CHANCE FAVORS THE WELL PREPARED.

AS A RESULT OF STUDYING THE INTERACTION OF LIGHT WITH GASEOUS ATOMS, SCIENTISTS CAME UP WITH THE QUANTUM THEORY THAT DESCRIBES HOW ELECTRONS ARE ARRANGED IN ATOMS. QUANTUM THEORY TALKS ABOUT WHY ELECTRON SHELLS AND SUBSHELLS FILL THE WAY THEY DO. 4 QUANTUM NUMBERS ARE USED TO DESCRIBE THE PROBABLE POSITION OF ELECTRONS IN ATOMS. NO TWO ELECTRONS CAN HAVE THE SAME 4 NUMBERS. THE FIRST DESCRIBES THE SHELL. THE SECOND DESCRIBES THE SUBSHELL. THE THIRD BREAKS THE SUBSHELL DOWN INTO ORBITALS.

THE FOURTH DESCRIBES THE ELECTRON SPIN. WE ARE ONLY GOING TO BE CONCERNED WITH SHELLS AND SUBSHELLS. THE FIRST SHELL CAN ONLY HAVE ONE SUBSHELL, AND THAT IS AN s SUBSHELL WHICH CAN HOLD ONLY 2 ELECTRONS. THE SECOND SHELL CAN HAVE TWO SUBSHELLS – AN s AND A p. A p SUBSHELL CAN HOLD 6 ELECTRONS. THE THIRD SHELL CAN HAVE THREE SUBSHELLS – AN s, A p, AND A d. A d SUBSHELL CAN HOLD 10 ELECTRONS. FINALLY, ANY SHELL BEYOND THE THIRD CAN HAVE 4 SUBSHELLS – AN s, A p, A d, AND AN f (14 electrons).

THE FOLLOWING TABLE GIVES THE INFORMATION ON THE SHELLS AND SUBSHELLS: Principal Quantum Number (shell) Number of Subshells Type of Subshell n = 111s (2 electrons) n = 222s (2 electrons) 2p (6 electrons) n = 333s (2 electrons) 3p (6 electrons) 3d (10 electrons) n = 444s (2 electrons) 4p(6 electrons ) 4d(10 electrons) 4f (14 electrons)

The first shell (n=1) can only hold 2 electrons, so there are only 2 elements in the first period. The maximum number of electrons in an outer shell other than the first is 8. 2 electrons in the first shell or 8 electrons in any other outer shell represent a very stable configuration. This is called the octet rule. This will be very important when we talk about chemical bonding. Quantum theory talks about why electron shells and subshells fill the way they do.

THE IDEA OF A PERIODIC TABLE WAS DEVELOPED BY SEVERAL SCIENTISTS AT ABOUT THE SAME TIME. MENDELEEV WAS THE FIRST TO PUBLISH IN HE SUGGESTED THAT SUCH A TABLE COULD BE CONSTRUCTED BY ARRANGING THE ELEMENTS IN ORDER OF INCREASING ATOMIC MASS. LATER, SCIENTISTS REALIZED THAT IT SHOULD BE IN ORDER OF INCREASING ATOMIC NUMBER.

PERIODIC LAW – WHEN ELEMENTS ARE ARRANGED IN ORDER OF INCREASING ATOMIC NUMBER, THERE IS A PERIODIC REPETITION OF THEIR PHYSICAL AND CHEMICAL PROPERTIES. NOW, IN THE PERIODIC TABLE, THE VERTICAL COLUMNS ARE CALLED FAMILIES OR GROUPS. THE HORIZONTAL ROWS ARE CALLED PERIODS.

ALL OF THE ELEMENTS IN A GIVEN COLUMN OR FAMILY WILL HAVE SIMILAR OUTER ELECTRON CONFIGURATIONS. FOR EXAMPLE, TAKE THE 1 ST COLUMN – THE ALKALI METALS. H – 1s 1 Li – 2s 1 Na – 3s 1 K – 4s 1 Rb – 5s 1 Cs – 6s 1 ALL HAVE 1 ELECTRON IN THE OUTER SHELL.

THE OUTER SHELL IS IMPORTANT. IT IS THE ELECTRONS IN THE OUTERMOST SHELL THAT WILL BE INVOLVED IN CHEMICAL REACTIONS. THEY ARE CALLED VALENCE ELECTRONS. THEY WILL FORM CHEMICAL BONDS THROUGH LOSS, GAIN, OR SHARING.

TAKE ANOTHER EXAMPLE – THE SECOND COLUMN, THE ALKALINE EARTH METALS: Be – 2s 2 Mg – 3s 2 Ca – 4s 2 Sr – 5s 2 Ba – 6s 2 THEY ALL HAVE TWO ELECTRONS EACH IN THEIR OUTER SHELLS.

OR, TAKE GROUP 7A, THE HALOGENS. THEY ALL HAVE 7 ELECTRONS IN THE OUTER SHELL. F – 2s 2 2p 5 Cl – 3s 2 3p 5 Br – 4s 2 4p 5 I – 5s 2 5p 5 At – 6s 2 6p 5 THE GROUP NUMBER FOR THE A GROUP ELEMENTS WILL TELL YOU HOW MANY ELECTRONS ARE IN THE OUTER SHELL – VALENCE ELECTRONS.

GROUPNAMEELEMENTS 1AAlkali MetalsLi, Na, K, Rb, Cs, Fr 2AAlkaline Earth Metals Be, Mg, Ca, Sr, Ba, Ra 7AHalogensF, Cl, Br, I, At 8ANoble GasesHe, Ne, Ar, Kr, Xe, Rn

THE ELEMENTS ARE GROUPED ACCORDING TO ELEMENT TYPE AS WELL – METALS, NONMETALS, AND METALLOIDS. THE METALS TO THE LEFT OF THE PERIODIC TABLE, STARTING WITH THE LIGHT BLUE, DARK BLUE, BLUE-GRAY, AND ORANGE. THE NONMETALS ARE TO THE RIGHT OF THE PERIODIC TABLE AND ARE GREEN, YELLOW, AND ORANGE. THE METALLOIDS ARE IN PURPLE.

METALS ARE ALL SOLIDS, WITH ONE EXCEPTION (MERCURY), AT ROOM TEMPERATURE. THEY HAVE A SHINY SURFACE WHEN FRESHLY CUT OR POLISHED. THEY ARE GOOD CONDUCTORS OF HEAT AND ELECTRICITY. MOST CAN BE PULLED INTO WIRES (DUCTILE) OR ROLLED INTO SHEETS (MALEABLE).

NONMETALS ARE BRITTLE SOLIDS OR GASES AT ROOM TEPMERATURE. BROMINE IS THE ONLY LIQUID. THEY ARE NOT GOOD CONDUCTORS OF ELECTRICITY OR HEAT (WITH THE EXCEPTION OF CARBON). METALLOIDS HAVE PROPERTIES BETWEEN THOSE OF METALS AND NONMETALS. SILICON WOULD BE A GOOD EXAMPLE. IT LOOKS LIKE A METAL, BUT IT IS BRITTLE.

THE ELEMENTS IN DARK BLUE IN THE CENTER OF THE PERIODIC TABLE ARE CALLED THE TRANSITION METALS. AS YOU MOVE ACROSS A PERIOD, ELECTRONS ARE BEING ADDED TO THE d SUBSHELLS. FOR EXAMPLE, STARTING WITH ELEMENT 21, SCANDIUM, AND MOVING ACROSS TO ZINC, ELECTRONS WOULD BE ADDED TO THE 3d SUBSHELL.

IN THE LANTHANIDE SERIES (STARTING WITH ELEMENT 58) ELECTRONS ARE GOING INTO THE 4f SUBSHELL. THE LANTHANIDE SERIES ELEMENTS ARE SOMETIMES REFERRED TO AS THE RARE EARTH ELEMENTS. IN THE ACTINIDE SERIES (STARTING WITH ELEMENT 90) ELECTRONS ARE GOING INTO THE 5f SUBSHELL.