1c Flashcards
Ions
Ions are charged particles. They can be single atoms (e.g. Na+) or groups of atoms (e.g. NO3-). Ions form when atoms lose or gain electrons.
The number of electrons lost or gained is the same as the charge on the ion. When atoms lose or gain electrons to form ions, they end up with full outer shells - this makes the ions very stable.
cations
Positive ions (cations) form when atoms lose electrons - they have more protons than electrons.
anions
Negative ions (anions) form when atoms gain electrons - they have more electrons than protons.
ionic bonding
When metals react with non-metals, electrons are transferred from the metal atoms to the non-metal atoms. The metal atoms lose electrons to become positively charged ions (cations) with a full outer shell of electrons. The non-metal atoms gain electrons and become negatively charged ions (anions) with a full outer shell of electrons.
The oppositely charged ions are strongly attracted to each other, and this strong electrostatic attraction holds the ions together in the ionic compound. This is known as ionic bonding.
Dot and cross diagrams
Dot and cross diagrams are used to show what happens during ionic bonding. The electrons in one type of atom are represented by dots, and the electrons in the other type of atom are represented by crosses. This means you can tell which atom the electrons in an ion originally came from. To show the charge on each ion, you use a big square bracket and a + or −.
Arrangment of ions in ionic bond NaCl
Ionic compounds have very strong electrostatic forces of attraction between oppositely charged ions in a giant lattice structure.
[Na][Cl] [Na][Cl]
[Cl] [Na][Cl] [Na]
[Na][Cl] [Na][Cl]
Na+ & Cl- are each surrounded by oppositely charged ions attracted by strong electrostatic forces of attraction in a regular lattice
ways of representing Ionic compounds:
- Dot and cross diagrams
- 3D models
- Ball and stick models
ways of representing Ionic compounds - Dot and cross diagrams
shows:
how ionic compounds are formed
where the electrons in the ions come from
doesn’t show:
structure of the compound
relative sizes of the ions
how ions are arranged.
ways of representing Ionic compounds - 3D models
shows:
relative sizes of the ions
how ions are arranged.
only lets you see the outer layer of the compound - doesn’t let you see inner layer
doesn’t show:
structure of the compound
how ionic compounds are formed
where the electrons in the ions come from
ways of representing Ionic compounds - Ball and stick models
shows:
relative sizes of the ions - not always to scale
how ions are arranged.
only lets you see the outer layer of the compound - doesn’t let you see inner layer
doesn’t show:
structure of the compound
how ionic compounds are formed
where the electrons in the ions come from
Ball and stick models of ionic compounds also suggest that there are gaps between the ions, when in reality there aren’t.
Properties of ionic compounds -Melting and boiling point
ionic compounds all have high melting points and high boiling points due to the giant lattice structure forming strong electrostatic attraction between the ions. It takes a large amount of energy to overcome this attraction and break the many strong bonds.
Properties of ionic compounds - solubility
ions are attracted to the polar water molecules and the attraction breaks lattice apart so it is soluble
Properties of ionic compounds - Electrical conductivity
solid ionic compounds can’t conduct electricity because the ions are bonded together in a lattice. when they’re melted or dissolved, the ions are free to move, and they’ll carry electric charge.
ionic Property - hardness
+- + - + - - layer attracted to other layer
- + - + - + - layer
A force makes the layer slide
There is repulsion between ions with the same charge
+ - + - + - layer repelled from other layer
‘
‘
- + - + - + therefore they are hard but brittle
Properties of ionic compounds
they are hard but brittle
high melting points and high boiling points
generally soluble
solid ionic compounds can’t conduct electricity
melted or dissolved ionic compounds can conduct electricity
What is covalent bonding?
A covalent/molecular bond is formed when a pair of electrons is shared between two atoms. Atoms share electrons in their outer shells with each other to get full outer shells - both atoms involved in the bond end up with one extra electron in their outer shell.
The positively charged nuclei of the bonded atoms are attracted to the shared pair of electrons by electrostatic forces, making covalent bonds very strong.
where do Covalent bonds occur?
Covalent bonds occur between non-metal atoms. This can either be in non-metallic elements, e.g. Cl2 or O2, or in compounds of non-metals, e.g. H₂O or CH4
ways of representing covalent bonding - Dot and cross diagrams
In dot and cross diagrams, the shared electrons can be drawn in the overlap between the outer orbitals of the two atoms. Dot and cross diagrams are useful for showing which atoms the electrons in a covalent bond come from, but they don’t show the relative sizes of the atoms, or how the atoms are arranged in space.
ways of representing covalent bonding - Displayed formulas
A displayed formula is a two-dimensional representation of a molecule that shows the covalent bonds as single lines between atoms. This is a great way of showing what atoms something contains, as well as how they are connected in large molecules. However, they don’t show the 3D structure (shape) of the molecule, which atoms the electrons in the covalent bond have come from or the correct scales of the atoms.
ways of representing covalent bonding - 3D models and ball and stick models
3D models show the atoms in a molecule and how they are arranged in space so they show you the shape of the molecule. Ball and stick models show the bonds as well as the atoms. Other types of 3D model usually don’t.
A disadvantage of 3D models is that they can get confusing for large molecules that contain lots of atoms. Ball and stick models make it look like there are big gaps between the atoms in reality this is where the electron clouds interact. They also don’t show where the electrons in the bonds have come from, and sometimes the atoms are not shown to scale.
Simple covalent molecules
Simple covalent molecules are made up of only a few atoms joined by covalent bonds. Hydrogen, hydrogen chloride, methane, water, oxygen and carbon dioxide are all examples of simple covalent molecules, and you need to know about the bonding in them all.
Simple covalent molecules are tiny. They generally have sizes around 10 ^-10 m they’re not much bigger than individual atoms. The bonds that form between atoms in these molecules are generally about 10 ^-10 m too.
Properties of simple molecules - Electrical conductivity
Covalent substances made up of simple molecules don’t conduct electricity in any state - there are no ions or free electrons so there’s nothing to carry an electrical charge.
Properties of simple molecules - Melting and boiling points
Simple molecular substances have low melting and boiling points, so they are mostly gases or liquids at room temperature (but they can be solids).
The reason for the low melting and boiling points is that, although the atoms within the small molecules form very strong covalent bonds with each other, the forces of attraction between the molecules (intermolecular forces) are very weak. It’s only the weak intermolecular forces that need to be overcome to melt or boil a simple molecular substance - not the much stronger covalent bonds between the atoms. Overcoming these weak intermolecular forces doesn’t take much energy, so the melting and boiling points are low.
Properties of simple molecules - solubility in water
poor - no charged particles present to be attracted to the polar water molecules.
Polymers
Polymers are molecules made up of long chains of covalently bonded carbon atoms. They’re formed when lots of small molecules called monomers join together. A famous example is poly(ethene).
What are giant covalent structures?
- Giant covalent structures are made up of lots of atoms that are all bonded to each other by strong covalent bonds. They have very high melting and boiling points as lots of energy is needed to break the covalent bonds. They generally don’t contain charged particles, so they don’t conduct electricity (apart from graphite and graphene, which do conduct electricity). Giant covalent structures aren’t soluble in water, either..
- Diamond and graphite are both carbon-based giant covalent structures.
Diamond
- In diamond, each carbon atom forms four strong covalent bonds with other carbon atoms.
- This forms a very rigid structure, making diamond very hard, so it’s used to strengthen cutting tools (e.g. saw teeth and drill bits).
- Diamond also has a very high melting point because the strong covalent bonds take a lot of energy to overcome.
- It doesn’t conduct electricity because it has no free electrons or ions.
- industrial use - Drill gits due to hardness & jewellery due to shininess
Graphite
- each carbon atom is joined to three others within a layer
- Strong covalent bonds throughout the layer
- weak intermolecular forces between layers
soft due to weak intermolecular forces between layers - This makes graphite soft and slippery, so it’s ideal as a lubricating material.
Graphite has a high melting point because the covalent bonds in the layers need a lot of energy to break.
In graphite, only three out of each carbon’s four outer electrons are used in bonds, so each carbon atom has one electron that’s delocalised (free) and can move. This means that graphite conducts electricity, so can be used to make electrodes.
graphite industrial use - pencil ‘lead’ and lubrication due to Layers ability to slide & used as electrodes due to good electrical conduction
Graphene
Graphene is a sheet of carbon atoms joined together in hexagons. It’s basically a single layer of graphite. The sheet is just one atom thick, making it a two-dimensional compound.
The network of covalent bonds makes graphene very strong. It’s also incredibly light, so can be added to composite materials to improve their strength without adding much weight.
Like graphite, graphene contains delocalised electrons, so it can conduct electricity through the whole structure. This means it has the potential to be used in electronics.
Fullerenes
Fullerenes are hollow molecules of carbon, shaped like tubes or balls. They’re mainly made up of carbon atoms arranged in hexagons but can also contain pentagons (rings of five carbon atoms) or heptagons (rings of seven carbon atoms).
Buckminsterfullerene
Buckminsterfullerene was the first fullerene to be discovered. It’s got the molecular formula Co and forms a hollow sphere containing 20 hexagons and 12 pentagons. It’s a stable molecule that forms soft brownish-black crystals.
Nanotubes
Nanotubes are fullerenes which are tiny carbon cylinders. The ratio between the length and the diameter of nanotubes is very high. They’re good conductors of heat and electricity.
Uses of fullerenes
- In medicine
- As catalysts
- Strengthening materials
Uses of fullerenes - In medicine
Fullerenes can be used to ‘cage’ other molecules. The fullerene structure forms around another atom or molecule, which is then trapped inside. This could be used to deliver a drug to where it is needed in the body in a highly controlled way.
Uses of fullerenes - As catalysts
Fullerenes have a huge surface area, so they could help make great industrial catalysts - individual catalyst molecules could be attached to the fullerenes (the bigger the surface area the better).
Uses of fullerenes - Strengthening materials
Nanotubes have a high tensile strength (they don’t break when stretched) so can be used to strengthen materials without adding much weight. For example, they can be used to strengthen sports equipment that needs to be strong but also lightweight, such as tennis racket frames.
The structure of metals
Metals consist of a giant structure. The atoms in a metal are arranged in a regular pattern (see Figure 1). Metals are said to have giant structures because they have lots of atoms. Exactly how many depends on how big the piece of metal is.
Bonding in metals
in a metallic structure positive metal ions (consisting of the nucleus and inner shell electrons) all held together by outer shell electrons which become delocalised. This means that they aren’t associated with a particular atom or bond - they’re free to move through the whole structure (see Figure 2). There are strong forces of electrostatic attraction between the positive metal ions and the negative electrons and these forces, known as metallic bonding, hold the metal structure together.
Properties of metals - High melting and boiling points
The electrostatic forces between the metal atoms and the delocalised sea of electrons are very strong, so need lots of energy to be broken.
Properties of metals - Conductivity
Metals have delocalised electrons that are free to move through the whole structure. Because of this, they are much better conductors of thermal energy and electricity than most non-metals. The electrons carry the current or the thermal energy through the structure.
Properties of metals - Malleability
Metals consist of atoms held together in a regular structure. The atoms form layers that are able to slide over each other - see Figure 3. This means they are malleable - they can be hammered or rolled into flat sheets or pulled into a wire.
Properties of metals - Density
The densities of metals are generally higher than those of non-metals. The ions in the metallic structure are packed close together, so there aren’t as many gaps in the structure as in non-metals.
Physical properties of metals and non-metals
All metals undergo metallic bonding, which causes them to have similar basic physical properties. Non-metals tend to form covalent bonds, so they don’t tend to exhibit the same properties as metals.
Non-metals form a variety of different structures, so have a wide range of chemical and physical properties. They tend to be dull looking, more brittle, have lower boiling points (they’re not generally solids at room temperature), don’t generally conduct electricity and often have a lower density than metals.
Metals and non-metals also have different chemical properties. Non-metals can be found on the top and right-hand side of the periodic table, so their outer shells are generally over half-filled and they tend to gain electrons to form full outer shells. Metals are found at the bottom and left-hand side of the periodic table, so their shells are generally under half-filled. They tend to lose electrons to gain full outer shells.
