase in nuclear charge.
What is a covalent bond?
A covalent bond is the strong electrostatic attraction between the nuclei of two atoms and the bonding pair of electrons.
How can covalent bonds be formed?
Covalent bonds are formed by the overlap of two atomic orbitals each containing a single electron.
What are the two types of covalent bond and how do they form?
An end on overlap of orbitals leads to the formation of a sigma bond (this can occur between two s-orbitals p, two p-orbitals or an s-orbital and a p-orbital (though an s and a p interaction can only occur between different elements, which leads to bond polarity). The second type of covalent bond is the pi bond, which can only form upon the sideways overlap of two p-orbitals (pi bonds can’t form between s orbitals as they are spherical, so they will only ever have one area of overlap, unlike the pi bond, which requires two areas of overlap, achieved by the p orbitals’ dumbbell shape).
How do pi bonds form?
Pi bonds can only form after the formation of a sigma bond. As a result of this, pi bonds only exist between atoms that are joined by double or triple covalent bonds.
Which area of the molecule has the highest electron density for each type of covalent bond?
Sigma bonds lead to high electron density in the middle of the molecule, (between the two nuclei) as this is where the bonding pair of electrons is held. Pi bonds lead to high electron density both above and below the molecule as the sideways overlap of orbitals means that electrons are held at opposite sides of the molecule.
What is the bond length of a covalent bond?
The bond length is the distance between the nuclei of the two atoms that are covalently bonded together.
How is the strength of a covalent bond measured?
The strength of a covalent bond is determined by the amount of energy required to break one mole of the bond in a gaseous state.
What is the relationship between bond length and bond strength for covalent bonds?
In general, for bonds that are of a similar nature, 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 (bonding pairs).
What is electronegativity?
The ability of an atom to attract a bonding pair of electrons.
What is the general trend in electronegativity down groups and across periods?
In general, electronegativity decreases down a group and increases from left to right across a period.
What is the scale of electronegativity used at A Level?
The Pauling electronegativities are used at A Level, at which Fluorine, the most electronegative element, is given a value of 4.0 and Caesium, the least electronegative element is given a value of 0.7. The more electronegative an element is, the greater the number of its Pauling electronegativity.
