4. Chemical Bonds

Question 1

Octet rule

Answer 1

• atoms of groups 1A to 7A elements react in such a way that they gain, lose or share sufficient electrons to achieve a completed valence shell
• period 3 elements have 3s, 3p + 3d orbitals + can accomodate more than 8 valence electrons. Some elements, particularly phosphorous + sulfur, use the 3d orbitals + are therefore exceptions to the octet rule.

Question 2

Ion

Answer 2

• an atom that has gained or lost one or more electrons so that it has a negative or positive charge
• the properties of ions are very different to those of their atoms as they are a completely different chemical species

Question 3

Anion

Answer 3

• a negatively charged ion

Question 4

Cation

Answer 4

• a positively charged ion
• elements of groups 1A, 2A and 3A form only one type of cation

Question 5

Monatomic ion

Answer 5

• contain only one atom

Question 6

Polyatomic ions

Answer 6

• contain two or more atoms

Question 7

Naming cations

Answer 7

• for monatomic cations, the name of the cation is the name of the metal followed by ‘ion’
• for polyatomic cations, the charge is indicated by a roman numeral in parenthesis immediately after the name of the metal, followed by ‘ion’
• for common naming of polyatomic cations, the suffixes -ous (smaller charge) and -ic (larger charge) are often added to the stem of the Latin name for the element

Question 8

Naming anions

Answer 8

• for monatomic anions, the suffix -ide is added to the stem of the name
• for polyatomic anions, the prefix di- and tri- are used to indicate more than one hydrogen
• for common naming of polyatomic anions, the prefix bi- is used for one hydrogen

Question 9

Chemical bond

Answer 9

• forces of attraction that link atoms together

Question 10

Electronegativity

Answer 10

• a measure of an atoms attraction for the electrons it shares in a chemical bond
• the propensity for an atom to maximise its ‘sharing’ of electrons in a covalent bond
• the more electronegative an atom, the more it will ‘hog’ the shared electron
• increase from left to right across a period because of the increasing positive charge on the nucleus which leads to a stronger attraction for electrons in the valence shell
• increase from bottom to top within a group as the decreasing distance of the valence electrons from the nucleus leads to stronger attraction between the nucleus + its valence electrons
• trends in the periodic table are the same as ionization energy trends for the same reason

Question 11

Metallic nature

Answer 11

• a willingness to give away electrons
• trends in the periodic table are opposite to those of ionisation energy and electronegativity

Question 12

Low ionisation energy = low electronegativity = high metallic nature ==> octet theory

Answer 12

Group 8A elements (noble gases) are stable as their valence shell is full

Question 13

Ionic bond

Answer 13

• chemical bond resulting from the attraction between positive + negative ions
• involves the transfer of one or more valence electrons from an atom of lower electronegativity to the valence shell of an atom of higher electronegativity
• strong + rigid
• compound formed is called an ‘ionic compound’
• most likely to occur if the difference in electronegativity between the participating atoms is approximately 1.9 or more
• ionic compounds have a definite ratio of one ion to another
• naming convention: cation followed by anion
• usually form between a metal + non metal

Question 14

Covalent bond

Answer 14

• a chemical bond resulting from the sharing of electrons between two atoms
• usually occur between two non metals or between a non metal + a metalloid
• forms a molecule or polyatomic ion
• most likely to occur if the difference in electronegativity between the participating atoms is approximately less than 1.9
• the valence shell of each atom is filled by the sharing of electrons

Question 15

Non polar covalent bond

Answer 15

• electrons shared equally between atoms whose difference in electronegativity is less than 0.5

Question 16

Polar covalent bond

Answer 16

• electrons shared unequally between atoms whose difference in electronegativity is between 0.5 and 1.9
• shared electrons are pulled to spend most of their time arond the more electronegative atom creating a partial positive atom + a partial negative atom (polarity)
• called a dipole

Question 17

Lewis structure

Answer 17

• a formula for a molecule or ion showing all pairs of bonding electrons as single, double or triple bonds and all non bonded electrons as pairs of lewis dots

Question 18

Structural formula

Answer 18

• a formula showing how atoms in a molecule or ion are bonded to each other
• similar to a lewis structure except that a structural formula typically shows only bonding pairs of electrons

Question 19

Bonding electrons

Answer 19

• valence electrons involved in forming a covalent bond ie. shared electrons

Question 20

Non bonding electrons

Answer 20

• valence electrons not involved in forming covalent bonds ie. unshared electrons

Question 21

Single bond

Answer 21

• a bond formed by sharing one pair of electrons
• represented by a single line between two atoms

Question 22

Double bond

Answer 22

• a bond formed by sharing two pairs of electrons
• represented by two lines between the two bonded atoms
• most common between atoms of C, N, O, and S

Question 23

Triple bond

Answer 23

• a bond formed by sharing three pairs of of electrons
• represented by three lines between the two bonded atoms
• most common between atoms of C and N

Question 24

Hydrogen bond

Answer 24

• an extra strong dipole-dipole attraction that occurs between molecules in which hydrogen is covalently bonded to nitrogen, oxygen or flourine
• an intermolecular force

Question 25

Metallic bond

Answer 25

• whereby atoms happily share their electrons with many others to form positive ions amidst a sea of electrons
• makes metals malleable, ductile + especially good conductors
• bonding is quite weak, hence malleability + ductility
• the sea of electrons can move about easily to conduct heat + electricity

Question 26

London dispersion forces

Answer 26

• a temporary polarization of electron density in one atom creates positive and negative charges, which in turn induce temporary positive and negative charges in an adjacent atom. The intermolecular attractions between the temporary induced positive end of one dipole + the negative of another are called london dispersion forces
• exist between all molecules
• range in strength depending on mass, size + shape of interacting molecules. In general, strength increases as the mass + number of electrons in a molecule increase

Question 27

Ionization energy measures the amount of energy necessary to remove an electron from an atom. Electronegativity measures how tightly an atom holds the electrons that it shares with another atom.

Answer 27

Nitric oxide has 11 valence electrons and therefore does not obey the octet rule.

Question 28

Naming binary covalent compounds

Answer 28

1. Name the less electronegative element (the first element in the formula)
2. Name the more electronegative element by adding the suffix -ide to the stem name of the element
3. Use prefixes di-, tri-, tetra- etc to show the number of atoms of each element

Question 29

Resonance Theory

Answer 29

• a theory that many molecules and ions are best described as a hybrid of two or more Lewis contributing structures

Question 30

Contributing structure

Answer 30

• representations of a molecule or ion that differ only in the distribution of valence electrons
• also known as resonance structures or resonance contributors

Question 31

Resonance hybrid

Answer 31

• a molecule or ion described as a composite or hybrid of a number of contributing structures
• is more stable than any one of its hypothetical contributing structures

Question 32

Double headed arrow

Answer 32

• a symbol used to show that the structures on either side of it are resonance contributing structures:

Question 33

Drawing contributing structures

Answer 33

1. All contributing structures must have the same number of valence electrons
2. All contributing structures must obey the rules of covalent bonding
3. The positions of all atoms must be the same in each contributing structure, only the position of valence electrons differ

Question 34

Bond angle

Answer 34

• the angle between two bonded atoms + a central atom

Question 35

VSEPR model

Answer 35

• valence shell electron pair repulsion model
• the valence electrons of an atom may be involved in the formation of bonds or they may be unshared. Each combination creates a negatively charged region of electron density around a nucleus. Because like charges repel, the variuos regions of electron density spread out so that each is as far away as possible from the others
• unshared electron pairs repel adjacent bonding electron pairs more strongly than the bonding pairs repel one another
• 4 regions of electron density = tetrahedral distribution = 109.5° bond angle
• 3 regions of electron density = trigonal planar distribution = 120° bond angle
• 2 regions of electron density = linear distribution = 180° bond angle

Question 36

Determining polarity

Answer 36

• a molecule will be polar if it has polar bonds + its centres of partial positive charge and partial negative charge lie at different places within the molecule