My advice would be to use a stream cipher.

First of all, it is easy to implement, because you won't have to think about dividing into blocks, padding, etc..

Secondly, the idea of stream ciphers is very easy: you generate a  pseudorandom sequence of bits out of the private key. Then you XOR this sequence with a plaintext. You will only have to code the generation, the rest is elementary. 

Finally, there are very easy stream ciphers, e.g.  RC4 http://en.wikipedia.org/wiki/RC4;  another example is Salsa20 http://en.wikipedia.org/wiki/Salsa20

And when I say "simple" I mean not only the amount of lines of code, but also the ease of debugging/ flaws search.

simple algorithm to encrypt/decrypt a text file
Elements of group 6A, compared to 5A, require less ionization energy due to the paired electrons of 6A.

But a question arises: why does group 7A, compared to group 6A, require more ionization energy in spite of the paired electrons in group 7a too.

Regardless of the general trend that the ionization energy increases as we go from left to right in periods, why didn't we apply the same procedure as the first comparison?
