atoms 

and 

the periodic table


We are all star stuff

90% of the atoms in the known universe are Hydrogen, formed almost 14 billion years ago when the Universe was formed.

As Hydrogen forms into stars, the massive gravity in the center of stars causes the Hydrogen to fuse into heavier atoms.

Outside of Hydrogen, every other atom in your body and in the rest of the Universe came from stars.

So, as Carl Sagan once famously said, "We are all star stuff!"

atoms

Atoms are so small that every time you exhale, your breath contains 10 billion trillion of them:
10 000 000 000 000 000 000 000 000

Atoms are so small that they cannot be seen with light: they are smaller than the wavelength of visible light.

Atoms are made up of subatomic particles. Protons and neutrons in the nucleus make up most of the mass, and electrons surround the nucleus and are much less massive.

atoms

Atoms are mostly empty space.

If the atom was the size of a baseball stadium, the nucleus would be the size of a grape in the middle, and each electron would be the size of a grain of salt. 
(and even this isn't quite correct -- you can't really describe the "size" of an electron)

Why don't atoms pass right through each other if they are so much empty space?

The electric field!

the elements

Any material made of only one type of atom is classified as an element.


We know of only about 90 atoms found in nature, and twenty more or so have been made in laboratories.


We list atoms in the Periodic Table, which was first formulated in 1869 by Dmitri Mendeleev.

the periodic table


Elements song by Tom Lehrer: http://www.privatehand.com/flash/elements.html

protons and neutrons

Protons, which are almost 2000 times more massive than electrons, are in the nucleus of atoms.  The number of protons in the nucleus is equal to the number of electrons surrounding the nucleus, and the two quantities of opposite charge balance each other.

Neutrons are almost the same size as protons, but they have no charge.

We call protons and neutrons nucleons.

The atomic number of an atom is the total number of protons; e.g., Hyrdogen's atomic # is 1, Helium's is 2, etc.

isotopes and 

atomic mass

Atoms can have different numbers of neutrons. E.g.:
Most hydrogen atoms have no neutrons.
A small percentage of hydrogen atoms have 1 neutron.
An even smaller percentage have 2 neutrons.


isotopes

Because atoms generally behave based on their charge, extra or fewer neutrons do not change an element chemically, and isotopes of atoms behave the same chemically.

Example: sugar containing carbon atoms with seven neutrons (Carbon 13) is digested the same as sugar with six neutrons (Carbon 12).

On a periodic table, the atomic mass is presented as an average of all the various isotopes, based on the abundance on Earth.

How to read an atomic symbol


On the periodic table (note the avg. of isotopes):

check question

The atomic number of an element matches the number of

  1. protons in the nucleus of an atom.
  2. electrons in a neutral atom. 
  3. both of the above.
  4. none of the above.



check question

The atomic number of an element matches the number of

  1. protons in the nucleus of an atom.
  2. electrons in a neutral atom. 
  3. both of the above.
  4. none of the above.

Comment:
When the atomic number doesn’t match the number of electrons, the atom is an ion.

check question

A nucleus with an atomic number of 44 and a mass number of 100 must have

  1. 44 neutrons.
  2. 56 neutrons. 
  3. 100 neutrons.
  4. none of the above.

check question

A nucleus with an atomic number of 44 and a mass number of 100 must have

  1. 44 neutrons.
  2. 56 neutrons. 
  3. 100 neutrons.
  4. none of the above.

Comment:
Be sure to distinguish between neutron and nucleon. Of the 100 nucleons in the nucleus, 56 are neutrons. A neutron is a nucleon, as is a proton.

the periodic table


the periodic table


the periodic table


the periodic table



spectroscopic atom identification

Spectroscope:
  • an instrument that separates and spreads light into its component frequencies
  • allows analysis of light emitted by elements when they are made to glow—identifies each element by its characteristic pattern

Each element emits a distinctive glow when energized and displays a distinctive spectrum.

The atomic spectrum is like a fingerprint for an atom.
(interactive spectrum demo)

Identifying Atoms with a Spectroscope

Spectral lines of various elemens:


check question

The hydrogen spectrum consists of many spectral lines. How can this simple element have so many lines?

  1. One electron can be boosted to many different energy levels.
  2. The electron can move at a variety of speeds. 
  3. The electron can vibrate at a variety of frequencies.
  4. Many standing electron waves can fit in the shell of the hydrogen atom.

check question


The hydrogen spectrum consists of many spectral lines. How can this simple element have so many lines?

  1. One electron can be boosted to many different energy levels.
  2. The electron can move at a variety of speeds. 
  3. The electron can vibrate at a variety of frequencies.
  4. Many standing electron waves can fit in the shell of the hydrogen atom.


an element discovered by spectroscopy!


the quantum hypothesis

Max Planck, German physicist, hypothesized—warm bodies emit radiant energy in discrete bundles called quanta. Energy in each energy bundle is proportional to the frequency of radiation.

Einstein stated that light itself is quantized. A beam of light is not a continuous stream of energy but consists of countless small discrete quanta of energy, each quantum called a photon.

the quantum hypothesis

Is light a wave, or a stream of particles?

Light can be described by both models—it exhibits properties of both a wave or a particle, depending on the experiment.

The amount of energy in a photon is directly proportional to the frequency of light:

E ~ f

check question


Which of these has the greatest energy per photon?

  1. Red light.
  2. Green light. 
  3. Blue light.
  4. All have the same.
 

check question


Which of these has the greatest energy per photon?

  1. Red light.
  2. Green light. 
  3. Blue light.
  4. All have the same.

Explanation:
In accord with E ~ f, the highest frequency light has the greatest energy per photon.

check question

Which of these photons has the smallest energy?

  1. Infrared.
  2. Visible. 
  3. Ultraviolet.
  4. All have the same.

check question

Which of these photons has the smallest energy?

  1. Infrared.
  2. Visible. 
  3. Ultraviolet.
  4. All have the same.

Explanation:
In accord with E ~ f, the lowest frequency radiation has the smallest energy per photon.

the quantum hypothesis


Bohr’s model explains why atoms don’t collapse:

  • Electrons can lose only specific amounts of energy equivalent to transitions between levels.
  • An atom reaches the lowest energy level called the ground state, where the electron can’t lose more energy and can’t move closer to the nucleus.

the quantum hypothesis


Planetary model of the atom:


Photons are emitted by atoms as electrons move from higher-energy outer levels to lower-energy inner levels. The energy of an emitted photon is equal to the difference in energy between the two levels. Because an electron is restricted to discrete levels, only lights of distinct frequencies are emitted.

 

the shell model


the shell model

Shell model showing the first three periods of the periodic table


the atomic nucleus and radioactivity

radioactivity


Elements with unstable nuclei are said to be radioactive.

When they break down, they eject energetic particles and emit high frequency electromagnetic radiation (gamma rays).
The process is called radioactive decay.
Radiation is not new, and is a natural process.

alpha, beta, and gamma rays

All elements with an atomic number greater than 83 (Bismuth) are radioactive.

These elements release three types of radiation, but only one of these (gamma rays) are electromagnetic radiation.

radioactivity


alpha particles

  • consists of two protons and two neutrons (a helium nucleus)
  • loses energy quickly during interaction
  • can be stopped easily by a few pieces of paper due to its large mass and double positive charge
  • does not normally penetrate lightweight material (paper, clothing)
  • causes significant damage to the surface of a material (living tissue) due to great kinetic energy
  • picks up electrons and becomes harmless helium when traveling through air
  • is deflected in the presence of magnetic or electric fields

beta particles

  • is an ejected electron from a neutron
  • has both a smaller mass and electric charge than an alpha particle, and moves faster
  • loses energy at a slower rate in air and travels farther before  stopping
  • can be stopped by several sheets of aluminum foil
  • penetrates fairly deeply into skin (potential for harming or killing living cells)
  • once stopped, becomes an ordinary electron
  • is deflected in the opposite direction to an alpha particle in the presence of magnetic and electric fields

gamma rays

  • are high-frequency electromagnetic radiation
  • are emitted when a nucleus in an excited state moves to a lower energy state
  • are more harmful than alpha or beta particles
  • are most penetrating because they have no mass or charge
  • are pure energy, greater per photon than in visible or ultraviolet light and X-rays
  • are unaffected by magnetic and electric fields, and therefore interact via direct hit with an atom

check question


Define:

  • alpha particle
  • beta particle
  • gamma ray

check question

Define:

  • alpha particle
  • beta particle
  • gamma ray

alpha particle: helium nucleus (2 protons, 2 neutrons)

beta particle: electron

gamma ray: high frequency electromagnetic radiation

Keeping the nucleus together


Given what we know about electric forces, what might concern you about a nucleus filled with protons and neutrons?

the strong

nuclear force

Nucleons (protons and neutrons) are kept together in the nucleus by the strong nuclear force.
The strong nuclear force is another fundamental force (along with gravity and electricity), but it is only strong over very short distances.

the strong nuclear force and the nucleus

The presence of neutrons helps hold the nucleus together.

the strong nuclear force and the nucleus

But in large nuclei, far-apart neutrons are less effective in holding a nucleus together, leading to radioactive decay!

half life and transmutation

Half-life:

  • is the rate of decay for a radioactive isotope.
  • is the time required for half of an original quantity of an element to decay.
  • is constant and independent of any physical or chemical change the atom may undergo.
  • can be calculated at any given moment by measuring the rate of decay of a known quantity using a radiation detector.

check question

A certain isotope has a half-life of one day. This means the amount of that isotope remaining at the end of two days will be

  1. zero.
  2. one eighth. 
  3. one quarter.
  4. half.



check question

A certain isotope has a half-life of one day. This means the amount of that isotope remaining at the end of two days will be

  1. zero.
  2. one eighth. 
  3. one quarter.
  4. half.

Comment:
And after three days, the amount remaining will be one eighth.

half life and transmutation

Radioactive isotopes decay at a rate characteristic of each isotope. Rates are described by half-life.

The shorter the half-life of a substance → the faster it disintegrates and the more active the substance.

transmutation

Transmutation of elements:
  • the changing of one chemical element to another occurring in
  •      natural events—natural transmutation
  •      artificially in labs—artificial transmutation


transmutation

Natural transmutation:

Alpha emission from a nucleus:
  • mass number decreases by 4 
  • atomic number decreases by 2
  • resulting atom belongs to an element two places back in periodic table

transmutation

Beta emission from a nucleus:
  • no change in mass number—no loss in nucleons
  • atomic number increases by 1
  • resulting atom belongs to an element one place forward in periodic table
  • a neutron turns into a proton, and ejects and electron!

There's also an antineutrino ejected, as well



artificial transmutation

Ernest Rutherford’s cloud-chamber experiment in 1919 succeeded in transmuting a chemical element. After bombardment of nitrogen gas with alpha particles from a radioactive piece of ore, he found traces of oxygen and hydrogen.

Since then, synthetic elements have been produced from atomic numbers 93 to 115, all with short half-lives.

check question

When an element ejects an alpha particle, the atomic number of the resulting element

  1. reduces by 2.
  2. reduces by 4. 
  3. increases by 2.
  4. increases by 4.

check question

When an element ejects an alpha particle, the atomic number of the resulting element

  1. reduces by 2.
  2. reduces by 4. 
  3. increases by 2.
  4. increases by 4.

Explanation:
An alpha particle (a helium nucleus) has atomic number 2. So, ejection of an alpha particle means a loss of two protons. Hence, the atomic number of the element is lowered by two.

cloud chambers

Used to "see" trails of ions created during transmutation

Many new particles were discovered with cloud chambers.

radiometric dating

  • Carbon dating used for plants and animals
  • Based upon ratio of carbon-12 to carbon-14
  • Half-life of carbon-14 is about 5760 years
  • Method not exact due to fluctuations in Earth’s carbon-14 production

  • Nonliving things dated through uranium and lead isotopes
  • Uranium-238 decays to lead-206
  • Uranium-235 decays to lead-207


check question

An archaeological dig uncovers an ancient spear whose wooden handle contains 1/8 the radioactive carbon of a fresh piece of wood.  About how old is the spear?

  1. 11,520 years old
  2. 17,280 years old
  3. 46,080 years old
  4. 23,040 years old

check question

An archaeological dig uncovers an ancient spear whose wooden handle contains 1/8 the radioactive carbon of a fresh piece of wood.  About how old is the spear?

  1. 11,520 years old
  2. 17,280 years old
  3. 46,080 years old
  4. 23,040 years old

Explanation: 
If 1 half-life reduces the C-14 by 1/2, then 2 reduces it by 1/4 and 3 reduces it by 1/8, so the answer is: 
3 x 5760 years = 17,280 years

nuclear fission

Amazing book you must read at some point in your life:

nuclear fission

Nuclear fission is a process in which the nucleus of an atom splits into smaller parts (lighter nuclei)

A neutron hitting a U-235 nucleus may cause it to stretch and fly apart. 


nuclear fission

Energy released is in the form of kinetic energy of fission fragments that fly apart from one another, with some energy given to ejected neutrons, and the remainder given to gamma radiation.


chain reactions

A chain reaction is a self-sustaining reaction in which the products of one reaction event stimulate further reaction events.


critical mass

Critical mass:
the minimum mass of fissionable material in a reactor or nuclear bomb that will sustain a chain reaction

  • You must have a mass large enough to sustain fission
  • At or above critical mass, in a large quantity of atoms, an enormous explosion can occur.


check question

Uranium-235, uranium-238, and uranium-239 are different

  1. elements.
  2. ions. 
  3. isotopes.
  4. nucleons.

check question

Uranium-235, uranium-238, and uranium-239 are different

  1. elements.
  2. ions. 
  3. isotopes.
  4. nucleons.

Comment:
     U-235 is the isotope that can undergo fission.

Mass–Energy Equivalence:

E = mc2

Albert Einstein in the early 1900s:
 
  1. discovered that mass is congealed energy.
  2. formulated the famous equation, E = mc2 , which is the key to understanding why and how energy is released in nuclear reactions.

e=mc2

Relationship of equation terms:

  • The more energy associated with a particle, the greater is the mass of the particle.
  • The mass of a nucleon outside the nucleus is greater than the mass of the same nucleon bound in the nucleus.
  • The greater mass of the nucleon is evident by the energy required to pull the nucleons apart from one another.


e=mc2


fission reactor


1kg of Uranium provides more energy than 30 freight-cars of coal.

nuclear fusion


Nuclear fusion is the combination of nuclei of light atoms to form heavier nuclei with the release of much energy.
  • Any nuclear transformation that moves nuclei toward iron releases energy.
  • Iron is the “nuclear sink” for energy production.

CHECK QUESTION

When energy is released by the process of fission or fusion, the total mass of the material after the event is

  1. less.
  2. the same. 
  3. more.
  4. none of the above.

CHECK QUESTION

When energy is released by the process of fission or fusion, the total mass of the material after the event is

  1. less.
  2. the same. 
  3. more.
  4. none of the above.
Explanation:
The name of the game for energy release is to lose mass. All reactions, nuclear or chemical, lose mass when energy release is the outcome. For chemical reactions, which are much less energetic than nuclear reactions, the mass lost is so small that it is typically ignored.

atoms and the periodic table

By Chris Gregg

atoms and the periodic table

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