Chemistry Notes Subatomic Particles

Basic Particles in the Atom RECALL: Protons – large mass – positive charge Neutrons – large mass – no charge Electrons – virtually no mass – negative charge

Discovery of the Particles Recall: Thomson and the electron Rutherford and the proton What about the neutron?

Discovery of the Particles Discovery of the Neutron Scientist since Rutherford realized that protons alone could not account for the mass of the nucleus. –Mass of H + = 1amu (H + is only a proton) –Mass of He 2+ = 4 amu Discovered in 1932 by James Chadwick –Cambridge, England

Discovery of the Particles Chadwick's experiment –Shot alpha particles at Be, this ejected radiation from the Be in the form of particles The ejected particles (neutrons) were not effected by an electric or magnetic field. Inference  particles are neutral –The ejected particles dislodged protons from the wax. Chadwick counted the number of protons and reasoned that it was the same number of particles coming from the Be. –Inference  the size of protons and neutrons are nearly the same Chadwick released alpha particles at a beryllium plate. Alpha particles are made of one proton and one neutron. When an alpha particle hit the beryllium, the proton bounced off and the neutron went through. These neutrons hit a sheet of paraffin and knocked high energy protons from it. He then detected and counted the protons. For each proton coming from the paraffin, he knew there was one particle of mysterious radiation coming from the beryllium. He used this data to deduce the existence of the neutron. Chadwick released alpha particles at a beryllium plate. Alpha particles are made of one proton and one neutron. When an alpha particle hit the beryllium, the proton bounced off and the neutron went through. These neutrons hit a sheet of paraffin and knocked high energy protons from it. He then detected and counted the protons. For each proton coming from the paraffin, he knew there was one particle of mysterious radiation coming from the beryllium. He used this data to deduce the existence of the neutron.

Discovery of the Particles Purpose of the Neutron Recall: protons are particles with a positive charge; they want to repel. This makes the nucleus unstable. The nucleus being comprised (in part) of protons needs a force to hold it together – the strong force –Only powerful at a very short range The strong force is more powerful than the electrostatic force at a close range

Discovery of the Particles Purpose of the Neutron (cont.) Neutrons also possess this strong force but with no charge. This allows them to hold the nucleus together more tightly than the protons alone can accomplish. As a nucleus gets larger, it gets more electric force pushing out the protons. –A larger nucleus needs more neutrons. –The lighter elements are about half proton and half neutron. The heaviest elements have many more neutrons than protons.

Summary Do we know each of the three elementary particles of the atom? –How discovered With what experiments / person? –Their charge –Their location in the atoms

Particles of the Atoms Different numbers of subatomic particles alter the behavior of atoms. Protons = define the atom!!! –If Z = 4 you always have Be Different #’s of neutrons create isotopes –C – 12 and C -14 Different #’s of electrons than protons create ions –Na and Na +

Atoms and the Periodic Table Each atom is represented on the PT Atomic # - represents the atom’s protons –Smaller of the two #’s next to the symbol –Protons define the atom EX: all carbon atoms have 6 protons no matter how many neutrons or electrons they have ATOMIC #

Atoms and the Periodic Table Mass # - mass of atom –Tells you the # of protons and neutrons –Larger of the two numbers next to the symbol Mass # Atomic masses are weighed averages of isotopes Ex: naturally occurring lithium is composed of 2 isotopes: (7.4%) Li-6 and 92.6% Li-7 Averaging masses  6.941

Atoms and the Periodic Table The units for the mass # are amu’s Atomic Mass Units Defined as 1/12 the mass of a carbon-12 atom 1 amu is approximately equal to the mass of 1 proton (1.0073) or 1 neutron (1.0087)

Determining the # of Particles in an Atom Determining Protons Atomic # = # of Protons Determining Electrons In a neutral atom the # of protons = the # of electrons –Therefore the Atomic # also tells the # of electrons –Ions will have a symbol w/ a “+” or “-” sign. This indicates how many electrons are lost or gained. EX: Al 3+ aluminum’s has 13 electrons minus the 3 it lost, so it now has 10 electrons total.

Determining the # of Particles in an Atom Determining the number of neutrons Subtract the atomic # (# of protons) from the mass # (# of protons and neutrons) EX: What is the number of neutrons in a carbon atom? Mass # of C = (round to 12) Atomic # of C = 6 # of neutrons in C = 6 The number of neutrons do not always equal the number of protons ( ex U  238 – 92 = 146 neutrons )

Atoms and Subatomic Particles Recall: Protons define the atom! Know: Atoms with the same # of protons but different # of neutrons are called isotopes. –Ex: Carbon 12 and Carbon 14

Atoms and Subatomic Particles Know: –Atoms that are not neutral due to losing or gaining electrons are called ions.

Atoms and Subatomic Particles Ions: Charged Atoms Positive ions are called cations –Sodium loses 1 e- and becomes Na +1 Negative ions are called anions –Chlorine gains 1 e- and becomes Cl -

Atoms and Subatomic Particles Valance Shell Electrons Electrons of the outermost energy level Most likely involved on bonding Most likely to leave atom Elements in the same group on the PT have the same # of valance shell electrons –Group 1 elements all have 1 electron in their valance shell

Atoms and Subatomic Particles Valance electrons can be represented with Lewis Dot Structures (symbols)

Subatomic Particles Summary Subatomic particles –Protons / neutrons / electrons Atoms and the PT –Atomic # / mass # Determining protons / neutrons / electrons Atoms can have isotopes and ions Valance electrons can be represented by Lewis Dot Structures