Atomic theory 9 Flashcards by Christine Aherne

Define covalent bonding - Intramolecular

Covalent bonding occurs when electrons are shared between atoms. Atoms share to attain eight electrons in the outer shell (usually). This type of bonding normally occurs between non-metal atoms.

How well did you know this?

Not at all

Perfectly

Are electrons normally localised or delocalised?

Localised

How well did you know this?

Not at all

Perfectly

What does localised mean?

It means that they are not free to move from one atom to the next, but are held between the two atoms.

How well did you know this?

Not at all

Perfectly

Single covalent bond e.g HCl

H + CL = HCl
Cl: 3.16
H: 2.20
3.16 - 2.2 = 0.96 = Polar covalent
Cl = S- (higher E.D), H = S+
Single (only sharing 1) polar covalent bond

How well did you know this?

Not at all

Perfectly

Double covalent bond e.g O₂

O + O = OO = O₂
O: 3.44 - 3.44 = O = Pure covalent
Equal sharing, double pure covalent bond, there is a temporary separation of charge
S-/S+ changes every second

How well did you know this?

Not at all

Perfectly

Triple covalent bond e.g N₂

N + N = NN = N₂
N: 3.04 - 3.04 = 0 = Pure covalent
Triple pure covalent bond 
S-/S+ = Changes every second
Note : The high energy triple bond between nitrogen atoms in a molecule of nitrogen makes the gas unreactive

How well did you know this?

Not at all

Perfectly

When covalent bonding occurs it involves the ………… of orbitals.

Overlapping

How well did you know this?

Not at all

Perfectly

Define sigma bonding

When orbitals overlap ‘head-on’. This can occur between two S orbitals, an ‘s’ and a ‘p’ orbital or two ‘p’ orbitals. (To get full marks draw a diagram from notes)

How well did you know this?

Not at all

Perfectly

Define pi bonding

When orbitals overlap ‘side-ways’. This can occur between two ‘p’ orbitals only. (To get full marks draw diagram from notes)

How well did you know this?

Not at all

Perfectly

(!!) Sigma bonds always form …….. pi bonds

Before

How well did you know this?

Not at all

Perfectly

HCl is a….

Single sigma polar covalent bond

How well did you know this?

Not at all

Perfectly

Why does HCl form a sigma bond?

Because the electron in the 1s orbital of H will overlap with the electron in the 3pz orbital of Cl. As they are overlapping ‘head-on’ the we form a sigma bond.

How well did you know this?

Not at all

Perfectly

Cl₂ is a….

Single sigma pure covalent bond

How well did you know this?

Not at all

Perfectly

Why does Cl₂ form a sigma bond?

Each Cl atom will share the electron in the 3pz orbital, resulting in a ‘head-on’ overlap, thus the formation of a sigma bond.

How well did you know this?

Not at all

Perfectly

All …….. bonds are sigma bonds

Single

How well did you know this?

Not at all

Perfectly

Check notes at the end of pg 3

Not done

5. Done and understood

How well did you know this?

Not at all

Perfectly

All double bonds =

Sigma and a pi bond

How well did you know this?

Not at all

Perfectly

All triple bonds =

Sigma and two pi bonds

How well did you know this?

Not at all

Perfectly

Why are sigma bonds stronger than pi bonds?

As there is more overlapping in a sigma bond.

How well did you know this?

Not at all

Perfectly

What does the degree of sharing dependant on?

The degree of sharing is dependant on the attraction of an atom for electrons.

How well did you know this?

Not at all

Perfectly

What do you get if the atoms have the same attraction for the electrons?

A pure covalent bond

How well did you know this?

Not at all

Perfectly

What do you get if the atom’s attractions differ?

A polar covalent bond

How well did you know this?

Not at all

Perfectly

Define pure covalent bonding

A pure covalent bond occurs when electrons are shared equally BETWEEN atoms.

How well did you know this?

Not at all

Perfectly

Why do both atoms of H form a pure covalent bond?

Both atoms of H want to share one electron and as both atoms of H have the same attraction towards these electrons the bond that is formed is a pure covalent bond as the electrons are shared equally between the two atoms.

How well did you know this?

Not at all

Perfectly

Check the end of page 5 notes

Checked

Define polar covalent bond

A polar covalent bond occurs when electrons are shared unequally between atoms.

Name 4 properties of covalent compounds.

1. Usually gases or liquids at r.t 2. Pure covalent compounds do not conduct electricity. 3. Generally insoluble in polar water. 4. Low melting and boiling points.

Why are covalent compounds usually gases or liquids at room temperature?

Due to weaker intermolecular forces of attraction between molecules when compared to ion compounds.

Why do pure covalent compounds not conduct electricity?

As there are no ions

Why can some polar covalent compounds conduct electricity?

Due to the slight charges

Why do covalent compounds have low melting and boiling points?

Due to the very weak intermolecular forces of attraction as very little energy is needed to break these factors.

What is ammonia at room temperature?

Gas

Why is ammonia a gas at room temperature?

Due to weaker Van Der Waals forces

What are the 3 most electronegative elements?

F, N and O

What type dipole and intermolecular forces does ionic bonding have?

- Permanent dipole | - Ionic attractions

What type dipole and intermolecular forces do pure covalent bonds have?

- Temporary dipole | - Van der Waals forces

What type dipole and intermolecular forces do polar covalent bonds have?

- Permanent dipole | - Dipole-dipole attractions

What type dipole and intermolecular forces do water, ammonia and hydrogen fluoride have?

- Permanent dipole | - Hydrogen bonding

What happens in dative covalent bonding?

One atom gives both the electrons to the bond.

Define polar molecule

This term means that the slight positive and slight negative poles of a molecule are separated by a distance. (= do not coincide)

Are all molecules with polar bonds polar molecules?

What are pi bonds characterised by?

- Side to die overlap. | - Electron density above and below the intermolecular axes.

What is the formation of a sigma bond due to?

(a) the s - s overlap (b) the s - p overlap (c) the p(z) - p(z) overlap

How many types of covalent bonding are there depending on whether the electrons are shared between the atoms equally or unequally?

Two

What do we get if the atoms have the same attraction for electrons?

A pure covalent bond.

What do we get if the attractions differ?

A polar covalent bond.

What is covalent bonding an example of?

Intra-molecular bonding.

Why is covalent bonding an example of intra-molecular bonding?

As the bond occurs between atoms within a molecule.

Water is a v-shaped molecule. What does this cause?

It to be polar

What leads to a molecule being polar?

Lack of symmetry

What type of molecule is ammonia?

Polar

Define electronegativity (L.C)

The relative power of attraction an atom of an element has for the shared pair of electrons in a covalent bond.

Predict the type of bond formed between carbon and chlorine atoms in a CCl₄ molecule (L.C)

Polar covalent

Why are polar molecules often called dipoles?

As there are two poles of opposite charge within the molecule.

State and account for the shape of a tetrachloromethane molecule. (L.C)

- Tetrahedral | - Four pairs electrons and no lone pairs

Is the bonding in BF₃ ionic?

Why is the bonding in BF₃ not ionic as expected?

As the energy needed to remove the three outer electrons in B is too great, so sharing occurs.

Define electronegativity (L.C)

The relative power of attraction an atom of an element has for the shared pair of electrons in a covalent bond.

Why is there an increase in electronegativity value moving from gallium to germanium in the periodic table? (L.C)

- Nuclear charge increasing | - Atomic radius decreasing

Use electron pair repulsion theory to predict the shape of a molecule of boron trifluoride. (L.C)

- Trigonal planar

How do you show that a molecule has x bonding?

By finding the electronegativity differences.

Define electronegativity (L.C)

The relative power of attraction an atom of an element has for the shared pair of electrons in a covalent bond.

Define electronegativity (L.C)

The relative power of attraction an atom of an element has for the shared pair of electrons in a covalent bond.

How many (i) sigma bonds, (ii) pi bonds, result from sharing of the valence electrons between the atoms in a molecule of nitrogen. (L.C)

(i) 1 sigma | (ii) 2 pi

Distinguish between intramolecular bonding and intermolecular forces. (L.C)

Intramolecular : forces between atoms in molecules | Intermolecular : forces between molecules

Explain in terms of intramolecular bonding and intermolecular forces, why the boiling point of hydrogen (20 K) is significantly lower than that of oxygen (90.2 K). (L.C)

- hydrogen smaller | - weaker intermolecular forces

Explain in terms of intramolecular bonding and intermolecular forces, why iodine has a very low solubility in water. (L.C)

- Iodine is pure covalent | - Water is a polar solvent

Explain in terms of intramolecular bonding and intermolecular forces, why when a charged rod is held close to a thin stream of water flowing from a burette, the stream of water is deflected. (L.C)

Charge on rod attracts opposite charge on polar water molecule.

Distinguish between sigma (σ) and pi (π) covalent bonding. (L.C)

Sigma - When orbitals overlap 'head-on'. This can occur between two s orbitals, an 's' and a 'p' orbital or two 'p' orbitals. Pi - When orbitals overlap 'side-ways'. This can occur between two 'p' orbitals only.

Give one reason why electronegativity values exhibit general increase across the second period of the periodic table. (L.C)

Increase in nuclear charge / decrease in atomic radius

Show that the ammonia molecule has polar covalent bonding. (L.C)

- There is an electronegativity difference between N and H. - N with greater attraction - H with smaller attraction

Describe the processes involved when ammonia dissolves in water. (L.C)

- Hydrogen bonds between slightly negative O of water and H of ammonia and between slightly positive H of water and N of ammonia. - Breaking of hydrogen bonds in water - Forming of hydrogen bonds between ammonia and water.

Distinguish between sigma (σ) and pi (π) covalent bonding. (L.C)

State two factors that cause electronegativity values to increase across a period in the periodic table of the elements. (L.C)

- Increasing effective nuclear charge | - Decreasing atomic radius

State which of the following compounds contain intermolecular hydrogen bonds, HCl, H₂O, NH₃. Justify your answer. (L.C)

H₂O and NH₃ | - Hydrogen bonded to a small highly electronegative element (bonded to F, O or N)

Suggest a reason why the boiling point of ammonia (-33°C) is significantly lower than that of water (100 °C) (L.C)

- Weaker hydrogen bonding in ammonia | - Stronger hydrogen bonding in water

Explain the term intermolecular forces (L.C)

Attractive forces(repulsive) between molecules

A thin stream of liquid is flowing from the burette. A stream of water is deflected towards a positively charged rod whereas a stream of cyclohexane is undeflected. Account for these observations. (L.C)

- Polarity of water causes attraction to charged rod | - Non-polarity of cyclohexane means it is not affected by charged rod

Use electronegativity values to predict the type of bond expected between hydrogen and sulphur. (L.C)

- Weakly polar - Almost non-polar - Covalent bond

Write the chemical formula for hydrogen sulphide. (L.C)

H₂S

Distinguish between sigma and pi covalent bonding.(L.C)

Define atomic covalent radius. (L.C) (same as atomic radius)

The atomic radius is obtained by getting half the distance between the centres of singly bonded atoms of the same element.

Under what circumstances can ionic compounds conduct electricity? (L.C)

In water / in the molten state

A thin stream of liquid is flowing from the burette. A stream of water is deflected towards a positively charged rod whereas a stream of cyclohexane is undeflected. Explain what would happen in the case of the stream of water if the positively charged rod were replaced by a negatively charged rod. (L.C)

Stream of water still attracted to rod as molecules arrange themselves with positive pole towards rod.