Question

what new characteristic did john dalton add to the model of the atom?
an atom is a round, solid mass.
an atom has a massive, positive center.
an atom is made up of smaller particles.
an atom can join with other kinds of atoms.

Explanation:

John Dalton's atomic model proposed that atoms are round, solid masses (the "billiard ball" model). The idea of an atom having a massive, positive center (nucleus) came later (Rutherford's model). The concept that an atom is made up of smaller particles (electrons, protons, neutrons) was also developed after Dalton. And while atoms can join with other kinds of atoms (a key aspect of chemistry), Dalton's main new characteristic in his model was the round, solid mass description.

Answer:

An atom is a round, solid mass.