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Elements a.k.a atoms
Robert Boyle first defined an element as a substance which could no longer be broken down into other substances Each element has unique properties
Many early scientists speculated how the element or atom was structured
Theory of atomic structure evolved from early thoughts to today’s atom
What do you know about the atom? Take a moment and create a concept
web about the atom. Work on your own. You have about 5 minutes. Jot down everything you can connect
to atoms.
Atoms
Timeline of the Atom
1808 - Dalton first proposed a theory on atoms
Discovery of the electron by J.J. Thompson in the late 1890’s
1910 -Lord Kelvin’s “plum pudding theory
1912 –Bohr model of the hydrogen atom
1932 – Rutherford and his coworker Chadwick identified the neutron
Mid-1920’s – wave mechanical model
Rutherford’s gold foil experiment leads to atomic nucleus and in 1919 the introduction of the proton
Democritus (460-370 B.C.) understood that if you cut a stone in two pieces, each piece contained the same material as the original stone. He also believed that you could do this an infinite number of times. He called these infinitesimally small pieces of matter atomos, meaning "indivisible.“
Who started…
John Dalton (1766 – 1844) was an English scientist who made his living as a teacher in Manchester.
Dalton’s Atomic Theory (p.88) Elements are composed of atoms All atoms of a given element are identical Atoms of a given element are different from those of
any other element Atoms of one element combine with atoms of other
elements to form compounds Law of Constant Composition: all samples of a
compound have the same proportion of the elements as in any other sample of that compound
Atoms are indivisible in a chemical process. all atoms present at the beginning of a chemical
process must also be present at the end of the process.
atoms are not created or destroyed, they must be conserved.
atoms of one element cannot be turned into atoms of another element
Atomic Structure History
Discovery of the Electron 1st atomic particle identified In 1897, J.J. Thomson used a cathode ray
tube to deduce the presence of a negatively charged particle.
Cathode ray tubes pass electricity through a gas that is contained at a very low pressure. This creates a beam of negatively charged particles bent by an electric field.
Conclusions from the Study of the ElectronCathode rays have identical properties regardless of
the elemental gas used to produce them. Therefore, all elements must contain identically
charged particles (electrons). Atoms are neutral, so there must be positive particles
in the atom to balance the negative charge of the electrons
Electrons have so little mass that atoms must contain other particles that account for most of the mass
An electron is a tiny, negatively charged particle
The next step…. What is the positive charge?
William Thomson’s (Lord Kelvin’s) Atomic Model
Lord Kelvin believed that the electrons were like plums embedded in a positively charged “pudding,” thus it was called the “plum pudding” model (easier to think of as “chocolate chips" in chocolate chip cookie dough.)
Rutherford’s Gold Foil Experiment
Rutherford shot α (alpha) particles at a thin sheet of gold foil (think: bullet = alpha particles, target atoms = gold foil)
α particles are positively charged gold atoms are about 50 larger than α
particles. Particles were fired at a thin sheet of gold
foil Particles hit on the detecting screen (film)
were recorded
(a) The results that the metal foil experiment would have yielded if the plum pudding model had been correct. (b) Actual results known as Rutherford’s model.
Over 98% of the particles went straight throughAbout 2% of the particles went through but were deflected by large anglesAbout 0.01% of the particles bounced off the gold foilMost of the volume of the atom is empty space
Rutherford’s Conclusion: A Nuclear Model
The atom contains a tiny dense center called the nucleus
the volume is about 1/10 trillionth the volume of the atom
The nucleus is essentially the entire mass of the atom (extremely dense)
The nucleus is positively charged the amount of positive charge of the nucleus balances
the negative charge of the electrons
The electrons move around in the empty space of the atom surrounding the nucleus
Finally, the neutron.. Discovered in 1932 by Chadwick based on the
idea from Rutherford Has no charge Is located in the nucleus Mass a mass of 1 amu (actually, it’s slightly
larger than a proton but for our work the mass is the same)
So, the structure of the atom ……. The nucleus was found to be composed of two
kinds of particles Some of these particles are called protons
◦ charge = +1◦ mass is about the same as a hydrogen atom
Since protons and electrons have the same amount of charge, for the atom to be neutral there must be equal numbers of protons and electrons
The other particle is called a neutron◦ has no charge◦ has a mass slightly more than a proton
The Modern Atom We know atoms are composed of three
main atomic particles - protons, neutrons and electrons
The nucleus contains protons and neutrons
The radius of the atom is about 100,000 times larger than the radius of the nucleus
A nuclear atom viewed in cross section.
Particle Charge Mass # Location
Electron -1 0 Electron cloud
Proton +1 1 Nucleus
Neutron 0 1 Nucleus
Summary of Atomic Particles
Going beyond the electron, proton, and neutron
Element # of protons
Atomic # (Z)
Carbon 6 6
Phosphorus
15 15
Gold 79 79
Atomic Structure - protons
The number of protons in an atom of a given element is the same as its atomic number (Z).
(Z) is found on the Periodic Table, whole # for each element
Atomic Structure - neutrons
Mass number = protons + neutrons; always a whole number.
# of Neutrons = mass number - # of protons
Atomic mass – decimal number in each element’s box on the periodic table. If you round the atomic mass of an element to the closest whole number you generally get the mass # for that element.
Atomic Structure - Electrons # of Electrons = # of protons if the atom is neutral If the chemical symbol is written with a charge,
representing an ion, the charge indicates the number of electrons that have been added or removed from the atom. If the ion has a positive charge (cation), subtract that
charge from the # of protons to get the number of electrons.
If the ion has a negative charge (anion), add that charge number to # of protons to get the number of electrons.
# of Electrons = # protons – charge Charge = # protons - # electrons
Representing atomic particles in atoms The number of each type of atomic particle
(proton, neutron, electron) is determined by using symbols.
There are several different ways to write an element: Atomic symbols Nuclear symbols
Atomic Symbols include the element symbol and a charge if any. C – neutral carbon C+4 – carbon cation C-4 – carbon anion
Charge (if any)
Cl-1 –38 Fluorine-18
Nuclear Symbols
Element symbol
Mass number (p+ + no)
Atomic number (number of p+)
U238
92
+
Mass number
Mass number
Element name
Element symbol with charge
Isotopes atoms of an element with the same number of protons but different numbers of neutrons
Isotope Protons
Electrons
Neutrons
Nucleus
Hydrogen–1
(protium)
1 1 0
Hydrogen-2
(deuterium)
1 1 1
Hydrogen-3
(tritium)
1 1 2
AMU When we think about the mass of an atom, we
use atomic mass units (amu). A proton is 1 amu A neutron is 1 amu
Add up protons and neutrons to get the mass number (not atomic mass) Why? Most elements in nature have isotopes All these isotopes contribute to the average
atomic mass (listed on the table)
Determining Average Atomic Mass
The average atomic mass seen on the periodic table is a combination of all an element’s isotopes and their abundance.
To determine the average atomic mass for an element, you must
1. Multiply the percentage (percent abundance) of each isotope of the element by its mass number.
2. Add the products of the multiplications together.3. Divide by 100.4. Your answer should be very close to the atomic
mass of the element for that element
Average Atomic Mass Examples Find the average atomic mass of each of the
following elements from their percentages and mass numbers.
69.17% 63Cu and 30.83% 65Cu
5.85% Fe-54, 91.75% Fe-56, 2.12% Fe-57 and 0.28% Fe-58
Nuclear Reactions What you just did was write nuclear reactions.
Typical reactions are decay reactions and capture reactions.
What did you notice about the products of the reactions? The products of the reactions are isotopes of the
element. Nuclear reactions produce different particles that are
not elements. You need these particles to balance out the protons
and neutrons in the nuclei. In a nuclear reaction, the atomic number (Z) and the
mass number (A) are conserved.
Radioactive decay
Radioactive decay is a natural process. Only a handful (~200) of the known isotope nuclei (2000) do
not decay A nuclei may spontaneously kick out a particle, forming
a new element. There are four common particles:
The alpha particle - α (gold foil) The beta particle – β The gamma particle – γ The positron particle
In many cases, the process does not stop at one step, but rather a combination of steps. This is known as a decay series.
Nuclear Transformations Nuclear transformation is the changing of one element
to another (modern alchemy!!!) using larger atoms
Rutherford observed 1st transformation in 1919
Marie Curie and her husband another transformation (14 years later)
Electron capture is the “natural” nuclear transformation
Man-made elements are made by bombarding two nuclei together
Radioactivity Review Radioactivity is expressed as either a decay
process or a capture process
Decay processes can be connected as a chain
Nuclear transformations use larger atoms to create different elements than what you started with
Review of nuclear symbols Mass number is in upper left of symbol Atomic mass is in lower left of symbol
Nuclear particles are written in the nuclear format
U238
92
The alpha particle (α)
The alpha particle is actually a helium nucleus. It is the weakest of the decay particles. A alpha particle has a mass number of 4 and an
atomic number of 2. If an α particle is added to a nuclei, the mass number
will increase by 4 and the atomic number by 2. If an α particle is released from a nuclei, the mass
number will decrease by 4 and the atomic number by 2.
For example:
The Beta particle (β)
The beta particle is an electron. It has no mass. It does have a charge
Examples include:
Sometimes a nucleus will grab an electron that is close. This is called electron capture.
The Gamma Particle (γ)
This is also know as the gamma ray. This is a high energy photon of light. Picked up by specific detectors. It is the strongest (most dangerous) of the decay
particles. Example:
Mass # and the atomic # totals must be the same for reactants and the products.
3919K 35
17Cl + ___
20682Pb 0
-1e + ___
Balancing Nuclear Equations
Alpha decay of Cu-68
Gamma emission of Thorium-235
Positron emission of P-18
Astatine-210 releasing 3 neutrons
Electron capture with Ti-45
Writing Balanced Nuclear Equations
Radioactive isotopes or nuclides all decay because they are unstable, some just breakdown much faster than others
Geiger counter or scintillation counters are used to detect particles.
Half-life – amount of time for half of the original sample to decay
For two samples of the same isotope, regardless of the sample size, after one half-life, only half of the original amount of sample remains.
Half-Life and Nuclear Stability
Isotopes Half-Live Carbon – 14 5730 years Sodium – 24 15 hours Bismuth – 212 60.5 seconds Polonium – 215 0.0018 seconds Thorium – 230 75400 years Thorium – 234 24.1 days Uranium – 235 7.0 x 108 years Uranium – 238 4.46 x 109 years
Sample Half-lives
Working with half-life A material has t1/2 = 10 minutes. If you begin
with 16g, how long will it take to decay to 2 g?
Begin with 16 g, 1 half life gets you to 8 g. 2 half lives get you to 4 g. 3 half lives get you to 2 g. So, 3 x 10 minutes = 30 minutes.
A material has t1/2 = 150 years. If you begin with 100 g, how long will it take to decay to 3.125 g?
A material has t1/2 = 15 minutes. How much material is left after 75 minutes if you begin with 100 g? Calculate the number of half-lives used = 75/15 =
5 Run through 5 half lives:
After 1 – 50g After 2 – 25 g After 3 – 12.5 g After 4 – 6.25 g After 5 – 3.125 g
A material has t1/2 = 6.2 years. How much material is left after 24.8 years if you begin with 14 g?
What is the half-life of a material that decays from 16 g to 2 g over 20 minutes?
Determine the number of half lives: 16 → 8 8 → 4 4 → 2 3 half lives spent
Divide the time by the number of half-lives: 20/3 = 6.67 minutes
What is the half-life of a material that decays from 125 g to 3.9 g over 100 years?
Uses of Nuclear Chemistry Medicine
X-rays and MRIs Chemical tracers
Energy Reactors generate electricity for homes, etc.
Destruction Bombs
Fusion – combining two smaller nuclei into one heavier, more stable nucleus.
32He + 1
1H 42He + 0
1e
Fission – splitting a large unstable nucleus into two nuclei with smaller mass numbers.
20984Po 125
52Te + 8432Ge
Fission versus Fusion
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