Why are the atoms of different elements different?

Consider the following stetements in reference to the structure of atom 1. Isopes are the atoms of the same element having same atomic number but different mass - numbers . 2. Isobars are the atoms of different elements having different atomic number of different atomic number but same mass - number . Which of above statement is/are correct ?

In what respect do the atoms of different elements differ from each other?

Fill in the blanks: - Atoms of different elements differ in mass and ____________ properties.

What do we call the atoms of different elements with same number of neutrons.

Show that atomic nuclei of different elements have different sizes.

Assertion (A) : The isotopes are the atoms of the same element with different atomic masses. They require different positions in the periodic table. Reason (R) : Variable number of neutrons in the nuclei of the atoms of same elements leads to the different atomic masses.