SC3. Students will use the modern atomic theory to explain the characteristics of atoms.
a. Discriminate between the relative size, charge, and position of protons, neutrons, and electrons in the atom.
b. Explain the relationship of the proton number to the element’s identity.
c. Explain the relationship of isotopes to the relative abundance of atoms of a particular element.
The Development of the Atomic Theory
THE INDIVISIBLE ATOM
Greek - Democritus (450 BC)
- Atom was indivisible
- Theorized the existence of the atom
- Also, theorized that there were just four 'elements' - fire, water, air, earth
Benjamin Franklin ( 1760’s)
- discovers that an object can have an electrical charge.
- Charge can be positive or negative
VIDEODISC - Electroscope
1. Can you explain what is happening?
John Dalton (1803)
- Atom was indivisible
- All elements are composed of atoms
- The same atoms for one element are exactly alike
- Atoms are neither created or destroyed in a chemical reaction
- In a chemical reaction, atoms are separated, combined, or rearranged
- Different atoms combine in simple whole number ratios to form compounds (Law of Definite Proportions)
THE DIVISIBLE ATOM
Sir William Crookes (1880)
- determined that rays were traveling from one end to another in the cathode ray tube
- the cathode ray tube led to the invention of the television
- others experimented with the cathode ray tube and discovered that the type of gas had no effect so the ray must be a part of all matter. A magnet deflected the ray, so it must be composed of charged particles, and it deflected toward the positive, so the charged particles must be negative.
J. J. Thomson (late 1890's)
- discovered the electron using the cathode ray tube
- determined that the electron was smaller than a hydrogen atom. This was a shocking discovery. Many thought Dalton was wrong.
- Knew the atom was neutral and the electron was negative, so there must be positive material with a lot more mass. Said the atom was a positive pudding-like material throughout which negatively charged electrons were scattered - Plum Pudding or Chocolate Chip Cookie Model ( Page 94, figure 4-9).
Robert Millikan (1909)
- Knew the electron was negative, but actually determined the charge (as a measurement). Was able to use this measurement to determine the mass of an electron. It was 1/1840 of a hydrogen atom
VIDEODISC – Rutherford’s Experiment
1. What is an alpha particle?
2. What did Rutherford expect to see? What did he actually see?
3. What is Rutherford’s model of the atom?
Ernest Rutherford (1909) (1919)
- Did a famous gold foil experiment (the alpha scattering experiment)
- Calculated that the atom was mostly empty space through which electrons move.
- Concluded that the atom has a small, dense, positively charged, centrally-located nucleus surrounded by negatively charged electrons
- By 1919 he had refined the concept of the nucleus. Called the positive particles protons.
Rutherford and James Chadwick (1932)
- showed the nucleus also had a neutron.
- The neutron was basically equal in mass to the proton but had no electrical charge.
SUBATOMIC PARTICLES:
Fill in the blanks with the appropriate information.
Relative How to determine
Particle Location Charge Mass (amu) the number in a neutral atom
PROTON
______
NEUTRON
______
ELECTRON
The atom at this point: is spherically shaped with a dense, centrally located, positively charged nucleus surrounded by one or more negatively charged electrons in an electron cloud. Most of the atom consists of fast moving electrons traveling through empty space surrounding the nucleus. Electrons are held within the atom by an attraction to the positive nucleus. The nucleus has neutral neutrons and positive protons and 99.9% of the mass. Since atoms are electrically neutral, the number of protons must equal the number of electrons.
Henry Moseley (1912)
- Discovered that atoms of each element contain a unique positive charge.
- The number of protons in an atom identifies it.
- ATOMIC NUMBER = number of protons (and number of electrons). It is unique for each element
The MASS NUMBER is the number of neutrons and protons (called nucleons) in the nucleus. This means that you can calculate the number of neutrons in an atom by taking the mass number and subtracting the number of protons.
ISOTOPES
Isotopes are atoms of the same element that have a different number of neutrons. They differ in mass, but the atom’s chemical behavior are the same. To identify an isotope, you write the element’s name and follow it with its mass number. Examples: Carbon-12, Uranium-238. It can also be identified by writing
C U
The mass of individual atoms is too small to measure so the mass of an atom is compared to a standard, Carbon-12 – Carbon-12 = 12 amu. AMU stands for atomic mass unit. The ATOMIC MASS is the weighted average of the isotopes of an element and can be calculated if you know the isotope’s mass numbers and the percentage abundance of each. For more practice, page 103.
PRACTICE
The atomic mass of iridium-191 is 191.0 amu and iridium-193 is 193.0 amu. The percentage abundance for each is 37.58% (iridium-191) and 62.62% (iridium-193). Calculate the average atomic mass of iridium.
SC1. Students will analyze the nature of matter and its classifications.
a. Relate the role of nuclear fusion in producing essentially all elements heavier than hydrogen.
NUCLEAR CHANGE
Review: Remember that a chemical reaction involves a change of one or more substances into new substances. Chemical reactions involve only an atom’s electrons. The nucleus remains unchanged.
Characteristics of Chemical and Nuclear Reactions
Chemical Reactions
1. Occur when bonds are broken and formed
2. Atoms remain unchanged, although they may be rearranged
3. Involve only valence electrons
4. Associated with small energy changes
5. Reaction rate is influenced by pressure, temperature, concentration, and catalysts.
Nuclear Reactions
1. Occur when nuclei emit particles and/or rays
2. Atoms are often converted into atoms of another element
3. May involve protons, neutrons, and electrons
4. Associated with large energy changes
5. Reaction rate is not normally influenced by pressure, temperature, and catalysts.
Nuclear reactions involve a change in the nucleus. Some substances spontaneously emit radiation called RADIOACTIVITY. They do this because their nuclei are unstable. The rays and particles emitted are called RADIATION. Unstable nuclei lose energy by emitting radiation in a spontaneous (does not require energy) process called radioactive decay. The atom undergoes decay until it becomes stable. By emitting radiation, atoms of one element change into atoms of another element. Elements with an atomic number greater than 83 are radioactive. Elements with #1-82 have isotopes that may be radioactive.
VIDEODISC – Hot Pocket Change
1. What type of radiation does this emit?
2. What is your hypothesis on why lead stops the radiation and wood does not?
HISTORY
1896 Henri Becquerel
Discovered mysterious rays coming from uranium. Called it RADIATION. Radioactivity is the spontaneous emission of radiation from an element.
1898 Marie and Pierre Curie discovered these rays in Radium and Polonium
TYPES OF RADIATION
1. Alpha radiation, a
Made of alpha particles. Is composed of two protons and two neutrons. Has a 2+ charge and a mass of 4 amu. Has the least amount of energy of any of the radiation – is stopped by paper. A new element is created when alpha decay happens. The mass number and the atomic number change.
Alpha decay: Ra à Rn + He Radium-226 decays to radon-222 and an alpha
particle
2. Beta radiation, b
made of fast moving electrons. Has a –1 charge and a mass of 1/1840. An electron is emitted during beta decay because it has been removed from a neutron, leaving behind a proton. Is stopped by foil and has more energy than alpha radiation.
Beta decay: Na à Mg + b Sodium-22 decays into magnesium-22 and a beta
particle
3. Gamma radiation, g
high energy radiation that possesses no mass. Has no charge. Usually accompanies alpha or beta radiation. Is slowed down by lead or concrete. Accounts for most of the energy lost in a radioactive decay process.
Gamma decay: U à Th + He + g Uranium-238 decays into thorium-234 and an alpha
particle and two gamma rays of different frequencies
NUCLEAR STABILITY
The ratio of neutrons and protons determines nuclear stability. Too few or too many neutrons make the nucleus unstable. It needs to lose energy to become stable. So it decays. Few radioactive elements are found in nature – they have already decayed into stable atoms.
The STRONG NUCLEAR FORCE acts only on subatomic particles that are extremely close together. It overcomes the electrostatic repulsion between positive protons. Neutrons add an attractive force within the nucleus because they are not positive or negative. They are also subjected to the strong nuclear force. They help hold the nucleus together.
To determine stability, you need to calculate the neutron to proton ratio. If you were to plot these on a graph (neutrons on the y-axis and protons on the x-axis), you would get a band of stability for elements 1-82. All non-radioactive elements are within this band. Radioactive elements are found outside the band (page 811, figure 25-11).
After radioactive decay, the new atom is now positioned more closely, if not within, the band of stability.
RADIOACTIVE SERIES
A series of nuclear reactions that begins with an unstable nucleus and results in the formation of a stable nucleus is called a radioactive series. Uranium-238 goes through a radioactive series and eventually become lead-206 (page 814).
RADIOACTIVE DECAY RATES
Naturally occurring (radioactive isotopes) radioisotopes are not uncommon on earth. Even though radioisotopes have been decaying for 15 billion years (lifespan of earth), there are still some left to decay. The difference in different isotopes decay rates provides the explanation. Radioactive decay rates are measured in half-lives. A HALF-LIFE is the time it takes for one half of the radioactive sample to decay. Different isotopes have different half-lives. Examples include uranium-238 (4.46 x 109 years), carbon-14 (5730 years), radon-222 (3.8 days) and polonium-214 (163.7 microseconds).
If you were given a 10.0g sample of carbon-14, how much would be stable after 3 half-lives?
radioactive
stable
original sample 1st half-life 2nd half-life 3rd half-life
5.0g stable 7.5g stable 8.75g stable
THREE TYPES OF NUCLEAR REACTIONS
1. SPONTANEOUS - a single nucleus releases energy and particles of matter
2. FISSION - nucleus is split in two
3. FUSION - two nuclei join together – this is how all elements are made.
*ALWAYS involve a change in the nucleus and a release of energy
WRITING AND BALANCING NUCLEAR EQUATIONS PRACTICE
Remember when balancing these equations that mass number and atomic number are conserved.
1. Write the nuclear equation for the alpha decay of polonium-208.
2. Write the nuclear equation for the beta decay of copper-66.
SC3. Students will use the modern atomic theory to explain the characteristics of atoms.
b. Relate light emission and the movement of electrons to element identification.
WAVE NATURE OF LIGHT
Rutherford’s model of the atom did not explain how the electrons of an atom are arranged in the space around the nucleus. His model did not take into account the chemical behavior among various elements. In the early 1900’s it was observed that certain elements emitted visible light when heated in a flame. Analysis of the emitted light revealed that an element’s chemical behavior was related to the electron arrangement in the atom.
Electromagnetic radiation is a form of energy that exhibits wavelike behavior as it travels through space. Radiant energy, which includes visible light, is shown in the electromagnetic spectrum.
Lowest Highest
Frequency frequency
104 1022
Radio Microwave Radar Infrared Visible Light Ultraviolet X-ray Gamma
longest ROYGBIV shortest
wavelength à increasing energy à wavelength
104 10-12
Wave Characteristics:
1. wavelength, l measured from crest to crest, measured in m, cm, nm
2. frequency, u measured by number of waves per second, hertz, Hz, waves/s
3. amplitude height of a wave crest or trough
4. speed, c 3.00 x 108 m/s in a vacuum. Speed is always the same. Waves can
have different wavelength and frequency. Wavelength and frequency are inversely related.
speed = wavelength x frequency c = lu
White light can be refracted into its component colors and each color has its own wavelength.
PRACTICE:
1. Calculate the speed of a wave whose wavelength is 1.5 meters and whose frequency is 280 hertz.
2. Find the wavelength of a wave whose speed is 5.0 m/s and whose frequency is 2.5 hertz.
3. The speed of light is 3.00 x 108 m/s. Red light has a wavelength of 7.0 x 10-7m. What is its frequency?
Particle Nature of Light
Wave model cannot explain why heated objects emit only certain frequencies of light at a given temperature or why some metals emit electrons when colored light of a specific frequency shines on them.
1900 Max Planck – Matter can gain or lose electrons only in small specific amounts called quanta. A quantum is the minimum amount of energy that can be gained or lost. Glowing objects emit light, which is a form of energy.
Energyquantum = hu h = Planck’s constant = 6.626 x 10-34 J·s
The energy of the radiation increases as the frequency increases. This explains why ultraviolet light has more energy than violet light.
PRACTICE:
1. Calculate the energy of a gamma ray photon whose frequency is 5.02 x 1020 Hz.
2. What is the difference in energy between a photon of violet light with a frequency of 6.8 x 1014 Hz and a photon of red light with a frequency of 4.3 x 1014 Hz?
Photoelectric Effect
Electrons (photoelectrons) are emitted from a metals’ surface when light of a certain frequency shines on the surface. Increased intensity results in more electrons being ejected from the metal’s surface. (Example: a solar calculator converts the energy of light shining on it to electrical energy.) In 1905, Einstein proposed that electromagnetic radiation had wavelike and particle-like natures. The photon is a particle of electromagnetic radiation with no mass that carries a quantum of energy.