How might a geneticist write alleles to show that a tall pea plant has one allele for tall stems and one allele for short stems?

T- tall stem
t- short stem
so, to represent what you asked for, it would be written Tt. Keep in mind, T is the dominant allele, and t is the recessive allele, in this example.

The proton has a positive charge while the neutron has no charge they both make up the nucleus of the atom while the elections have a negative charge and surround the nucleus of the atom. the protons and electrons are used to calculate the atom's mass whereas the neutrons don't affect the mass