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 He2+ = 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 ParticlesPurpose 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 AtomsDifferent 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 AtomDetermining Protons• Atomic # = # of ProtonsDetermining 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: Al3+ aluminum’s has 13 electrons minus the 3 it lost, so it

now has 10 electrons total.

Determining the # of Particles in an AtomDetermining 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 = 12.011 (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 ParticlesRecall:• 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 ParticlesIons: 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 ParticlesValance electrons can be represented with Lewis Dot Structures (symbols)

Subatomic ParticlesSummary• 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