Science Chemistry Molecular Substance and Intermolecular Forces

Lesson 1: Introduction to molecular substances

Substances are either a metal, non- metal or both

There are 2 types of Molecules

1. Molecular Elements

All elemental molecules are made up of atoms of a single element. This means they contain 2 atoms of the same element and are therefore called… diatomic e.g. Hydrogen (H2)

2. Molecular Compounds

   Molecules of compounds have atoms from different elements. e.g. Water (H2O)

Binary molecular compounds are those that contain exactly 2 elements

Greek prefixes are used to indicate the number of atoms of each element present

There are 4 rules of naming binary covalent compounds:

1. First element is named in full

2. Second element is shortened and the -ide suffix is added

3. Use a prefix, note mono- is never used for the first element

4. If the name of the 2nd element begins with a vowel and the prefix for the name ends with a or o these letters are dropped.

Lesson 2: Lewis structure of atoms and simple molecules

Electron Dot Diagram of Atoms

Electron dot diagrams only show the outer-shell electrons in atoms and molecules. It shows the number of electrons available for bonding

Remember:

• For groups 1 and 2, the number of valence electrons is the group number

• For groups 13 - 18, the valence electrons is the group number minus 10

Unpaired electrons = Bonding electrons

Paired electrons = Lone pairs

Covalent Bonding

Covalent bonding = Is a chemical bond that involves sharing electron pairs between atoms. They’re formed when non-metallic atoms combine together to form covalent substances

Atoms share electrons in covalent bonding to form a full outer shell

When 2 non-metal atoms bond together they form a covalent bond sharing electrons to make the molecule more stable

Electron Dot Diagram of Molecules

Summary

• Atoms achieve a full outer shell to become …? (stable)

• Covalent bonds form bonds between non metals by … electrons (sharing)

• Electrons are shared … … (in pairs)

• Each shared pair represents a … … (covalent bond)

• Covalent bonds can be, single, double or triple bonds

Lesson 3: Valence Shell Electron Pair Repulsion (VSEPR) Theory

VSEPR theory is used to determine the shape of molecules

The key idea is that the valence electron pairs repel each other. Therefore they are arranged as far apart as possible.

There are 5 basic molecular shapes

Lesson 4: Representations of Molecules

Different type of molecule diagrams

1. The electron dot diagram helps to determine the type of bonds but does not show the size or shape of molecules.

2. The valence structure can show single, double or triple bonds using lines. Lone pairs are shown with dots or lines. Does not show size or shape of molecules.

3. In structural formulas bonding pairs are shown with lines/dashes. Lone pairs are left out. 3D structure may be required.

4. The ball and stick model displays the shape of the molecule including single, double and triple bonds It shows the shape but not the size of atoms.

5. The space-filling model shows the whole atom including the electron cloud. Shows the size and position of atoms but not types of bonds or angles.

Lesson 5: Polarity of Covalent Bonds

Electronegativity = A measure of how strongly the atoms attract electrons in chemical bonds

Non polar occurs

• Difference is less or equal to 0.4 electronegativity

• When atoms are the same element

• Similar electron negativity e.g. Cl and N(3.16-3.04 = 0.12)

• Bonding electrons are equally distributed

• Equal distribution of electrons

Polar occurs

• Difference between 0.4-1.7 electronegativity

• Different electronegativities therefore the atoms won’t be shared equally

• The bigger the electronegativity difference the more polar

Ionic bond is when its greater than 1.7

Lesson 6: Polarity of Covalent Molecules

Polar molecules:

One end has a permanent partial negative charge and the other partial positive. These charged ends are called dipoles (meaning 2 poles)

Polarity of diatomic molecules

Recap: What is a Diatomic molecule? When only 2 atoms are connected by a covalent bond. Which can be single, double or triple bond.

The polarity of a diatomic molecule is the same as the polarity of the covalent bond

Same as above

Polyatomic molecules

Recap: Are made up of 3 or more atoms.

An unbalanced distribution of electrons can result in a dipole same as diatomic molecule

To determine the polarity, note 2 things:

What are the 2 things? Polarity of bonds and the symmetry of molecules

1. The polarity of the bonds

   If at least 1 covalent bond is polar the electrons are unevenly distributed 
   hence consider the entire distribution of electrons to determine if polar

2. The symmetry of the molecule

   Asymmetric: Distribution of electrons will be unbalanced and a dipole will form
   Symmetric: Distribution of electrons will be balanced and no dipole is formed

Lesson 7: Intermolecular Forces - Part A

Strength of intermolecular bonds/forces:

• The stronger the bonds the more energy need to break them

• Therefore the stronger the bond the higher the melting and boiling points

Dipole - dipole force

ONLY occurs between TWO POLAR MOLECULES

PERMANENT partial POSITIVE charge of ONE molecule will be attracted to the NEGATIVE of another

THE STRENGTH of these interactions determines PHYSICAL CHANGES

Hydrogen Bond

Is a SPECIAL type of DIPOLE-DIPOLE bonding that occurs between 2 MOLECULES it is 10X STRONGER

1 MOLECULE must contain a HYDROGEN atom that is directly/covalently bonded to an OXYGEN, NITROGEN OR FLUORINE ATOM

THE (N,O,F) must contain a LONE pair

Water!!!!!!!!!! Mermaid!!!!!!!!!!!!!!!

• Can form 4 hydrogen bonds with 4 other water molecules

  Properties:

• High melting and boiling points due to the H-bonding so more energy is needed to break the bonds

Liquid form = water molecules are closely packed together but are not fixed in place

Solid form = The water molecules form 4 hydrogen bonds with its neighbouring molecules and forms a HEXAGONAL CRYSTALLINE LATTICE. Therefore the molecules are further spread out from each other resulting in the expansion of ice

Water requires more energy to be absorbed for an increase in temperature compared to other fluids. This is = Heat capacity for water

Lesson 8: Intermolecular Forces - Part B

Dispersion force!!!!!!!!!!!!!

Dispersion forces exist between ALL molecular compounds, regardless of polarity

Formation of temporary dipole:

• For non polar molecules the electrons are distributed around the molecule evenly. However as they are constantly moving there are points in time where more electrons are on one side creating a more negative and more positive side.

• When a molecule forms a temporary dipole it can encourage neighbouring molecules to make one too. They align their temporary dipoles with opposite attractions forming the dispersion force. This is similar to dipole-dipole forces but is generally weaker due to it being temporary

Strength of dispersion force:

• Molecules with larger molecular mass = more electrons

• Which means it is easier to produce a larger temporary imbalance of electrons = stronger temporary dipole bonds

• Therefore the strength of dispersion forces increases as the molecular mass increases

• Usually DF is weaker than D-D and H-D. However in really large molecules the dispersion force could dominate

To calculate the molar/molecular mass use the number below the symbol and then add up the numbers of each atom. Include 1 decimal place and the units g/mol

Lesson 9: Solubility of Molecular Substances

Glossary :

Solute = a substance like sugar

Solvent = a liquid like water

Solubility = If the solute dissolves in the solvent

Miscible = when 2 liquids are fully mixed in any proportion

Immiscible = when 2 liquids are never fully mixed in any proportion

The “Rule of Thumb” for predicting solubility or miscible vs immiscible is “like dissolves in like”. Polar dissolves in polar. Non polar dissolves in non polar.

The interactions between different polarity molecules would be weaker than the intermolecular bonding within the polar molecule. Therefore the bonds in the molecule would stay together and not interact with other molecules.