Chemistry · Structure Of Atom · NEET
Chemical properties are decided by electrons, mainly the outer (valence) electrons. The number of electrons in a neutral atom equals the number of protons (the atomic number Z). Isotopes of an element have the SAME atomic number Z, so they have the same number of electrons and the same electron arrangement. Same electrons means same bonding, same reactions, same valency. That is why NCERT says: 'all the isotopes of a given element show same chemical behaviour.'
Neutrons sit inside the nucleus. The nucleus takes almost no part in ordinary chemical reactions. Reactions happen when the outer electrons of atoms interact, share, or transfer. Neutrons have no charge and stay locked in the nucleus, so they do not touch electrons of other atoms. NCERT states neutrons 'have very little effect on the chemical properties of an element.' So adding a neutron changes the mass, but not the way the atom reacts.
Physical properties like mass, density, rate of diffusion, boiling point and melting point depend on the mass of the atom. Isotopes have different numbers of neutrons, so they have different masses (different mass number A). A heavier isotope moves a little slower and is denser. So isotopes differ slightly in physical properties (mass-based) but stay identical in chemical properties (electron-based).
Yes, chemically they are all hydrogen. Protium (1 proton, 0 neutrons), deuterium (1 proton, 1 neutron) and tritium (1 proton, 2 neutrons) all have 1 electron, so all react as hydrogen. That is why heavy water (D2O) is chemically similar to normal water (H2O). They differ only in physical properties because of the extra neutron mass. (Note: very light atoms like H show a small 'isotope effect' in reaction rate, but the type of reaction stays the same.)
Practically yes, and this is a common NCERT-style question (exercise 3.25). Ionisation energy depends on how tightly the nucleus holds the outermost electron. Isotopes have the same nuclear charge (same Z) and the same electron configuration, so the first ionisation enthalpies are essentially the same. The tiny neutron-mass difference does not change how strongly electrons are held.
The number and arrangement of electrons, especially the valence electrons. Since electrons = protons = atomic number Z in a neutral atom, the atomic number Z is the true identity of an element. Two atoms with the same Z are the same element and react the same way, no matter how many neutrons they carry. This is why the modern Periodic Table is ordered by atomic number, not by atomic mass.
Try the real previous-year questions from this chapter — each with the answer and a full solution.
Yes. Isotopes of an element always have the same atomic number Z (same number of protons). They differ only in mass number A because they have different numbers of neutrons.
In a neutral atom, the number of electrons equals the number of protons (Z). Isotopes have the same Z, so they have the same number of electrons and the same electron configuration.
Yes. Valency depends on the valence (outer) electrons. Isotopes share the same electron arrangement, so they have the same valency and form the same compounds.
Mostly yes. Isotopes have the same chemical properties but slightly different physical properties (mass, density, diffusion rate) because their masses are different. For NEET, remember: same chemistry, different physics.
Because chemical properties depend on electrons, which are set by the atomic number Z. Since all isotopes of an element share the same Z, arranging by Z keeps chemically similar atoms together. Atomic mass varies between isotopes, so it is a less reliable basis.
