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 X g AND A CHARGE OF +1 (A UNIT POSITIVE CHARGE). A NEUTRON HAS A MASS SIMILAR TO THAT OF A PROTON, X g, AND NO CHARGE. IT IS NEUTRAL. AN ELECTRON HAS A MASS OF 9.11 X 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 MASSAmuLocation Charge Neutron1.67 x Nucleus0 Proton1.67 x Nucleus+1 Electron x Shells 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. ATOMIC NUMBER 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 = FeSODIUM = NaLEAD = 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 = = 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 cm IN DIAMETER AND HAS A MASS OF 3.3 X g, OR Dia = cm Mass = g OTHER ATOMS RANGE IN SIZE FROM ABOUT HALF THAT OF GOLD TO TWO TIMES AS LARGE.