Lecture Notes: Atoms, Conservation Laws, and Isotopes

Originally thought of as the smallest, indivisible unit of matter.
99.9999% empty space.
Currently, 118 known atoms, potentially more stable ones up to around 126.
Changing chemical composition involves rearrangement of atoms.
Originally, there was no method to determine the size or appearance of an atom or identify individual elements.

During a chemical reaction, mass cannot be created or destroyed (Law of Conservation of Mass).
Proposed by scientists through observation of elements combining, noting no mass change during heating.
Consistent Mixture of Components: When materials are made, the ratio of components is consistent.
Proust's experiment with copper carbonate showed that lab-made and naturally occurring samples have the same composition.
Chemical synthesis has multiple pathways to a product, but the final composition remains the same.
Ratios were originally worked out based on mass (e.g., water as a one:eight ratio of hydrogen to oxygen).

If a substance contains specific amounts of elements, the same ratio applies even with different masses.
Example provided: Given 2 grams of a substance with 0.93 grams of iron and 1.07 grams of sulfur, calculate the amount of sulfur in 4.19 grams of iron in the same material based on proportion.
Application: Determining how much iron can be extracted from a ton of ore to estimate potential profit.

Chemists use chemical equations as shorthand notation.
Starting materials (reactants) are on the left, products on the right, with an arrow indicating the transformation.
Symbols for elements (H for hydrogen, O for oxygen), some Greek in origin (Pb for lead - plumbonium)

Position of numbers (subscripts and superscripts) is critical.
O2 indicates two oxygen atoms joined together, while 2O indicates two individual oxygen atoms.
Chemical equations must be balanced, reflecting the conservation of mass.
Example: Splitting water (H2O) to form hydrogen (H2) and oxygen (O2) requires balancing: $2H<em>2O \rightarrow 2H</em>2 + O_2$

Sulfur dioxide (SO2) is a gas, while sulfur trioxide (SO3) is a solid, suggesting different atomic arrangements.
In SO2, one gram of sulfur interacts with one gram of oxygen, while in SO3, the ratio is one:1.5.
Different properties indicate that the arrangement of atoms affects chemical properties.

Experiments showed that something could change photographic plates at a distance, suggesting the atom is not the end-all.
Cathode ray experiments showed the emission of negatively charged particles (electrons) from a heated source.
If negatively charged particles are ejected from an atom, what remains must be positively charged to maintain electrical neutrality.

Plum Pudding Model: original belief that the atom was basically a pudding and the positive charges were spread out in there and the electrons were spread out in there.

In 1909, Millikan calculated the charge on an electron: $1.5924 \times 10^{-19}$ coulombs (current value is $1.6 \times 10^{-19}$ coulombs).
The mass of an electron is $10^{-31}$ kg.

Alpha particles fired at a thin gold foil.
Most particles passed straight through, but a small fraction (0.0125%) bounced back.
This suggested that all positive charge is concentrated in the middle of the atom, with the rest being empty space.
The electrons orbit the nucleus like planets around the sun (although quantum mechanics provides a more nuanced picture).

The nucleus contains positively charged protons.
To balance the mass, there must also be neutrons.
Atoms are made up of electrons, protons, and neutrons.
Electrons are in different levels, which is significant because electrons in an atom are different depending on where they actually are.
The mass of the neutron and the mass of the proton is pretty much exactly the same.

Carbon is the element of life. It crops up everywhere.
Atomic mass is defined relative to carbon, with carbon-12 assigned a mass of 12.
This simplifies calculations, as it's easier to work with 12 rather than $10^{-27}$ .
Atomic number (Z) is the number of protons in the nucleus; mass number (A) is the total number of protons and neutrons.
For oxygen (atomic number 8), the mass number is typically 16 (8 protons + 8 neutrons).

Different forms of the same element with varying numbers of neutrons.
Hydrogen exists as hydrogen, deuterium (one neutron), and tritium (two neutrons).
Isotopes have different chemical properties (e.g., deuterium oxide (D2O) is denser than H2O).
Most elements have multiple isotopes, making calculation of molecular weight complex.

Carbon-12 (six protons, six neutrons) is the standard.
Carbon-13 (six protons, seven neutrons) is used in resonance spectroscopy.
Carbon-14 (six protons, eight neutrons) is radioactive and used for radiodating.

Knowing the exact mass of materials is crucial for chemical synthesis.
Calculations must account for the contribution of each isotope to the overall mass of an element.