Chapter 2: Mainly About Compounds

Question 1

Ions

Answer 1

Positively or negatively charged particles formed when atoms lose or gain electrons.

Question 2

Ionic bonding

Answer 2

• The type of chemical bonding which involves the outright transfer of electrons from one atom to another
• The bonding consists of electrostatic attraction between the positive and negative ions formed by this transfer of electrons

Question 3

Empirical formulae

Answer 3

• A formula that gives the ratio by atoms of elements in a compound, rather than the actual numbers of atoms in a molecule
• Formulae for ionic compounds always empirical (because there are no molecules)

Question 4

Cations and anions

Answer 4

Cations - positive ions

Anions - negative ions

Question 5

Group 1 metals

Answer 5

• Li, Na, K, Rb, Cs
• Alkali metals
• All tend to lose one electron
• Therefore form singly charged positive ions
• Form ionic compounds

Question 6

Group 2 metals

Answer 6

• Be, Mg, Ca, Sr, Ba
• Alkaline earth metals
• Tend to lose two electrons
• Therefore form doubly charged positive ions
• Form ionic compounds

Question 7

Group 7 elements (non-metals)

Answer 7

• F, Cl, Br, I
• Halogens
• All tend to gain one electron
• Therefore form singly charged negative ions
• Form ionic and covalent compounds

Question 8

Group 6 elements (non-metals)

Answer 8

• O, S, Se, Te
• Tend to gain two electrons
• Form doubly charged negative ions
• Form ionic and covalent compounds

Question 9

Covalent bonds

Answer 9

• Formed between pairs of atoms by the atoms sharing electrons
• Strong covalent bonding forces hold atoms together in a covalent molecule (intramolecular)
• E.g. Two chlorine atoms combine to form a chlorine molecule by sharing a pair of electrons with each atom contributing one electron to the shared pair. Shared pair occupies a volume of space that surrounds both atoms. By moving around both nuclei these electrons hold the atoms together and so form a chemical bond. Each chlorine atom ‘satisfied’ by complete configuration.

Question 10

Covalent molecular substances

Answer 10

Substances (elements or compounds) that are made up of simple covalent molecules

Question 11

Properties of ionic substances

Answer 11

• Always compounds
• Solids at room temp.
• High melting points (typically above 400C) and high boiling points (typically over 1000C)
• Hard and brittle
• As solids do not conduct electricity
• When molten or in aqueous solution they do conduct electricity

Question 12

Properties of covalent molecular substances

Answer 12

• Can be elements or compounds
• At room temp. generally gases or liquids; a few are solids
• Low melting points (generally below 200C) and low boiling points (generally below 400C)
• When solid they are soft (weak intermolecular forces)
• Pure covalent substances do not conduct electricity either as solids or liquids (because they’re a neutral species)
• In aqueous solution do not conduct electricity (unless they react with water to form ions)

Question 13

What makes ionic substances hard?

Answer 13

• The strong electrostatic attraction between pairs of ions makes ionic substances hard

Question 14

What happens when you apply a strong force to an ionic crystal?

Answer 14

• If orderly array of ions is disturbed by applying a strong force, ions of the same charge come close together and repel each other, causing the crystal to shatter. Ionic crystals therefore are brittle

Question 15

Why don’t solid ionic compounds conduct?

Answer 15

• Because in solid form the ions are tightly bound in an orderly array and are unable to move towards a charged electrode
• However when ionic substances melt, the orderly arrangement of ions is broken up and the ions can move freely towards a charged electrode and therefore conduct electricity

Question 16

Intermolecular forces

Answer 16

• The bonding forces holding atoms together within a covalent molecule are very strong, but the forces between one molecule and its neighbours are quite weak
• These weak forces between pairs of molecules are called intermolecular forces

Question 17

Effect of intermolecular forces on m.p and b.p

Answer 17

• Both melting and boiling overcome weak intermolecular forces by separating molecules from one another
• The stronger the intermolecular forces in molecular compounds, the higher are the melting and boiling points

Question 18

Covalent network solids

Answer 18

• Are solids in which the covalent bonding extends indefinitely throughout the whole crystal
• Also called covalent lattice solids or covalent lattices
• E.g. Carbon in the form of diamond - each carbon atom bonded to four other carbon atoms

Question 19

Lattice

Answer 19

An infinite orderly array of particles

Question 20

What happens when you melt a covalent lattice?

Answer 20

• The one big covalent crystal breaks up into many smaller pieces, which are able to move about relative to one another
• Melting covalent lattices involves breaking many strong covalent bonds
• This process requires a lot of energy and thus only occurs at high temperatures, typically well over 1000C (e.g. diamond 3550C)

Question 21

Why don’t covalent network solids conduct electricity?

Answer 21

• They do not contain ions
• All electrons are tied up either being held by individual atoms or being shared by pairs of atoms; none are free to roam around as in metals

Question 22

Ionic compounds

Answer 22

• No discrete (separate) molecules in ionic compounds, just an infinite array of positive and negative ions
• Held together by the electrostatic attraction between oppositely charged ions
• Ions are ‘locked’ into a crystalline network array with very strong force (hence a high melting point)

E.g. Sodium and chlorine combine to form the compound, sodium chloride

Question 23

Delocalised electrons

Answer 23

• Mobile sea of electrons
• Valence electrons break away from their atoms, leaving behind positive ions
• These free electrons no longer belong to particular atoms, but move randomly through the lattice, being shared by numerous positive ions
• Provide the chemical bonding that holds the crystal together
• The ability of the electrons to move freely causes metals to be good conductors, and to have a metallic lustre

Question 24

Properties of covalent network solids

Answer 24

• High melting and boiling points
• Do not conduct electricity
• Hard and brittle
• Covalent bonding throughout the crystal

Question 25

Properties of Metallic lattice solids

Answer 25

- Variable melting and boiling points - Conductors of electricity - Variable hardness - Malleable and ductile - Forces holding particles together: delocalised electrons (metallic bonding)

Question 26

Valency of an element

Answer 26

- When an element forms ionic compounds, the valency of the element is the charge that the ion of the element carries

Question 27

What happens when an ionic crystal is dissolved in water?

Answer 27

Due to water's polar nature, it attracts ions to its slightly positive end and slightly negative end, thus destroying the network and allowing solubility