AP Chemistry Unit Review

Unit 1: Atomic Structure and Properties

How it applies to our lives

In crime scenes investigation, forensic scientists often use mass spectrometry as a way to identify the unknown substance by measuring their mass and composition. This connects to the moles and molar mass concept since scientists have to convert the masses of these tiny particles into measurable amounts in grams and moles in order to determine what the substance is actually made of. Percent composition and empirical formula is also used as a way to identify the drug or unknown substance through breaking it down. Once it is known, chemists can match it to known compounds in their date base. An example of this would be that Michelle Evans, a forensic chemist who works for the Bureau of Alcohol, Tobacco, Firearms, and Explosives., investigated the burning of a bone of two young women who died in an explosion looking into the bones using a mass spectrometer to identify molecules from the explosion in this case. The results from the explosion were hydrocarbons and presents of potassium and chlorine that can signal compounds of potassium chlorate or potassium perchlorate.

Key Vocabulary

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Ground state
Photoelectron spectroscopy
Ions
Valence electrons
Excited state
Atomic numbers
Main Takeaways
Things like atomic radius, ionization energies, and electronegativity follow predictable trends across periods and down groups due to their nuclear charge and electron shielding. For example as you move from left to right across a period, the number of protons will increase which will create a stronger attraction pull from the nucleus. The increase in nucleus pull will cause the atomic radius to decrease while making it harder to remove an electron due to higher ionization energy and higher electronegativity.
Every atom consists of protons which define the elements, neutrons which can vary, and electrons which determine the charges. For example a normal Carbon is Carbon-12 which has 6 protons, 6 neutrons, and 6 electrons. If you change the number of neutrons to 7, then it will change to Carbon-13. So the elements stay the same while the neutrons change. However if you remove 2 electrons instead, the atoms will end up gaining a +2 electrical charge and become C2+. While changing the number of protons will change the elements as a whole.
1 mole equals 6.022 x 10^23 particles which is Avogadro's number and molar mass is connected to grams to moles which allows us to convert between grams, moles, and particles. We can also use this to find Percent comp and determine empirical and molecular formulas. For example if there is an unknown powder which has 40% Carbon, 67% Hydrogen, and 53.3% Oxygen by mass, we can use those percentages as grams. Use the molar mass to convert them into moles by finding the whole number mole ratio to calculate the empirical formula of CH2O. Then use total Molar mass to find molecular formulas such as glucose C6H12O6.
The binding energy measured is equal to the energy needed to remove the electrons and electrons that are closest to the nucleus have higher binding energy. So this is because electrons are negative while protons are a positive charge. Opposite charges attract, making the electrons near the inner shell experience a way stronger electrical pull than the one further away from it. An example can be a lithium atom which has 2 core electrons in its inner shell and 1 valence electrons in its outer shell. If you rip one of the inner core electrons we will require way more energy than taking out the other valence ecotrons located in the outer shell.
In Photoelectron Spectroscopy, the heights of each peak show the number of electrons in that sublevels.The heights are relative to each other. So peaks with higher binding energy will be closer showing that it has stronger attraction to the nucleus and require more energy to remove. While the Mass Spectroscopy is based on different isotopes of the same element with different masses.
Common Misconceptions
Many people believe that a high atomic number means there will be a larger atomic radius. This is wrong because atomic radius actually decreases across a period because the number of protons are increasing which creates a stronger attraction that pulls the electrons closer to the nucleus. The atomic radius actually increases down a group due to an extra electron shell being added. So it places the other electron farther away from the nucleus.
Unit Quiz

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