CHE 349 Physical Chemistry for Life Sciences Lecture 20 Atom the smallest unit of an element An atom consists of protons neutrons and electrons REMEMBER Protons have a positive charge electrons have a negative charge and neutrons have a neutral charge e g no charge Ex A helium atom has two protons and two neutrons in the nucleus 2 protons 2 neutrons 4 00 atomic mass units amu Two electrons are on the outside of the nucleus circling the center of the atom NOTE The electrons in an atom are arranged in shells that surround the nucleus with each successive shell being farther from the nucleus in other words the electrons in an atom are arranged in shells that orbit the nucleus Subshell a subdivision of electron shells separated by electron orbitals 2 2 2 Ex 1s 2s Orbit the fixed path on which electron moves or revolves around the atom s nucleus Orbital a three dimensional region within which there is a 95 percent probability of finding the electron If you add energy to an electron it can undergo an increase in energy level or transition to a higher orbit within an atom This means that you now have a 95 chance of locating that electron in either the first shell the one closest to the nucleus or the second shell Transition of Energy Throughout the Nucleus of An Atom Pathway 1 The first energy level or first shell in the lowest energy level s orbital is called 1s 2 If you add more energy to the electron in 1s shell that electron will transition into the second energy level or second shell in the S orbital this shell is called 2s If you add more energy to the electron in the 2s shell that electron will transition into the 3 If you add more energy to the electron in the higher second energy shells this shell is called2 2 the higher second energy shells this shell is called2 2 2 If you add more energy to the electron in the the higher second energy shells this shell is called If you add more energy to the you add more to that d shell it will transition into the f shell 2 4 5 6 shell that electron will transition into shell that electron will transition into shell that electron will transition into the d shell and if PARAMAGNETISM VS DIAMAGNETISM NOTE For paramagnetic substances the magnetic field generated by the dipoles or moments of the atom are aligned in the same direction as the external outside magnetic field For diamagnetic substances the magnetic field generated by the dipoles or moments of the atom are aligned in the opposite direction as the external outside magnetic field Bond Order represents the number of chemical bonds between two atoms 12 B O Formula NOTE Bond Order does NOT need to be a whole number average atomic number If your 8 If your 8 Draw the MO diagram for 2 2 of is Since the two atoms are of the same element the average atomic number equal to 6 Carbon has six total electrons but four valence electrons So on the left hand of the MO orbitals you would put in 2 electrons in the 2s orbital and 2 electrons in the 2p orbital one electron per line On the right hand side of the MO orbitals you would put in 2 electrons in the 2s orbital and 2 electrons in the 2p orbital one electron per line You have eight electrons in total for 4 electrons on the left hand side and 4 electrons on the right hand side You will then add those electrons to the molecular orbitals in the middle You start with and work your way up until you ve added the total number of shells 2 2 NOTE When drawing a Molecular Orbital MO diagram you typically apply Hund s Rule Hund s Rule states that electrons will occupy degenerate orbitals orbitals with the same energy singly before pairing up meaning that you fill the lower energy orbitals first placing one electron in each before pairing them B O 12 6 2 12 4 42 2 The bonding orbitals are the ones that aren t asterisked The antibonding orbitals are the ones which are asterisked Since all the electrons are paired in the molecular orbital 2 2 2 2 2 is diamagnetic