IUPAC nomenclature: putting the number before or in the middle of the name
The name “2-pentanone” and “pentan-2-one” describe the same compound. The name “2-pentanone” is in accordance with obsolete IUPAC Recommendations. The relevant rule of the IUPAC Nomenclature was changed in the 1993 IUPAC Recommendations:

R-0.1.2 Position of locants
Locants (numerals and/or letters) are placed immediately before the part of the name to which they relate, except in the case of traditional contracted forms (see R-2.5).

The current version of Nomenclature of Organic Chemistry – IUPAC Recommendations and Preferred Names 2013 (Blue Book) reads as follows:

P-14.3.2 Position of locants
Locants (numerals and/or letters) are placed immediately before that part of the name to which they relate, except in the case of the traditional contracted names when locants are placed at the front of their names.

Examples:

hex-2-ene (not 2-hexene)
cyclohex-2-en-1-ol (not 2-cyclohexen-1-ol)
pentan-2-one (not 2-pentanone)
(propan-2-yl)benzene

Examples for traditional contracted names with locants at the front of the name:

2-naphthyl (not naphth-2-yl), the preferred name is naphthalen-2-yl
2-pyridyl (not pyrid-2-yl), the preferred name is pyridin-2-yl