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.