Isotopy, Radioactivity and Allotropy

CSEC Chemistry - Grade 10

What are isotopes?

Different atoms of the same element that have the same number of protons and electrons but different numbers of neutrons.

What is isotopy?

The occurrence of atoms of the same element that have the same number of protons and electrons but different numbers of neutrons.

Why do isotopes of an element have the same chemical properties?

They have the same number of protons and electrons.

Why do isotopes of an element have slightly different physical properties?

They have different numbers of neutrons.

What is relative abundance?

The percentage of each isotope of an element in a sample of the element.

What is relative atomic mass (A_r) ?

The average mass of one atom of an element compared to one-twelfth the mass of an atom of carbon-12.

Define ‘radioactive isotope’.

An isotope which has an unstable nucleus which decays spontaneously to a more stable form by emitting particles and radiation.

Define ‘half-life’.

The time taken for half of the nuclei in a sample of a radioactive isotope to undergo radioactive decay.

What are the 3 main types of particles/radiation emitted by radioactive isotopes?

Alpha, beta and gamma particles

Describe alpha particles

They consist of 2 protons, 2 neutrons and have a charge of +2

Describe beta particles

They consist of an electron and have a charge of -1

Describe gamma radiation

A form of high energy electromagnetic radiation and has neither mass nor charge.

This radioisotope is used to diagnose and treat thyroid disorders (goitre, hyperthyroidism, thyroid cancer).

Iodine-131 (I-131)

This radioisotope is used in dating to determine the age of fossils, bones, wood, and archaeological remains

Carbon-14 (C-14)

Name two radioisotopes used as fuel in nuclear reactors and atomic bombs.

Uranium-235 (U-235) and Plutonium-239 (Pu-239)

This radioisotope is to power spacecraft, satellites, and deep-space probes, In the past, it also used to be used to power pacemakers for the human heart.

Plutonium-238 (Pu-238)

This radioisotope is used for sterilizing medical equipment and food preservation (killing bacteria, insects).

Cobalt-60 (Co-60)

This radioisotope is used in smoke detectors.

Americium-241 (Am-241)

This radioisotope is used to trace leaks in underground pipelines.

Sodium-24 (Na-24)

Name three isotopes of hydrogen.

Protium, deuterium and tritium.

This stable (non-radioactive) isotope of hydrogen is known as ‘heavy water’ as is used in nuclear reactors.

Deuterium (²H)

A nuclear reaction in which two light nuclei combine (fuse) to form a heavier nucleus, releasing a large amount of energy.

Nuclear fusion

A nuclear reaction in which a heavy nucleus splits into two (or more) lighter nuclei, releasing energy and neutrons.

Nuclear fission

What are allotropes?

Different structural forms of the same element in the same physical state.

What is allotropy?

The existence of different structural forms of the same element in the same physical state.

Why do allotropes have the same chemical properties?

They are comprised on the same element.

Why do allotropes have different physical properties?

Differences in how their atoms are bonded.

Name the four different solid structures/lattices.

Ionic, metallic , simple molecular, giant molecular

What is a lattice structure?

A lattice is a regular, repeating three-dimensional arrangement of particles (atoms, ions, or molecules).These particles are held together by bonds (ionic, covalent, or metallic) or by intermolecular forces.

This type of lattice is made up of cations and anions held together in a regular, repeating arrangement by strong ionic bonds.

Example: Sodium chloride (NaCl)

Ionic lattice

This type of lattice is characterized by strong electrostatic forces of attraction between metal cations within a bulk of metal and the sea of mobile, delocalized electrons surrounding them.

E.g. copper, iron

Metallic lattice

This type of lattice is characterized by small molecules arranged in a regular, 3D way . Weak intermolecular forces between molecules hold them together.

Example: Iodine crystals (I₂), dry ice (solid CO₂).

Simple molecular lattice

This type of lattice is characterized by atoms joined together by covalent bonds in a giant continuous network.

E.g. Diamond and graphite (C), Silicon dioxide (SiO₂),

Giant covalent lattice

Differentiate between ionic crystals and simple molecular crystals in terms of melting point.

Ionic - HIGH melting point

Simple molecular - LOW melting point

Differentiate between ionic crystals and simple molecular crystals in terms of solubility.

Ionic - most dissolve in polar solvents (e.g. water)

Simple molecular - most dissolve in non-polar solvents (e.g. benzene)

Differentiate between ionic crystals and simple molecular crystals in terms of electrical conductivity.

Ionic - conduct electricity when molten or dissolved in water

Simple molecular - DO NOT conduct electricity

In what state do ionic compounds conduct electricity?

When molten or dissolved in water. They DO NOT conduct electricity in the solid state wince the electrons and ions are not mobile then.

What is the principle of “like dissolves like” in Chemistry?

Substances with similar types of intermolecular forces (or polarity) will dissolve in each other.

• Polar dissolves polar and ionic compounds,

• Non-polar dissolves non-polar substances (like fats and oils)

• Polar and non-polar do not mix (like oil and water).

Define polar substance

Molecules in which electrons are unevenly shared, creating regions of partial positive (δ⁺) and partial negative (δ⁻) charge.

E.g. Water (H₂O), ethanol (C₂H₅OH), acetone (CH₃COCH₃), hydrochloric acid, ammonia etc

Define non-polar substance

Molecules in which electrons are shared equally, so there are no permanent partial charges.

E.g. Hexane (C₆H₁₄), benzene (C₆H₆), carbon tetrachloride (CCl₄), O₂, N₂, Cl₂, CH₄ etc

Ionic & polar covalent substances dissolve in

polar solvents

Non-polar covalent substances dissolve in

non-polar solvents

List three characteristics of diamond (C)

• Very hard

• Insulator (does NOT conduct electricity)

• Transparent and shiny

• Each carbon atom is covalently bonded to 4 others in a tetrahedral arrangement.

• Giant covalent lattice

List three characteristics of graphite (C).

• Soft and slippery

• Good conductor of electricity

• Black, opaque and shiny

• Giant covalent lattice

• Each carbon atom is covalently bonded to 3 others in flat hexagonal layers

Diamond and graphite are both allotropes of carbon (C). Why can graphite conduct electricity but diamond cannot?

Graphite has delocalized electrons while diamond does not.

In order for electricity to flow, charged particles (either mobile ions or mobile electrons) need to be present.

What accounts for graphite'‘s use as a solid lubricant?

Hexagonal layers are held by weak van der Waals forces hence layers can slide against each other.

Why do diamond and graphite have very high melting and boiling points?

Very strong covalent bonding - extensive giant covalent lattice structure formed.

List uses of graphite.

Pencils ("lead"), solid lubricants, electrodes, batteries.

List uses of diamond

Cutting tools, drills, abrasives, jewelry.