IB SL Chemistry Unit 2

UNIT TWO ● Atomic structure ○ Atoms are made of nucleons and electrons ■ Nucleons are neutrons and protons ○ Electrons exist in separate energy levels named energy shells ● Calculating relative atomic mass ○ Bromine exists as two isotopes: bromine-79 and bromine-81. The relative atomic mass of bromine is 79.90. Calculate percent abundance of each isotope ■ Create a system of equations ■ Assign variable x to bromine 79 ■ 79x+81(100-x)=100 ■ Solve for x and get 55 ■ Bromine is 55% Br79 and 45% Br81 ● Atomic orbitals ○ Electrons exist in shells that are split up by energy. ○ The lowest energy level can carry two shells. ○ Each energy level is divided into subshells that each carry a different amount of electrons ○ Each energy level is divided into subshells that each carry a different amount of electrons/ pairs of electrons ■ S orbital carries 2 (1 pair of electrons) ■ P orbital carries 6 (3 pairs) ■ D orbital carries 10 (5 pairs) ■ F orbital carries 14 (7 pairs) ○ Energy levels can be noted as a value of N ■ First Level - n=1: Only has an S subshell ■ Second Level - n=2: has both S and P subshell ■ Third Level - n=3: has S P and D subshells ■ Fourth Level - n=4: has S P D and F subshells Given alongside is a 3D representation of the s, px, py, and pz shells and how they exist on the axes ● Pauli Exclusion principle ○ Wolfgang Pauli discovered this law on the orientation of electrons. ○ Electrons spin two ways, and two electrons will not pair together if they spin the same direction. ● Aufbau Principle ○ Electrons will occupy shells starting from the lowest shell to the highest energy shell. ○ Notice how the 4s shell fills up before 3d shell. 4s shell is a lower energy the shell than the 3d. ○ Chromium and Copper are the exceptions to the Aufbau Principle. ● Hund’s Rule ○ Electrons will fill up a shell one by one. ○ To help better understand, imagine kids on a bus. Every kid wants a seat to themselves, so they’ll find their own seat. Once every seat is taken up, then kids will start doubling up. ● Heisenberg uncertainty principle ○ We can’t ever know where an electron is. Our knowledge of their placements relies on the probability of an electron being in that shell. ● Electron Configuration ○ Electrons are written as the primary quantum number(n), with with the subshell and an exponent to dictate how many electrons are in the subshell ○ For example, Carbon has 6 electrons ■ It is written as 1s22s22p2 ○ Configuration of ions. When an atom gains or loses electrons, it becomes an ion. Electrons will always shed the electrons on the farthest shell. We’ll discuss this later. ○ For example, Sodium has an electron configuration of 1s22s22p63s ○ When it becomes the Na+ ion, the configuration will be 1s22s22p6 ● Noble Gas configuration ○ A shorter way to write out longer electron configurations. ○ For example, Iron has 26 electrons. ○ It’s full configuration is 1s22s22p63s23p64s23d6 ○ Iron can also be written using the nearest noble gas as a starting point ○ The noble gas configuration for Iron is [Ar]4s23d6 ● Chromium and Copper ○ Chromium should have a configuration of [Ar]4s23d4 ○ In reality, it has a configuration of [Ar]4s13d5 ○ Copper should have a configuration of [Ar]4s23d9 ○ In reality, it has a configuration of [Ar]4s13d10 ○ You won't need to know why, but just for your own knowledge, it is to bring further stability to the atom by completely filling an inner energy level. ● Light Spectrum ○ When light is passed through a prism, it emits a continuous spectrum that shows all the wavelengths of visible light. ○ absorption spectrum is the light absorbed by the reflected material. Characterized by a spectrum of visible light with noticeable black bands. ○ Emission spectrum is the light emitted out by the material. Characterized by bands of light on a black spectrum. ● Absorption spectrum of Hydrogen ○ When a gas becomes excited, electrons can jump energy shells. ○ From n=2 - n=6, hydrogen absorbs violet light ○ From n=2 - n=3, hydrogen absorbs red light ● Emission spectrum of Hydrogen ○ As energy levels come back down, hydrogen will emit light at the same frequency it absorbed them. ○ You must know this graph for any IB exams.