Something

element = {}
string = {}

fileHandle = open ('input.txt')
for x in string:
	string[x]= fileHandle.readline()

#	x = fileHandle.readlines()
#	append.string(x)	
	
#string[0] = 'Li'
#string[1] = 'Cs'
#string[2] = 'H_'
#string[3] = 'P-'
#string[4] = 'CA'
#string[5] = 'N_'
#string[6] = 'H_'

element[0] = 'H_'
element[1] = 'He'
element[2] = 'Li'
element[3] = 'Be'
element[4] = 'B_'
element[5] = 'C_'
element[6] = 'N_'
element[7] = 'O_'
element[8] = 'F_'
element[9] = 'Ne'
element[10] = 'Na'
element[11] = 'Mg'
element[12] = 'Al'
element[13] = 'Si'
element[14] = 'P_'
element[15] = 'S_'
element[16] = 'Cl'
element[17] = 'Ar'
element[18] = 'K_'
element[19] = 'Ca'

ptval = {}
ptval['H_'] = '001'
ptval['He'] = '002'
ptval['Li'] = '003'
ptval['Be'] = '004'
ptval['B_'] = '005'
ptval['C_'] = '006'
ptval['N_'] = '007'
ptval['O_'] = '008'
ptval['F_'] = '009'
ptval['Ne'] = '010'
ptval['Na'] = '010'
ptval['Mg'] = '011'
ptval['Al'] = '012'
ptval['Si'] = '013'
ptval['P_'] = '014'
ptval['S_'] = '015'
ptval['Cl'] = '016'
ptval['Ar'] = '017'
ptval['K_'] = '018'
ptval['Ca'] = '019'

for index, element in string.iteritems():
	if element in ptval.keys():
		print ptval[element]