Defining the Atom >

A neutron walks into a bar and asks how much for a beer. Bartender replies “For you, no charge”.

NUCLEAR CHEMISTRY(Sections 4.4, 25.1, & 25.2)

Review:

Atomic Number tells us: The identity of the element; the number

of protons (and number of electrons in a neutral atom)

Atomic Mass tells us: The average mass in amu of all naturally

occuring isotopes

Mass

REVIEW

• The number of ______ defines the element.

PROTONS (6 protons = carbon)• Elements with different numbers of

neutrons are called…ISOTOPES

REVIEW: Isotope Naming

• Element name – mass number• Ex. carbon-12 or neon-22

• Using the element symbol:

Chemical v. Nuclear Reactions• In a chemical reaction (CHEMICAL CHANGE), atoms gain, lose or share electrons and a new SUBSTANCE is formed.

Chemical v. Nuclear Reactions• In a nuclear reaction, the nucleus of an atom changes and becomes a new ELEMENT.

Nucleus Stability• What particles are in the nucleus?

• Protons (+ charge) and neutrons (neutral)

• STRONG NUCLEAR FORCE holds the neutrons and protons together.

Nuclear Reactions• In nuclear reactions, an unstable

nucleus changes (LOSES MASS and ENERGY) to become more stable.

• Stability of a nucleus depends on the ratio of protons

(p+) to neutrons (n0)

Radioactivity• No elements have stable nuclei for atomic

numbers above ~83• Unstable if too many or too few neutrons

Radioactivity• Marie Curie (1867-1934) was a

Polish scientist whose research led to many discoveries about radiation and radioactive elements. She won 2 Nobel prizes.

• In 1934 she died from leukemia caused by her long-term exposure to radiation.

• The element polonium that she helped discover is named after her homeland, Poland

Radioactivity

• Marie Curie showed that rays emitted by uranium atoms caused fogging in photographic plates.

• Marie Curie named the process by which materials give off such rays radioactivity.

• The penetrating rays and particles emitted by a radioactive source are called radiation.

Radioactivity

DEFINITIONS:• Radioactivity – the process

by which some substances spontaneously emit energy called radiation. Spontaneously: on its own; no

energy is required to start it

Radioactivity

DEFINITIONS:• Radiation - The penetrating

rays and particles emitted by a radioactive substance.

• Radioactive Decay – An unstable nucleus spontaneously loses energy by emitting radiation.

Radioactivity

Three main types of nuclear radiation:• Alpha radiation• Beta radiation • Gamma radiation

Alpha Radiation

• Alpha particles released• Contains 2 protons and 2 neutrons –

(helium nucleus no electrons)

• symbol: positive charge (2+)

Produced in large nuclei

Beta Radiation• Beta particle is released.

• A high speed electron leaves the nucleus

• or negative charge (1-)

Occurs in elements with more neutrons than protons: a neutron splits into a proton and electron. The proton stays in the nucleus and the electron is emitted.

Gamma Radiation• Gamma ray is released.

• High energy radiation• zero charge

Occurs if nucleus is in an excited state and it releases energy (gamma rays).Usually occurs with alpha or beta decay.

Summary – 3 Types of Radiation

Radiation Type Symbol Mass (amu)

Charge

Alpha 4 2+

Beta ~0 (very small) 1-

Gamma 0 0

Alpha particles can be stopped by paper. Beta particles can be stopped by wood Most (but not all) Gamma rays can be stopped by

lead or thick concrete.

Radiation

Alpha particles are the least penetrating. Gamma rays are the most penetrating.

Radiation

Damage Caused by 3 Types of Radiation

Radiation Type

Exposure Damage

Alpha Stopped by clothing and your outer skin layer.Inhaled or ingested

Lung cancer (i.e., radon gas)

Beta Can penetrate the first few

mm of skin or ingested

Burns, tissue damage, radiation sickness

Gamma Can pass through your

body

Cancer, radiation sickness, tissue

and organ damage

Radioactive Decay – Nuclear Equations• Nuclear equations written similarly to chemical reactions

• Mass numbers and atomic numbers are conserved

Alpha Decay• Alpha particles 2+ charge

• Mass number: 238 = 234 + 4• Atomic number: 92 = 90 + 2

Balancing Nuclear Equations

+ ___• Mass number: 230 = 226 + x

x = 230-226 = 4• Atomic number: 90 = 88 + y

y = 90-88 = 2

Beta Decay• Beta particles 1- charge

• Mass number: 228= 228 + 0• Atomic number: 88 = 89 + (-1)

+ ___

Balancing Nuclear Equations

+ ___• Mass number: 239 = 239 + x

x = 239-239 = 0• Atomic number: 93 = 94 + y

y = 93-94 = -1

Gamma Radiation• Gamma ray zero charge

• Mass number: 230 = 226 + 4• Atomic number: 90 = 88 + 2

Balancing Nuclear Equations

+ ___• Mass number: 137 = 137 + x

x = 137-137 = 0• Atomic number: 56 = 56 + y

y = 56-56 = 0

CHECK

1. Certain elements are radioactive because their atoms have

more neutrons than electrons. an unstable nucleus. a large nucleus. more neutrons than protons.

CHECK

2. An unstable nucleus releases energy by emitting radiation. thermal vibrations. a chemical reaction. giving off heat.

CHECK

3. Which property does NOT describe an alpha particle?

2+ charge a relatively large mass a negative charge low penetrating power

CHECK

4. When a radioactive nucleus releases a high-speed electron, the process can be described as

oxidation. alpha emission. beta emission. gamma radiation.

Half-LifeA half-life (t1/2): the time required for one-half of the nuclei of a radioactive sample to decay.

• After each half-life, half of the atoms have decayed into atoms of a new element.

Half-Life

Half-Life

Half-Life

The ratio of Carbon-14 to stable carbon in the remains of an organism changes in a predictable way that enables the archaeologist to obtain an estimate of its age.

Half-Life – Sample ProblemCarbon-14 emits beta radiation and decays with a half-life of 5730 years. If you start with a mass of 20,000 g of carbon-14, how long is 3 half-lives? How much mass remains?

3 half lives = 3 x 5730 yrs = 17,190 yrsRemaining mass = 20,000 g x x x = 2500 g

OR: 20,000 g10,000 g 1 half life

5,000 g 2 half lives2,500 g 3 half lives

Half-Life – Sample ProblemIron-59 has a half-life of 44.5 days. How much of a 200 mg sample will remain after 133.5 days?

Number of half lives: 133.5 days/44.5 days = 3 half lives

Remaining mass = 200. mg x x x = 25.0 mgOR: 200. mg

100. mg 1 half life50.0 g 2 half lives

25.0 g 3 half lives