Atomic Theory

Dalton's Atomic Theory:

Some basic laws that have been established by the year 1800:

• Law of definite composition (Berzelius): Every pure compound has a constant composition.
• Laws of conservation of mass (Lavoisier): Matter can neither be created or destroyed during a chemical change.

Dalton's Laws:

1. All matter is composed of extremely small particles called atoms.
2. All atoms of a given element are alike, but atoms of one element differ from atoms of any other element.
3. Compounds are formed when atoms of different elements combine in fixed proportion.
4. A chemical reaction involves a rearrangement of atoms. No atoms are created or destroyed or broken apart in a chemical reaction.

The law of definite composition is explained by the third statement. Water (H20) is always formed by two atoms of hydrogen joining with one atom of oxygen. Its elemental composition never changes, no matter were it was from or how it was synthesised. 

The fact that chemical changes involve the rearrangement of atoms explains why mass is conserved during a chemical reaction.

Atoms and Molecules:

• Atoms: The smallest part of an element that can take part in chemical changes.

Usually individual atoms are not found in nature except the noble gases which occur in atomic form.

• Helium, He used as an extreme coolant and in helium balloons
• Neon, Ne used in neon lights and neon lasers
• Argon, Ar used in welding and chemical industry as an inert gas
• Krypton, Kr used in automobile headlights (banned in a lot of European counties now because too bright)
• Xenon, Xe used in Xenon Ion Propulsion Engines (space missions)

• Molecules: The smallest particle of a substance that still has the properties of that substance.

Several elements exist as diatomic molecules.

• Hydrogen, H2 Used as a fuel and reactant and in small weather balloons
• Oxygen, O2 vital to sustain life, essential to carry out combustion.
• Nitrogen, N2 most abundant gas in the atmosphere, used as inert gas and to generate ammonia
• Fluorine,F2 most reactive of all elements. Used as a reactant in the manufacture of Teflon and refrigerants
• Chlorine, Cl2 Used as a disinfectant and for bleaching and in the manufacture of PVC
• Bromine, Br2 Important reactant.
• Iodine, I2 essential trace element in the human body (thyroid)

Atomic Structure:

Although Daltons atomic theory is still largely true 200 years after it was proposed it has a weak spot. Atoms are not indestructible. Around the turn of the 19th Century experimental observations were starting to accumulate that supported our modern view of atoms.

The Discovery of the Electron:

• JJ Thompson is credited with it's discovery in 1897.
• He determined the charge/mass ratio of the electron to me 1.76E-5 coulomb/g

Below, video 1 gives a good run down of Thompson's experiment that led to this discovery:

Video 1 - JJ Thomson's cathode ray tube experiment

Millikan’s Oil Drop Experiment:

After the mass to charge ratio of the electron was known:

• Millikan determined the charge of the electron with his oil drop experiment.
• Consequently, he concurrently found the mass of the electron because of Thompson's discovery earlier on of the charge/mass ratio. 

Below, in video 2, Millikan's oil drop experiment is explained:

Video 2 - Millikan’s Oil Drop Experiment

Discovery of Radioactivity:

• Pierre and Marie discovered and isolated radioactive elements.
• The chemical and physical properties of a radioactive element changes as it undergoes radioactive decay (transmutation). 
• Rutherford discovered 3 types of radiation while studying radioactive decay.
• α (alpha) particles: 2 proton and 2 neutrons; a helium nucleus emitted by some radioactive substances.
• β  (beta) particles: a fast-moving electron emitted by radioactive decay of substances.
• γ  (gamma) rays: penetrating electromagnetic radiation of a kind arising from the radioactive decay of atomic nuclei.

Rutherford and the Discovery of the Nucleus:

• Ernest Rutherford discovered the nucleus with his famous gold foil experiment.

Below, in video 3, Rutherford's gold foil experiment is explained:

Video 3 - Rutherford's gold foil Experiment

Summary:

Each of the roughly 100 elements found in nature is composed of the same three fundamental particles:

• The charge of -1 refers to the charge of an electron which is -1.602176462 Coulomb.
• 1 u (formerly amu for atomic mass unit) = 1.6605E-24 g = 1/12 of the mass of one 12C atom

Atoms have a small dense nucleus that holds almost the entire mass of the atom. Atoms are mostly empty space (Figure 1).

Symbolic Representation of an element:  

• The number of protons in the nucleus identifies the element.
• The number of protons = ATOMIC NUMBER (Z)
• The number of protons + the number of neutrons = MASS NUMBER (A)

Isotopes:

• Atoms of a particular element may have the same atomic number but have differing mass numbers. If they do, then they are called isotopes of that element.

For example the three isotopes of hydrogen (Figure 2):

Because the Isotopes of hydrogen are so important they have special names. 1H is the most abundant
isotope of hydrogen with 99.985% of all natural occurring hydrogen consisting of this isotope
independent of its chemical form (e.g. water, oil, cellulose). 2H (deuterium) is an isotope of hydrogen that is naturally occurring with 0.015 %.


Water made from deuterium and oxygen (D2O) has chemical properties very similar to ordinary water (H2O). It will react with active metals to generate hydrogen, it will react with nonmetal oxides to form acids. Because deuterium is much heavier than hydrogen the rate at which the reactions proceed might be noticeably different. The mass also affects some physical properties. 

Deuterium oxide which is also known as heavy water is used in nuclear reactors and as a solvent for nuclear magnetic resonance spectroscopy.