Atoms and Nuclear Chemistry

Atoms

Radioactivity

Half-Life

Atoms

An atom is the smallest particle of an element that has all of the properties of that element.

Composition of the Atom

Nucleus•Positively charged•Very Dense – most of the mass of the atom is found in the nucleus

Electron Cloud •Area surrounding the nucleus •Negatively charged•Occupies most of the volume of the atom

Subatomic ParticlesSubatomic

Particle Symbol Charge Location in Atom Mass

1.673×10-24 gProton p+, Positivein the

nucleus

in the nucleus

outside of the nucleus

Electron 9.109×10-28 g

Neutron

e-, Negative

n°, Neutral 1.675×10-24 g

The identity of an element is determined by the number of protons in the nucleus.

The chemical behavior of an element is determined by the arrangement of the electrons in the atom.

Atom Mass Units (Amu)

Special unit used for measuring the mass of an atom.

One amu is equal to 1/12 the mass of a carbon-12 atom.

Atom Mass Units (Amu)Masses of Subatomic Particles in Amu

Particle Mass (amu)

Proton 1.007276 ≈ 1

Neutron 1.008665 ≈ 1

Electron 0.0005486 ≈

Charge of an Atom

Atoms are electrically neutral.

The number of protons in an atom equals the number of electrons.

Ions

Ions are formed when an atom gains or loses electrons.• anion – negatively charged ion formed

when an atom gains electrons• cation – positively charged ion formed

when an atom loses electrons

We will talk more about ions later this year.

Atomic Number• determined by the number of

protons in the nucleus• found on the periodic table

Determine the atomic number for each of the following elements.

Li

N

Mg

3712

Mass Number• equal to the sum of the neutrons

and protons in the nucleus of an atom

• not given on the periodic table

Ways to indicate mass number:•Hyphen notation•Nuclear Symbol

Chlorine-35

Write the hyphen notation and nuclear symbol for the element containing 4 protons and 5 neutrons.

Beryllium-9

Determine the number of neutrons in Argon-40.

The atomic number for argon is 18. 40 - 18 = 22.

Complete the following table.Element(hyphen notation)

Nuclear Symbol

Atomic Numbe

r

Mass Numb

er

Number of

Protons

Number of

Neutrons

Number of

Electrons

Sodium -22 22

9 19 9

80 45

40 20

0

11 11 11 11

Fluorine-19 9 10

Bromine-80 35 35 35

Calcium-40 20 20 20

Hydrogen-1 1 1 1 1

IsotopesAll atoms of the same element have the same number of protons, but the number of neutrons may vary.

Isotopes are atoms of the same element which have different numbers of neutrons.

Complete the following table for the 3 commonly occurring isotopes of oxygen.

Isotope

Nuclear

Symbol

Number of

Protons

Number of

Electrons

Number of

Neutrons

Mass Numb

er

Mass (amu)

Oxygen – 16 15.99415

Oxygen – 17

16.999131

Oxygen - 18

17.999160

8 8 8 16

8 8 9 178 8 10 18

What structural characteristics do all oxygen atoms have in common? All oxygen atoms have the same number of protons and electrons.

What differences exist between the isotopes of oxygen?The mass number, number of neutrons and mass of each isotope are different.

The isotopes of an element do not differ significantly in their chemical behavior. Why do you think this is so?The chemical behavior is determined by the number of electrons and they all contain the same number of electrons.

Average Atomic MassThe average atomic for an element is given on the periodic table, but how was it determined?

In order to calculate the average atomic mass for an element, you must know the percent abundance and atomic mass for each of the isotopes of that element.

Example Problem

Isotope Atomic MassPercent

AbundanceOxygen – 16 15.99415 amu 99.762%Oxygen – 17 16.999131 amu 0.038%Oxygen – 18 17.999160 amu 0.200%

The average atomic mass of oxygen is given as 15.999 amu on the periodic table. Let’s see how that was calculated.

15.99415 amu × = 15.95608 amu

16.999131 amu x = 0.0064597 amu

17.99160 amu × = 0.03598 amu

15.95608 amu + 0.0064597 amu + 0.03598 amu = 15.999 amu

A certain element exists as three natural isotopes as shown in the table below.

Isotope Mass (amu)Percent

Abundance Mass Number

1 19.99244 90.51 20

2 20.99395 0.27 21

3 21.99138 9.22 22

Calculate the average atomic mass of this element to the nearest thousandth.

Identify the element.18.09515744 amu + 0.056683665 amu + 2.027605236 amu = 20.179 amu

Neon

19.99244 amu × = 18.09515744 amu

20.99395 amu × = 0.056683665 amu

21.99138 amu × = 2.027605236 amu

Carbon has three naturally occurring isotopes: Carbon-12 (12.000 amu), Carbon-13 (13.003 amu), and Carbon-14 (14.003 amu). Based upon the average atomic mass of carbon (12.011 amu), which isotope of carbon do you think is the most abundant in nature? Explain your answer.

Carbon-12 is the most abundant in nature. This answer is based on the fact that the average atomic mass of carbon is 12.011 amu which is closest to the mass of carbon-12.

An element has two naturally occurring isotopes. The mass of the first isotope is 64.9278 amu and the mass of the second isotope is 62.9296 amu. The average atomic mass of the element is 63.546 amu. Calculate the percent abundance of each isotope to two decimal places.Let x = the percent as a decimal of the first isotope.

1-x = the percent as a decimal of the second isotope.

(64.9278)x + (1-x)(62.9296) = 63.546

64.9278x + 62.9296 – 62.9296x = 63.546

1.9982x = 0.6164

x = 0.3085; 1-x = 0.6915

The percent abundances are 30.85% and 69.15%.

Discovery of RadioactivityRadioactivity was accidently discovered in 1896 by the French chemist Henri Becquerel.

Becquerel was studying the properties of fluorescent materials, substances that glow in the dark having been exposed to light. On one occasion, Becquerel placed the minerals he was studying along with some unexposed photographic film in a laboratory drawer. When he retrieved the film on a later date, he found that it was foggy.

Discovery of RadioactivityUpon further investigation, Marie Curie and her husband Pierre, were able to determine that the fogginess was caused by rays emitted by the uranium in the mineral samples. Marie Curie named the process by which materials give off such rays radioactivity; the rays and particles emitted by a radioactive source are called

radiation.

Ionizing vs. Nonionizing RadiationIonizing radiation – radiation with enough energy to produce ions by knocking electrons off some atoms of a bombarded substance. Alpha, beta, gamma, and X-rays are examples of

ionizing radiation. Ionizing radiation can cause changes to living cells.

Nonionizing radiation – not capable of ionizing matter. Radio waves and visible light are forms of

nonionizing radiation.

RadioisotopesRadioisotopes are isotopes that are radioactive because they have unstable nuclei.

Nuclear StabilityThe stability of the nucleus depends upon its ratio of neutrons to protons. Too many or too few neutrons lead to an unstable nucleus.

When the number of protons in stable nuclei is plotted against the number of neutrons, a beltlike graph is obtained. This stable nuclei cluster over a range of neutron-proton ratios is referred to as the band of stability.

Stable isotopes fall within the band of stability and have neutron to proton ratios of nearly 1:1 at the lower range and nearly 1.5:1 at the upper range. Such isotopes tend to be stable. Radioactive isotopes fall outside of the band of stability.

Band of Stability

Would you expect Helium-4 to be a stable isotope? Why or why not?Yes, the neutron to proton ratio is 1:1. (falls within the band of stability)Would you expect Carbon-14 to be a stable isotope? Why or why not?No, the neutron to proton ratio is 1.3:1 (falls above the band of stability)

What happens if a nucleus is unstable?

Unstable nuclei undergo spontaneous changes that change their number of protons and neutrons.

In this process, they give off large amounts of energy and increase their stability.

Reminder: The identity of an element changes when the number of protons changes.

Types of Radioactive DecayDuring radioactive decay, unstable atoms lose energy by emitting one of several types of radiation.

The three main types of radiation are alpha, beta, and gamma.

Properties of the Three Most Common Types of Radiation

Radiation Alpha Beta Gamma

Composition

alpha particles(helium nucleus)

beta particles(electron)

form of electromagnetic radiation

Symbol or

Mass 4 amunearly 0 amu(0.0055 amu) 0 amu

ElectricCharge

2+ 1- 0

Penetrating

Power

stopped by paper, wood, cloth, etc.

stopped by aluminumor other metals

stopped by lead

He2

4 e1-

01-

00

0

Alpha Emission (Decay)An alpha particle ( ) is composed of two protons and two neutrons bound together.

Alpha emission is restricted almost entirely to very heavy nuclei. In these nuclei, both the number of neutrons and the number of protons need to be reduced in order to increase the stability of the nucleus.

All nuclei with atomic numbers greater than 83 are radioactive. A majority of these undergo alpha emission.

Alpha Emission (Decay)

In a balanced nuclear equation, the total of the mass numbers and atomic numbers on each side of the equation must be equal.

Describe the change in the mass number.

The mass number decreases by 4.

Describe the change in atomic number.

The atomic number decreases by 2.

Beta Emission (Decay)Beta emission ( ) is the emission of electrons from the nucleus when a neutron is converted to a proton and an electron.

Beta emission occurs when the nucleus of an element has too many neutrons.

This is true of elements that fall above the band of stability.

Beta Emission (Decay)

Describe the change in the mass number.

The mass number stays the same.

Describe the change in atomic number.

The atomic number increases by 1.

What happens to the neutron to proton ratio?

The neutron to proton ratio decreases.

Gamma Emission (Decay)Gamma rays ( ) are high-energy electromagnetic waves emitted from a nucleus as it changes from an excited state to a ground state.

Gamma rays are produced when nuclear particles undergo transitions in energy levels.

Gamma emission usually follows other types of decay that leave the nucleus in an excited state.

Gamma Emission (Decay)

Positron Emission (Decay)Positron emission ( ) occurs when a proton is converted into a neutron.

A positron is a particle that has the same mass as an electron, but has a positive charge and is emitted from the nucleus during some kinds of radioactive decay.

Positron emission occurs when elements have too many protons to be stable.

This is true of elements that fall below the band of stability.

Positron Emission (Decay)

Describe the change in the mass number.

The mass number stays the same.

Describe the change in atomic number.

The atomic number decreases by 1.

What happens to the neutron to proton ratio?

The neutron to proton ratio increases.

Electron CaptureElectron capture ( ) occurs when an inner orbital electron is captured by the nucleus of its own atom.

The inner orbital electron combines with a proton and a neutron is formed.

Electron capture also occurs in atoms with too many protons.

Electron Capture

Describe the change in the mass number.

The mass number stays the same.

Describe the change in atomic number.

The atomic number decreases by 1.

What happens to the neutron to proton ratio?

The neutron to proton ratio increases.

Complete the following nuclear equations and identify the type of radioactive decay.

1. → + 2. →3. → +4. → +

beta decay electron capture

alpha decaypositron emission

Write nuclear Decay equations for each of the following.

1. alpha decay of polonium-210

2. beta decay of copper-66

3. oxygen-15 undergoes positron emission

4. argon-37 undergoes electron capture

Write nuclear Decay equations for each of the following.

5. potassium-38 undergoes positron emission

6. silver-106 undergoes electron capture

7. beta decay of zirconium-191

8. alpha decay of thorium-230

Radioactive Decay SeriesA radioactive decay series is a series of nuclear reactions that begins with an unstable nucleus and results in the formation of a stable nucleus.

Radioactive Decay Series

Decay SeriesWrite the series of reactions that represent the decay series for uranium-238.

TransmutationTransmutation – conversion of an atom of one element to an atom of another element.Transmutation may occur through radioactive decay.Induced Transmutation may also occur when high

energy particles (protons, neutrons, or alpha particles) bombard the nucleus.

Transuranium elements – elements in the periodic table with atomic numbers above 92. These elements have been synthesized in nuclear reactors and nuclear accelerators; which accelerate the bombarding particles to very high speeds.

TransmutationThe nuclear equation for the induced transmutation of aluminum-27 into phosphorus-30 by alpha particle bombardment is written below.

Which particle is emitted from the aluminum atom? neutron

Write the balanced nuclear equation for the induced transmutation of aluminum-27 into sodium-24 by neutron bombardment. An alpha particle is released in the reaction.

Write the balanced nuclear equation for the alpha bombardment of plutonium-239. One of the reaction products is a neutron.

Nuclear FusionIn nuclear fusion, light nuclei combine to form a heavier, more stable nucleus.

Nuclear fusion occurs in the sun where hydrogen nuclei fuse to make helium nuclei.

Nuclear Fusion as a Power SourceNuclear fusion produces more energy per

gram of fuel than nuclear fission.Potential fuels (hydrogen-2, hydrogen-3) are

inexpensive and readily available.Fusion products are usually not radioactive.Unfortunately fusion requires high

temperatures to initiate the reaction and once started no known structural materials can contain the reaction.

Nuclear FissionIn nuclear fission, fissionable isotopes split when bombarded with neutrons.

The isotopes release

neutrons that cause a

chain reaction.

Nuclear Fission as a Power SourceUranium-235 is typically used as the

source of fuel in controlled fission reactions that produce large amounts of energy.

A major problem associated with nuclear fission is the issue of how to contain, store and dispose of the nuclear waste produced.

Half-LifeToday’s Objectives

1. Define half-life and calculate the half-life of certain isotopes.

2. Use half-life information to determine the amount of a radioisotope remaining at a given time.