Nuclear Chemistry

Atoms and Radioactive Decay

Nuclear Chemistry

1. Review of Atomic Structure

Let’s Start By Reviewing Atomic Structure…

Atoms consist of three basic particles:•Protons (Positive)•Neutrons (Neutral)•Electrons (Negative)

Atomic Structure

The protons and neutrons are located in the nucleus.

The electrons orbit the nucleus.

Fluorine

9 P10 N

electrons

Atomic Structure…

Protons and Neutrons are particles called nucleons.

Protons and neutrons have nearly the same mass.

Nucleons have a mass nearly 2000 times that of electrons!!!

Atomic Structure

The positive protons attract the negative electrons and hold them in orbit around the nucleus.

Atoms are generally electrically neutral.

9 P

FluorineWhy?

Atomic Structure

9 P

Fluorine

Atoms are generally neutral because the number of positive protons equals the number of negative electrons.

Atomic Configurations…

The configuration of electrons in an atom’s highest energy level determine its chemical properties, and thus its bonding properties and conductivity.

6 P

Carbon

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2. Isotopes and Neutrons

Isotopes and Neutrons

Isotopes are atoms of an element that have the same number of protons and electrons, but different numbers of neutrons.

In other words, the number of neutrons varies among the atoms of that element.

Isotopes and Neutrons

The atomic mass found on the periodic table is an average mass of all the atoms of that element, taking into account the varying numbers of neutrons.

Neutrons do not play a role in determining the identity of an atom.

Isotopes and Neutrons

Hydrogen isotopes may consist of one proton; one proton plus one neutron; or one proton plus two neutrons

Isotopes of HydrogenThus the Average Atomic Mass of

Hydrogen is 1.00794 amu.

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3. Nuclear Forces

Nuclear Forces

The positive protons tend to repel each other through electrostatic repulsion.

But the presence of the neutrons provides a ‘nuclear force’, or strong force which holds the nucleus together and stabilizes it.

Neutron

Proton

Nuclear Forces

The nuclear force is only strong when the nucleons are close together.

As the number of protons increases, the electrostatic repulsion increases and the nuclear force weakens.

So, even more neutrons are needed to stabilize the nucleus.

Nuclear Forces

A stable neutron - proton ratio is 1:1.

As the number of protons increases, so does the number of neutrons. This keeps the nucleus stable. But, this increases the neutron – proton ratio.

Nuclear Forces

A neutron-proton ratio of 1.5 : 1 is at the limit of stability.

Nuclear Forces

•Atoms with more than 83 protons cannot reach stability even with their larger numbers of neutrons.•All elements beyond bismuth on the periodic table are unstable and undergo some sort of ‘decay’ in order to become stable.

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4. Radioactive Decay

Radioactive Decay

Unstable nuclei are those with a high neutron:proton ratio; this will result in decay or a change in the nucleus in order to become stable.

Transmutation: a change in the identity of an element as a result of a change in the number of protons.

Radioactive Decay

Atoms will decay by ejecting nucleons, or altering the nucleons into different particles by releasing one or more of the following:› Alpha rays› Beta rays› Gamma rays› Positron Emission› Electron Capture

Alpha Particles; Alpha Rays

Alpha particles consist of two protons and two neutrons, and are emitted during some kinds of radioactive decay.

Remember that protons determine the identity of the element : if an alpha particle is emitted, the identity of the element changes.

Alpha particles are often called a helium nucleus.

Helium nucleus 2+ charge

The atomic number will be reduced by two, and the mass number reduced by four.

Alpha Particles; Alpha Rays

Alpha rays are streams of alpha particles given off during nuclear decay.

Alpha rays are relatively slow and easy to stop – a piece of paper will stop alpha particles.› They will travel only a few centimeters before

stopping even if they do not encounter any matter.

› The positive charge of the alpha particle attracts electrons nearby and the particle becomes a harmless helium atom.

Beta Particles; Beta Rays A Beta particle is an electron created and emitted

when a neutron is transformed* into a proton and an electron during radioactive decay.

This action adds a proton and thus changes the identity of the atom.

The mass number stays the same.

*The proton and electron are not ‘inside’ the neutron. They are created at the time of the action.

Beta Particles; Beta Rays

Beta rays travel faster than alpha rays and can penetrate paper, but are generally stopped by thin sheets of metal such as aluminum.

Their negative charge causes them to interact with other atoms which slows their speed.

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Gamma Rays

Gamma rays are photons of electromagnetic radiation with high frequency and energy.

Gamma rays are given off when the nucleons undergo an abrupt energy difference.

Gamma rays have no mass – they are pure energy.

Gamma Rays

Because they have no charge and are high energy, gamma rays travel far and penetrate further than alpha or beta rays.

Thick concrete or lead is needed to stop gamma rays.

Gamma Rays

The release of gamma rays alone do not affect the identity of the atom since they have no mass and no charge.

But, gamma radiation may be released along with release of an alpha or beta particle.

Gamma Ray

Beta Particle

Positron Emission

A positron is a particle that has the same mass as that of an electron, but has a positive charge.A positron is emitted from the nucleus as a proton is converted into a neutron.The atomic number decreases by one but the mass number stays the same.

11

p10

n0

+1B

Positron

Electron Capture

The nucleus can ‘capture’ one of its own inner-orbital electrons if the atom is unstable due to too many protons.The electron will combine with a proton in the nucleus and form a neutron.The atomic number decreases by one but the mass number stays the same.

0-1

e 11

p 10

n

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5. Exposure to Radioactivity

Human Exposure to RadioactivityWe are exposed to low doses of radiation,

including gamma radiation, every day without ill effects.

• Radioactive decay heats the interior of Earth.• Radiation occurs in all of our

surroundings: air, water, soil.• Cosmic radiation reaches us

every day.

Human Exposure to Radioactivity

The largest dose of radiation we are normally exposed to comes from radon gas which emanates from the ground.

Exposure to Gamma Rays

In low doses with short exposure, gamma radiation is not harmful.

But if you are exposed to a high concentration, or for an extended period of time, gamma radiation may cause damage.

Dangers of Gamma Rays Gamma rays are the most dangerous

form of radiation to humans because the rays can travel through the body, exposing organs to damage by altering the molecules that make-up the body.

This molecular damage can result in genetic mutations, tumors, and other physical abnormalities.

Effects of Concentrated Exposure to Gamma Radiation

High-dose exposure may result in radiation burns, nausea, hair-loss, pre-mature aging, weakness, organ damage, and death within hours or a few months, depending on the dose.

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6. Use of Radioactivity

Making Use of Radioactivity

Humans have the ability to harness and manipulate radioactivity for:› National defense and weaponry› Medical diagnosis and treatments› Energy production› Radioactive Dating

National Defense and Nuclear Weaponry

Nuclear weaponry includes the nuclear bomb – the explosive energy coming from nuclear reactions, usually fission, but sometimes the combinations of fission and fusion.

Atomic bomb: fission bomb

Hydrogen bomb: thermonuclear bomb (uses fusion and fission)

National Defense and Nuclear Weaponry

Naval vessels are equipped to use nuclear propulsion.

Nuclear warheads attached to missiles can be launched if needed for national defense.

Nuclear Medicine

Gamma radiation can be safely used when directed at cancerous tumors for the purpose of killing cancer cells.

Gamma rays are used in the medical field for imaging purposes to diagnose diseases and tumors.

Nuclear Energy

The nuclear fission reaction is used in power plants to generate electricity for homes and industry.

Nuclear power plants supplied roughly 13% of the world’s electricity in 2012, according to the International Energy Agency.

Nuclear Energy

Nuclear power plants use nuclear fuel to create heat which boils water, creating steam, which turns the turbine to generate the electricity.

Radioactive Dating

Radioactive dating is a process used to determine the approximate age of an object.The amount of radioactive nuclides present in the object, such as a rock, can be measured. The half-life of the radioactive nuclide must be known, and from there, the age of the object can be estimated.

Half-life: the time it takes for half of a given amount of radioactive material to decay.

Carbon-14 is a radioactive nuclide often used to estimate the age of organic material

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7. Detecting and Measuring

Radioactivity

Radiation Detection

Approximate amounts of radiation can be detected by the following devices:•Film badge•Geiger-Muller counter•Scintillation counter

Radiation Detection

The film badge is worn on a lapel, the wrist, or finger, and detects the approximate accumulated dose of radiation over time.

People who work with radiation, such as x-ray technicians, will wear such a device.

Radiation Detection

The Geiger-Muller counter detects the approximate radiation present at any given time by counting electric pulses carried by gas ionized by radiation.

The Geiger-Muller counter would be used to detect an approximate radiation dose that a person has been exposed to, such as when the Fukushima nuclear power plant leaked radiation.

Radiation Detection

The scintillation counter can detect ionizing radiation. Some substances absorb ionizing radiation and emit visible light, or scintillate.

A scintillation counter would be used in border security, homeland security, and nuclear plant safety, among other things.

Information about radiation exposure - Hiroshima, Chernobyl, Three Mile Island

Information on Effects of Nuclear Weapons

For More Information…

Information on: What is Radioactivity? What is Radiation?