The proper name Russia is the only one known by non-Russian speakers who sometimes become confused with the fact that it has two derivational paradigms, particularly two adjectives русский - российский and two demonyms (words naming a resident of a country), русский - россиянин. Meanwhile, the existence of the two versions of this word is conditioned by not so subtle semantic differences emerged from not only the Russian history but also from the Russian mentality:
* Phonetic stress is underlined
Paradigm 1, universal:
Russian toponym - Русь /Rus/
Russian adjective - русский /russkiy/ - Russian
Russian masculine demonym - русский /russkiy/ - a Russian
Russian feminine demonym - русская /russkaya/ - a Russian
Paradigm 2: official
Russian toponym - Россия /Rosseeya/ - Russia
Russian adjective - российский /rosseeyskiy/ - Russian
Russian masculine demonym - россиянин /rossiyanin/ - a Russian citizen
Russian feminine demonym - россиянка /rossiyanka/ - a Russian citizen
Paradigm 1 signifies an attribute, which is why the nominalised adjective русский has also become the demonym. The nominalization русский relates to one’s cultural and spiritual background rather than to the place, where he or she resides.
For example, if I live in the United Kingdom, I may be referred as русская and not россиянка. Paradigm 1 describes one’s mentality, while Paradigm 2 is about formal nationality. Therefore, I may be referred as русская британка – British Russian in the end of the day because, no matter where I live, my mentality will remain Russian. Such ambiguity has been imprinted in the Russian language since as early as the 18th century. It demonstrates the deep cultural and spiritual awareness of the Russians (and this is the words from the Paradigm 1) that can be summarised by the following statement: “I differentiate between my Motherland and the state”. One may love the Motherland but state is not there to make people feel emotional.
Similarly, there are two ways to spell the toponym (name of a geographic location) Belarus.
Белоруссия is the name of the area, which is often used in conversational speech, and Беларусь is the official name of the country that is located in this area. The vowels a and о alternate in the two versions of this toponym and the rest of its paradigm derives from the noun Белоруссия. In both versions, the stem word is a compound of two roots. The toponym Беларусь combines a shortened collocation, Белая Русь – White Rus that is said to be given to the area for its vast silver-birch woods. The word бела is the short form of adjective that, incidentally, is older than its full version (compare with such expressions in Russian folklore as красна девица – beautiful girl, сине море – blue sea and бела ручка – pale hand). In the noun белорус, -о- is a connecting vowel typical for many Russian compound words.
I was born and spent my early childhood in Belarus, the place of gentle people, from what my memory has retained. For this reason, I prefer to avoid comments about the reasons for the two versions of the toponym. Maybe residents and expatriates of the country can enlighten us in their comments to this post.