How many electrons are located in an atom? Remember the atomic number refers to the number of protons (positive charges) and that all  atoms are neutral; therefore,  the number of electrons must be equal to the number of protons. 
(an atom with an atomic number of 19 contains 19 electrons; an atom containing 7 protons and 9 neutrons contains 7 electrons)

How are the electrons arranged in an atom? 

In orbit around the nucleus, the negatively charged electrons form an electron cloud. This cloud has distinct layers called shells. These shells have numerical designations. The shell closest to the nucleus is the first shell, the second closest the second and so forth. An older naming scheme has the closest shell called the K shell, the second closest the L shell, then the M and N shell and so forth in that order. Each of the shells have different energy levels. Electrons load or fill these shells in a definite fashion. There is a particular numerical limit or capacity that each shell has for electrons. 

The 1st shell (K shell) can hold a maximum of 2 electrons, the second shell (L shell) can hold a maximum of 8 electrons, the third shell (M shell) can hold a maximum of 18 electrons, the fourth shell (N shell) can hold a maximum of 32 and so on. Electrons will almost always fill an inner shell completely before going an outer shell. The first shell will fill before the second, the second before the third until all the available electrons are used. There is a formula that can be used to determine how the shells are filled. 

where 
X = the maximum number of electrons in the shell 
N = the shell number 

 example 1: Determine the maximum number of electrons possible in the 1st shell. (K shell) 

X = 2 * 1¹ 
X = 2

X = 2 * 2² 
X = 8

X = 2 * 4² 
X = 32

  
19. Based upon the above information, Lithium, which has an atomic number of 3, would have how many shells to fill and how many would go into each shell? 

a) 3 shells, 1, 2, and 3, with 1 electron in each 

b) 1 shell, the K shell with all 3 electrons in it 

c) 2 shells, with 2 electrons in the first and 1 electron in the second 

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20. Magnesium, whose atomic number is 12, has how many electron shells? 

a) 5 

b) 3 

c) 7 

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21. How many electrons would go into which shells for the element chlorine (atomic number = 17)? 

a) 2 electrons in the first, 8 electrons in the 2nd, and 7 electrons in the 3rd 

b) 2 electrons in the K shell, and 5 electrons each in the L and M shells 

c) 4 electrons in the 1st shell, and 2 electrons in each of the last 6 shells 

 Press here to check your answer. 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 

  
The following is a table of the 1st 20 elements showing their electron distribution. 

 

ELEMENT	SYMBOL	ATOMIC      NUMBER	(K SHELL)   1ST SHELL	(L SHELL)    2ND SHELL	( M SHELL)  3RD SHELL	(N SHELL)  4TH SHELL
Hydrogen	H	1	1
Helium	He	2	2
Lithium	Li	3	2	1
Beryllium	Be	4	2	2
Boron	B	5	2	3
Carbon	C	6	2	4
Nitrogen	N	7	2	5
Oxygen	O	8	2	6
Fluorine	F	9	2	7
Neon	Ne	10	2	8
Sodium	Na	11	2	8	1
Magnesium	Mg	12	2	8	2
Aluminum	Al	13	2	8	3
Silicon	Si	14	2	8	4
Phosphorus	P	15	2	8	5
Sulfur	S	16	2	8	6
Chlorine	Cl	17	2	8	7
Argon	Ar	18	2	8	8
Potassium	K	19	2	8	8	1
Calcium	Ca	20	2	8	8	2

Important: Even though the third and fourth shell can hold a maximum of 16 and 32 electrons, the maximum number of electrons they can hold when they are an outer shell is eight! What happens in filling these shells is: once you have placed eight electrons in the outer shell place the next two electrons in the next shell, making it the outer shell, than go back to the original shell and complete filling it. (Observe this pattern in the table above - potassium and calcium) 

 With the aid of the chart above draw the atomic structure of the first twenty elements showing the arrangement of the electrons in their proper shells. 

 22. Which of the following statements is NOT true concerning the arrangement of electrons in an atom? 

a) The first shell can only hold either one or two electrons. 

b) The second shell can hold anywhere from one to eight electrons. 

c) The third shell can hold anywhere from one to eight electrons when its an outer shell. 

d) The third shell can hold anywhere from one to sixteen when there is a fourth shell. 

e) All outer shells of an atom must have eight electrons. 

  
In the next lesson bonding will be explained. To understand how atoms bond to form molecules you will need to determine the number of electrons in an atoms outer shell or valence shell. 

 Read behavioral objective 9 and 10. 

 CHEMICAL SHORTHAND 

 The 103 or so elements that have been discovered have a variety of names some of which are quite long. Trying to use these names to refer to these elements on an everyday basis would not be very practical. The use of symbols to represent these elements is used makes for a far more efficient method when referencing them as individual elements or when they combine with each other forming compounds. 

 An abbreviation is used in place of the full name. This abbreviation usually is the first one or two letters of the elements name. Carbon, for instance, uses a capital C as its symbol. Oxygen uses a capital O and hydrogen uses a capital H.  There are some elements that do not follow this rule. Sodium uses Na as its symbol, deriving this from the Latin word Natrium. Potassium obtains its symbol, K, from the Latin word Kalium. 

 The following table illustrates the use of symbols for the first forty elements: 

 

ELEMENT	SYMBOL	ELEMENT	SYMBOL
Hydrogen	H	Scandium	Sc
Helium	He	Titanium	Ti
Lithium	Li	Vanadium	Va
Beryllium	Be	Chromium	Cr
Boron	B	Manganese	Mn
Carbon	C	Iron	Fe
Nitrogen	N	Cobalt	Co
Oxygen	O	Nickel	Ni
Fluorine	F	Copper	Cu
Neon	Ne	Zinc	Zn
Sodium	Na	Gallium	Ga
Magnesium	Mg	Germanium	Ge
Aluminum	Al	Arsenic	As
Silicon	Si	Selenium	Se
Phosphorus	P	Bromine	Br
Sulfur	S	Krypton	Kr
Chlorine	Cl	Rubidium	Rb
Argon	Ar	Strontium	Sr
Potassium	K	Yttrium	Y
Calcium	Ca	Zirconium	Zr

 
A molecular formula is where the number of atoms for each element present in a molecule is indicated by subscripts. 

 C₅₂H₁₂₈O₉N₄Mg is the molecular formula for chlorophyll. 

 This formula indicates that one molecule of chlorophyll consists of 52 carbon atoms, 128 hydrogen atoms, 9 oxygen atoms, 4 nitrogen atoms, and 1 magnesium atom. 

 23. Which elements are present in the following molecule? 

 MgCl₂ 

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24. The molecular formula for glucose is C₆ H₁₂ O₆. 

 Which elements are present in glucose? 

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A molecular formula is where the number of atoms for each element present in a molecule is indicated by subscripts. 

 25. Write a formula showing the following arrangement of atoms.  Table sugar, sucrose, contains twelve atoms of carbon, twenty two atoms of hydrogen and eleven atoms of oxygen. Indicate two molecules. 

 Press here to check your answer. 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 

This is the end of the second lesson. The two molecular formulas above demonstrate two molecules containing atoms bonded together by ionic or covalent bonding. The next lesson will explain how ionic bonds are formed. 

You must answer the following question correctly to go to the Biology 108N home page and to lesson three, ionic bonding. 

 26. Based upon the above information, oxygen, which has an atomic number of 8 and atomic weight of 16 amu would have how many protons, neutrons, electrons, electrons in outer shell? 

                     protons      neutrons       electrons        outer shell electrons 
 
            a )         24               8                   16                     6 
            b )           8               8                     8                    6 
            c )           8              16                    8                     4 

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