CONVERTING ROMAN NUMERALS TO ARABIC NUMBERS
What are the rules for writing roman numerals?
 In the Roman numeral system, the basic "digits" are the letters I, V, X, L, C, D, and M which represent the same numbers regardless of their position.
 Symbols are placed in order of value, starting with the largest values.
 When the higher numeral is placed before a lower numeral, the values of each Roman numeral are added.
 When smaller values precede larger values, the smaller values are subtracted from the larger values, and the result is added to the total.
 Do not repeat I, X, and C more than three times in a row. (Number 4 on a Roman numeral clock is usually written as IIII. )
 Symbols V, L, and D cannot appear more than once consecutively.
 Do not subtract a number from one that is more than 10 times greater: I may only precede V and X, X may only precede L and C, and C may only precede D and M.
I 
= 1 
XI 
= 11 
XXX 
= 30 
CL 
= 150 
II 
= 2 
XII 
= 12 
XL 
= 40 
CLIX 
= 159 
III 
= 3 
XIII 
= 13 
XLIX 
= 49 
CXC 
= 190 
IV 
= 4 
XIV 
= 14 
L 
= 50 
CC 
= 200 
V 
= 5 
XV 
= 15 
LX 
= 60 
CCC 
= 300 
VI 
= 6 
XVI 
= 16 
LXX 
= 70 
CD 
= 400 
VII 
= 7 
XVII 
= 17 
LXXX 
= 80 
D 
= 500 
VIII 
= 8 
XVIII 
= 18 
XC 
= 90 
DC 
= 600 
IX 
= 9 
XIX 
= 19 
XCIX 
= 99 
CM 
= 900 
X 
= 10 
XX 
= 20 
C 
= 100 
M 
= 1000 
When they needed to work with large numbers (4000 and above), the Romans often wrote a bar above a numeral, or parentheses placed around it, to indicate multiplication by 1000.
MMM 
= 3 000 000 
XLILXII 
= 41 062 
XMXVII 
= 19 007 
XMCXI 
= 11 111 
According to the old IUPAC recommendation before 1985, Roman numerals were used to denote the group in the periodic table of the elements. When the metal has more than one possible ionic charge or oxidation state the oxidation number (the same as the charge) of the metal ion is represented by a Roman numeral in parentheses immediately following the metal ion name. For example, FeO is iron(II) oxide and Fe_{2}O_{3} is iron(III) oxide.
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