Download THE NUCLEUS

Opportunities are usually disguised as hard work, so most
people don't recognize them.
- Ann Landers
ATOMS CAN BE SUBDIVIDED INTO SMALLER
PARTICLES IF WE HIT THEM WITH ENOUGH ENERGY.
THE THREE SUBATOMIC PARTICLES WE ARE GOING
TO BE CONCERNED WITH ARE:
ELECTRONS
PROTONS
NEUTRONS
HOW MANY OF THESE EXIST IN A GIVEN ATOM
DETERMINES THE PROPERTIES OF THE ATOM.
ALL ATOMS CAN BE DIVIDED INTO TWO
PARTS:
1. THE NUCLEUS - THIS IS THE CENTRAL
CORE OF THE ATOM, AND IT CONTAINS
PROTONS AND NEUTRONS.
2. SURROUNDING SHELLS OF ELECTRONS.
WE OFTEN USE THE “SOLAR SYSTEM MODEL”
OF THE ATOM TO DESCRIBE ITS
PROPERTIES.
A PROTON HAS A MASS OF 1.673 X 10-24 g AND A
CHARGE OF +1 (A UNIT POSITIVE CHARGE).
A NEUTRON HAS A MASS SIMILAR TO THAT OF A
PROTON, 1.675 X 10-24 g, AND NO CHARGE. IT IS
NEUTRAL.
AN ELECTRON HAS A MASS OF 9.11 X 10-28 g,
WHICH IS A GOOD BIT SMALLER THAN THE
MASSES OF NEUTRONS AND PROTONS. IT HAS A
CHARGE OF -1 (A UNIT NEGATIVE CHARGE).
USING THE MASSES OF THESE PARTICLES IN
GRAMS IS CUMBERSOME, SO WE USE THE
ATOMIC MASS UNIT (1 amu) FOR THE MASS OF
THE PROTON AND NEUTRON.
THE ELECTRON IS SO MUCH LIGHTER THAT WE
WILL ASSUME THAT ITS MASS IS 0 COMPARED TO
THAT OF THE PROTON AND NEUTRON.
PARTICLE
MASS
Amu
Location Charge
Neutron
1.67 x
10-24
1
Nucleus 0
Proton
1.67 x
10-24
1
Nucleus +1
Electron
0.0009 0
x 10-24
Shells
-1
outside
nucleus
ATOMS ARE NORMALLY NEUTRAL, SO THIS MEANS
THAT THEY HAVE TO HAVE THE SAME NUMBER OF
ELECTRONS IN SHELLS ABOUT THE NUCLEUS AS
THERE ARE PROTONS IN THE NUCLEUS.
IN OTHER WORDS, THE NUMBER OF NEGATIVE
CHARGES HAVE TO BALANCE THE NUMBER OF
POSITIVE CHARGES.
# positive charges = # negative charges
OR
# PROTONS = # ELECTRONS
ATOMS OF DIFFERENT ELEMENTS DIFFER
FROM EACH OTHER BY THE NUMBER OF
PROTONS IN THE NUCLEUS AND THE
NUMBER OF ELECTRONS IN SHELLS ABOUT
THE NUCLEUS.
IT IS THIS NUMBER THAT DETERMINES THE
PROPERTIES OF THE ATOMS OF AN
ELEMENT.
THIS NUMBER IS THE ATOMIC
NUMBER.
THE SIMPLEST ELEMENT IS HYDROGEN.
THE MOST COMMON FORM OF HYDROGEN HAS A
SINGLE PROTON IN THE NUCLEUS AND A SINGLE
ELECTRON IN A SHELL ABOUT THE NUCLEUS.
HYDROGEN IS THE MOST ABUNDANT ELEMENT IN
THE UNIVERSE.
IF WE COULD LOOK AT THE DISTANCES INSIDE AN
ATOM, WE WOULD QUICKLY REALIZE THAT AN
ATOM IS MOSTLY EMPTY SPACE.
IF WE WERE TO EXPAND A HYDROGEN ATOM SO
THAT THE PROTON WAS THE SAME SIZE AS A
HUMAN, THE ELECTRON WOULD BE AT A DISTANCE
OF ABOUT 2 MILES AND WOULD BE THE SIZE OF A
SMALL BIRD.
WE MENTIONED EARLIER THAT IT IS THE NUMBER
OF ELECTRONS (OR PROTONS) THAT
DETERMINES THE PROPERTIES OF A NORMAL
ATOM.
THIS NUMBER IS CALLED THE ATOMIC NUMBER.
WE’LL DEFINE IT AS THE NUMBER OF PROTONS IN
THE NUCLEUS, AND WE WILL USE THE SYMBOL Z.
Z = ATOMIC NUMBER = # PROTONS
NOW, ELECTRONS HAVE VERY LITTLE MASS
COMPARED TO THE MASSES OF NEUTRONS AND
PROTONS.
SO, THE MASS OF AN ATOM, OR THE ATOMIC MASS
WILL BE THE SUM OF THE PROTONS AND NEUTRONS
IN THE NUCLEUS.
WE’LL USE THE SYMBOL A FOR ATOMIC MASS.
A = ATOMIC MASS = #PROTONS + # NEUTRONS
RATHER THAN WRITE THE NAMES OF THE
ELEMENTS, WE USE A SHORTHAND METHOD FOR
DESIGNATING THE ELEMENTS.
FOR EXAMPLE, THE SYMBOL FOR HYDROGEN IS
H.
THE SYMBOL FOR HELIUM IS He.
THE FIRST LETTER WILL BE CAPITALIZED, AND IF
THERE IS A SECOND LETTER, IT WILL BE SMALL.
THE SYMBOLS ARE MOSTLY BASED ON THE
ENGLISH NAMES, BUT THERE ARE SOME
EXCEPTIONS.
IRON = Fe
SODIUM = Na
LEAD = Pb
THE PERIODIC TABLE IS ARRANGED IN ORDER OF
INCREASING ATOMIC NUMBER, Z.
FOR EXAMPLE, HELIUM, He, THE SECOND ELEMENT,
HAS AN ATOMIC NUMBER OF 2.
THIS MEANS THAT IT HAS 2 PROTONS IN THE
NUCLEUS AND TWO ELECTRONS IN SHELLS ABOUT
THE NUCLEUS.
THE ATOMIC MASS IS 4. THIS MEANS THAT IT HAS
TWO NEUTRONS IN THE NUCLEUS IN ADDITION TO
THE TWO PROTONS.
# NEUTRONS = A - Z = ATOMIC MASS - AT. NUMBER
THE THIRD ELEMENT IS LITHIUM.
LITHIUM HAS AN ATOMIC NUMBER, Z, OF 3.
A = ATOMIC MASS = 7
SO, # NEUTRONS = A - Z = 7 - 3 = 4
HOW MANY ELECTRONS DOES LITHIUM HAVE?
IF THE ATOMIC NUMBER, Z, IS 3, THIS MEANS IT HAS
3 PROTONS IN THE NUCLEUS.
THE NUMBER OF ELECTRONS HAS TO EQUAL THE
NUMBER OF PROTONS, SO THE NUMBER OF
ELECTRONS IS 3.
THE FOURTH ELEMENT IS BERYLIUM, Be.
Z = 4 AND A = 9
HOW MANY NEUTRONS DOES Be HAVE IN THE
NUCLEUS?
HOW MANY ELECTRONS DOES Be HAVE IN
SHELLS ABOUT THE NUCLEUS?
IT’S INTERESTING TO THINK ABOUT THE RELATIVE
ABUNDANCE OF THE ELEMENTS.
IF WE CONSIDER THE UNIVERSE, HYDROGEN IS BY
FAR THE MOST ABUNDANT ELEMENT. IF WE HAD
200 ATOMS, 182 OF THEM WOULD BE HYDROGEN.
NEARLY 18 WOULD BE HELIUM.
LESS THAN ONE HALF OF ONE ATOM WOULD BE
THE REST OF THE ELEMENTS.
AT THE BIRTH OF THE UNIVERSE, MOST EARLY
MATTER WAS HYDROGEN AND HELIUM.
THE REST OF THE ELEMENTS WERE FORMED IN
THE CORES OF STARS.
IF WE LOOK AT THE ELEMENTAL MAKE-UP OF THE
HUMAN BODY, THE COMPOSITION WOULD BE
QUITE DIFFERENT AS COMPARED TO THE
UNIVERSE.
IF WE HAD 200 ATOMS, 126 WOULD BE HYDROGEN
AND 61 WOULD BE OXYGEN. AFTER ALL, THE
HUMAN BODY IS LARGELY WATER.
BUT, THERE WOULD BE 19 CARBON ATOMS AND 3
ATOMS OF NITROGEN.
ALL OF THE OTHER ELEMENTS WOULD MAKE UP
THE REMAINING ATOM
IF YOU STARTED TO SUBDIVIDE GOLD, YOU WOULD
REACH A POINT WHERE YOU HAD REACHED THE
SMALLEST SUBDIVISION OR PARTICLE - THE ATOM.
THESE PARTICLES ARE TOO SMALL TO BE SEEN
WITH THE NAKED EYE, AND UNTIL RECENTLY, THEY
WERE TOO SMALL TO BE SEEN EVEN WITH THE
BEST ELECTRON MICROSCOPES.
A GOLD ATOM IS 3 X 10-8 cm IN DIAMETER AND HAS
A MASS OF 3.3 X 10-22 g, OR
Dia = 0.00000003 cm
Mass = 0.00000000000000000000033 g
OTHER ATOMS RANGE IN SIZE FROM ABOUT HALF
THAT OF GOLD TO TWO TIMES AS LARGE.