Name the particles which determine the mass of an atom.

The presence of electrons outside the nucleus and the nucleus at the centre of the atom was suggested by:

$(a)$ Both helium and beryllium have two electrons in their valence shells. Helium is a noble gas whereas beryllium is a metal. Justify.
$(b)$ Hydrogen exists in three isotopic forms. Why are the isotopes of hydrogen chemically alike?

$(a)$ Calculate the number of electrons,protons,and neutrons in an atom of an element with atomic number $20$ and mass number $40$. Write the electronic configuration and draw the structure of the atom.
$(b)$ An atom has complete $K$ and $L$ shells. Is this an atom of a metal,non-metal,or noble gas? Justify.

Write the symbols of two isotopes of uranium.

If an ion $M^{3+}$ contains $10$ electrons and $14$ neutrons, what are the atomic number and mass number of the element $M$?