The first step towards getting somewhere is to
decide you are not going to stay where you are.
THE STUFF THAT THINGS ARE MADE OF
YOUR BODY HAS SOMETHING IN COMMON WITH
STARS, FROGS, ROCKS, AND CLOUDS.
ALL CONTAIN MATTER.
MATTER IS ANYTHING THAT HAS MASS AND
TAKES UP SPACE.
IN EVERYDAY LIFE, WE USE THE TERMS MASS
AND WEIGHT INTERCHANGEABLY.
MASS IS ACTUALLY INDEPENDENT OF GRAVITY.
MASS IS A MEASURE OF HOW MUCH MATTER
SOMETHING CONTAINS.
REMEMBER:
Mass is a measure of how much matter
an object has.
Weight is a measure of how strongly
gravity pulls on that matter.
MATTER IS ANYTHING THAT HAS MASS AND TAKES
UP SPACE.
MATTER CAN EXIST IN FOUR DIFFERENT STATES:
1. SOLID - HAS A DEFINITE SHAPE AND VOLUME
2. LIQUID - HAS A DEFINITE VOLUME BUT
ASSUMES SHAPE OF CONTAINER
3. GAS - HAS NO DEFINITE SHAPE OR VOLUME ASSUMES SHAPE AND VOLUME OF CONTAINER
4. PLASMA - SIMILAR TO GAS, BUT ATOMS ARE
STRIPPED OF ELECTRONS
MOST MATTER IN THE UNIVERSE EXISTS AS
PLASMA.
THE STATE THAT MATTER EXISTS IN DEPENDS ON
TEMPERATURE.
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.
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
NUMBER.
ATOMIC
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.
IN OTHER WORDS, ATOMS ARE MOSTLY
EMPTY SPACE.
IF ATOMS ARE MOSTLY EMPTY SPACE, WHAT
DOES THIS SAY ABOUT YOU?
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
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?
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