Chapter 2
Topic 1
Atoms and the Periodic Table
Simple Atomic Theory
Basic Elements
The Greek philosophers Thales and Aristotle speculated that matter was composed of five elements :
Fire
Water
Air
Earth
Quintessence (pure, highly � concentrated essence)
Each object’s motion was predetermined by its natural or proper place.
Rock is earth therefore will fall to earth
Smoke is fire therefore will rise above air.
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ATOMOS
DEMOCRITIUS suggests the existence of the smallest piece of matter that is the ATOM
ATOMOS in Greek means indivisible, indestructable or individual.
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Atoms First
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An atom is the smallest quantity of matter that still retains the properties of matter.
An element is a substance that cannot be broken down into two or more simpler substances by any means.
A DVD collection can be separated into smaller numbers until you have just one DVD left. But a single DVD cannot be separated into smaller pieces that are still DVDs.
Atoms can also be divided smaller and smaller until eventually only a single atom remains. Dividing it any smaller would give pieces that are no longer an atom.
The Atomic Theory
In 1808, John Dalton formulated a precise definition of matter that we call atoms:
The Elements� or Smaller
What makes up an element?
What makes up the Atom?
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Atomic Man made up of individual atoms on a surface of platinum
Atoms First
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Dalton said that atoms, of which all matter consists, are tiny, indivisible particles.
We can expand Dalton’s theory:
Once a single atom has been obtained, dividing it smaller produces subatomic particles.
The nature, number, and arrangement of subatomic particles determine the properties of atoms, which in turn determine the properties of all things material.
We will start by examining the structure of atoms and the tiny subatomic particles that atoms contain.
Subatomic Particles and Atomic Structure
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In the late 1800’s, many scientists were doing research involving radiation, the emission and transmission of energy in the form of waves.
They commonly used a cathode ray tube, which consists of two metal plates sealed inside a glass tube from which most of the air has been evacuated.
Radioactivity
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Wilhelm Rontgen (1845-1923) discovered �X-rays. They were not deflected by magnetic or electric fields, so they could not consist of charged particles.
Antoine Becquerel (1852-1908) discovered radioactivity, the spontaneous emission of radiation. In 1896, he discovers radioactivity by accidentally exposing a photographic plate with Uranium ore. He called these rays Uranic rays.
Radioactive substances, such as uranium, can produce three types of radiation.
Radiation
Three types of decay particles produced by radioactive decay include:
Alpha (α) rays; positively charged, large mass
Beta (β) rays; negatively charge, small mass
Gamma (γ) rays; no charge, no mass; � very high energy waves
Structure of the Atom
Sir J.J. Thomson and his student Ernest Rutherford wished to discover the nature of the atom.
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The Structure of the Atom
When metal plates are connected to a high-voltage source, the negatively charged plate, or cathode, emits an invisible ray.
The cathode ray is drawn to the anode.
Cathode
(-)
Anode
(+)
The Structure of the Atom
Cathode rays may be deflected by magnetic or electric fields.
Thomson’s contributions:
He proposed the rays were actually a stream of negatively charged particles.
These negatively charged particles are called electrons.
By varying the electric field and measuring the degree of deflection of cathode rays, Thomson determined the charge-to mass ratio of electrons to be:
1.76×108 C/g.
(C is coulomb, the derived SI unit of electric charge.)
The Structure of the Atom
R. A. Millikan measured the charge of an electron with great precision.
Charged Particles
If the Electron is negative, does the atom have unbalanced charge resulting in a shocking affect?
There must be present in the atom a charged particle which neutralizes the charge of the electron.
The Proton was theorized to exist.
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Canal Tube Experiment
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1914: The Proton was proven to exist.
Thomson Plum Pudding
The first model of the atom was developed by J.J. Thomson in 1904
Rutherford does an experiment to prove this model correct in 1909.
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The Gold Foil Experiment
Ernest Rutherford used α particles to prove the structure of atoms.
The majority of particles penetrated the gold foil undeflected.
Sometimes, α particles were deflected at a large angle.
Sometimes, α particles bounced back in the direction from which they had come.
The Structure of the Atom
Rutherford proposed a new model for the atom:
Positive charge is concentrated in the nucleus
The nucleus accounts for most of an atom’s mass and is an extremely dense central core within the atom
A typical atomic radius is about 100 pm
A typical nucleus has a radius of about 5 x 10–3 pm
1 pm = 1 x 10–12 m
The Structure of the Atom
Protons are positively charged particles found in the nucleus.
Neutrons are electronically neutral particles found in the nucleus.
Neutrons are slightly larger than protons.
The Models of the Atom
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Chadwick’s Experiment (1932)�(1935 Noble Prize in Physics)
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H atoms - 1 p; He atoms - 2 p
mass He/mass H should = 2
measured mass He/mass H = 4
α + 9Be
1n + 12C + energy
neutron (n) is neutral (charge = 0)
n mass ~ p mass = 1.67 x 10-24 g
The Neutron
was proven to exist in 1932 by James Chadwick.
has a mass equivalent of 1.008665 u, slightly more mass than the proton
has no positive or negative charge, the balance of negative and positive charges remains constant with the number of protons and electrons.
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The Periodic Table - An Element
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Ag
107.87
Silver
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ATOMIC NUMBER - Z
The atomic number equals the number �of protons in the atom
Each atom has a unique number of protons.
SYMBOL
This is the shorthand letter that
allows chemists talk about different Elements
AVERAGE ATOMIC MASS
An averaged mass of the atom in grams per mole, representing all the isotopes and the abundance.
SYMBOLS
An isotope of any element is represented by a symbol which gives the name of the element, the atomic number of the element, the mass number of the isotope, and if needed the charge present on the atom.
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O
2 = no. of atoms
C = #p - #e�2- = charge
A = #p + #n mass no. = 18�
atomic no. = 8� Z = #p
Isotopes of Hydrogen
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Most elements have two or more isotopes, atoms that have the same atomic number but different mass numbers.
Protium or Hydrogen - 1
1 proton
0 neutrons
Deuterium or Hydrogen - 2
1 proton
1 neutron
Tritium or Hydrogen - 3�1 proton
2 neutrons
Isotopes of the same element exhibit similar chemical properties, forming the same types of compounds and displaying similar reactivities.
Activity 5
How Many?
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How Many?
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How Many?
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ISOTOPES
In 1914, T.W. Richards won a Nobel Prize for proving the existence of isotopes.
What is an isotope?
An isotope is an atom of an element that has the same number of protons but a different mass which results from a larger or smaller number of neutrons present in the nucleus
The mass number, A, represents the sum of the number of protons and the number of neutrons in a specific isotope of an atom.
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An ion is an atom, or group of atoms, that has a net positive or negative charge.
cation – ion with a positive charge
If a neutral atom loses one or more electrons
it becomes a cation.
anion – ion with a negative charge
If a neutral atom gains one or more electrons
it becomes an anion.
Na
11 protons
11 electrons
Na+
11 protons
10 electrons
Cl
17 protons
17 electrons
Cl-
17 protons
18 electrons
Ions - Cation
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Ions - An Anion
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How Many?
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The Element that has an atomic number of 50 is
A. Sn B. V
How Many?
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How Many?
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Activity 6
Nuclear Stability
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The nucleus is a small portion of the total volume of an atom
Example of a nucleus with 30 protons and 30 neutrons
Nuclear Stability
Principle factor for nuclear stability is neutron-to-proton
ratio (n/p)
Nuclear Stability
Principle factor for nuclear stability is neutron-to-proton ratio (n/p)
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Magic numbers
Symbolism
Proton = 11p
Neutron = 10n
Electron = 0-1e
Alpha (𝛂) particle:
Helium-4 nucleus: 42He2+
Beta (𝛃) particle: e-
Gamma (𝛄)
Positron (𝛃+) = 0+1e
Radiation is emitted from isotopes of certain unstable elements that spontaneously emit particles and energy from the nucleus.
Alpha Decay
Parent nuclide → daughter nuclide + He-4
23892 U → 23490Th + 42He
Beta Decay
The nucleus can emit a beta particle by changing a neutron into a proton
10n → 11p + 0-1e-
daughter nuclide → Parent nuclide
146C → 147N + 0-1e-
Gamma ray emission
Gamma rays are high energy photons or electromagnetic radiation with no charge and no mass
6128Ni (excited) → 6128Ni + 00𝝲
Gamma rays are generally emitted in conjunction with other types of radiation
Radioactive decay series
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