Chemistry - Chemical Bonding

What is the octet rule ?

The octet rule states that atoms tend to undergo chemical changes in order to reach an electron arrangement with eight electrons in their outer energy level (or valence shell)

What are the limitations to the octet rule ?

Elements near helium (e.g. hydrogen, lithium) want to have two electrons in their outer energy level

Many transition metals don’t follow the octet rule since they have variable valency

What is valency ?

The number of bonds an atom of an element forms when it reacts

What is the valence shell ?

The outermost energy level

What are valence electrons ?

The number of electrons in valence shell

What is ionic bonding ?

The force of attraction between oppositely charged ions

What does ionic bonding generally occur between ?

A metal and a non-metal

What are polyatomic ions ?

Covalently bonded groups of two or more atoms chemically bonded together that carry an overall electric charge, meaning they act as a single ion

What are examples of polyatomic ions ?

Ammonium – NH₄⁺
Hydroxide – OH⁻

Hydrogencarbonate – HCO₃

Sulfate – SO₄²⁻

Permanganate → MnO₄⁻

Hypochlorite → ClO⁻

Nitrate – NO₃⁻

Carbonate – CO₃²⁻

Sulfite → SO₃²⁻

Phosphate – PO₄³⁻

What does covalent bonding involve ?

The sharing of electrons to form covalent bonds

What does covalent bonding generally occur between ?

non-metals

How is a covalent bond formed ?

When electrons are shared between two atoms

What is electrostatic attraction ?

The attraction between particles due to their electric charge

What are intramolecular bonds ?

Bonds (between atoms) in a molecule

What are intermolecular forces ?

Forces of attraction that exist between molecules

What is electronegativity ?

The relative attraction that an atom in a molecule has for a shared pair of electrons in a covalent bond

What is the range of values on the Pauling Scale ?

0 to 4

What is system used to measure electronegativity ?

The Pauling Scale

What are the values that form a polar covalent bond ?

Numbers between 0.4 and 1.7

What are the values that form a non-polar (pure) covalent bond ?

Numbers less than 0.4

What are the values that form a ionic bond ?

numbers greater than 1.7

What is a polar covalent bond ?

A chemical bond where two atoms share electrons unequally

What is a non-polar (pure) covalent bond ?

A chemical bond where two atoms share electrons equally

How do we show partially negative and partially positive charges

δ+ and δ– ( delta positive and delta negative)

What is the most electronegative element ?

Fluorine

What is the least electronegative element ?

Francium

What are the electronegativity trends ?

Increases across a period and decreases down a group

How are ionic compounds made up ?

They are arranged in 3-D repeating structures called crystal lattices

What do covalent compounds as ?

Molecules

What is electrical conductivity ?

Refers to the ability of a substance to allow electricity to flow through it

What is electric current ?

A flow of charged particles

When will a substance conduct electricity ?

If it contains charged particles and if those particles are free to move

When will ionic compounds conduct electricity ?

When the ions are free to move - i.e. when the substance is molten (melted) or dissolved in aqueous solution

When do ionic compounds not conduct electricity ?

When in solid form since the ions are not free to move

Why do covalent compounds not conduct electricity ?

They are made of neutral molecules

Why do ionic compounds typically have high melting points ?

Due to their strong ionic bonds between their ions

Why do covalent compounds have relatively low melting and boiling points ?

The energy needed to separate molecules is the energy needed to weaken the intermolecular forces, not the covalent bonds in a molecule

What do ionic compounds with high melting and boiling points exist as at room temperature ?

solids

What do covalent compounds with lower melting and boiling points exist as at room temperature ?

liquids and gases

What is solubility ?

Refers to the ability of a substance to be dissolved

What is the solubility rule for covalent compounds ?

Like dissolves like - i.e polar solvents dissolve polar solutes, and pure (non-polar) solvents dissolve pure (non-polar) solutes

What is the solubility rule for ionic compounds ?

Soluble in polar solvents, insoluble in pure (non-polar) solvents

How are sigma bonds formed ?

When atomic orbitals overlap head-on

How are pi bonds formed ?

When atomic orbitals overlap sideways

Why are pi bonds weaker than sigma bonds ?

Sideways overlap is less efficient than head-on overlap

What are all single bonds ?

Sigma bonds

What is a double bond composed of ?

One sigma bond and one pi bond

How strong are double bonds ?

They are stronger than a single bond but not twice as strong, since pi bonds are weaker than sigma bonds

What is a triple bond composed of ?

One sigma bond and two pi bonds

How strong are triple bonds ?

Stronger than single and double bonds

What are delocalised electrons ?

Electrons shared between more than two atoms

What are the limitations of predicting bonding between atoms ?

Using electronegativity values is not always reliable

Not all bonds are purely ionic or covalent

Variable valency

Polyatomic ions