[RGraham9]

Corrections:
• if somehow (even if this is impossible scientifically) I am able to change one of the atoms to 6 protons and 7 neutrons instead  will the physical properties of the element change drastically.

@Borek : Yup , what I meant be identical element was another sample of the same element , made out of different isotopes. And the 99 atoms this is a reference to a previous example.


Forgive me for the mistakes.

[AWK]

C-12 and C-13 show different magnetic properties (used in nuclear magnetic resonance)

[RGraham9]

I will try my best to rephrase the questions again.
Question 1
• This is the definition of an Isotope correct ? "Isotopes are atoms of the same elements with different numbers of neutrons."
Question 2
• Due to the difference in neutrons between isotopes , different isotopes will have different mass numbers correct ?
Question 3
• Basically there is this statement / example in my guide book that goes " Isotopes have the same chemical properties but different physical properties."My question is , what this statement is trying to say or refer to is that if we have two identical elements / samples of the same element made out of different isotopes / atoms of same element with different neutron numbers , the physical properties of the two elements will be different from one another due to the difference in the nucleon number / mass number of the atoms making up the elements correct ?
Question 4
• How exactly does the mass number of an isotope in an element affect the physical properties of that element  ?
Question 5
• Lets say if I have an element that consists of 50 atoms. Each atom has 6 protons and neutrons. If I were to let's say change one of the 50 atoms to the point where that one atom has 6 protons and 7 neutrons , will there be a change in the physical properties of that element ? Will the change be drastic ?

I sincerely apologise for the previous poorly written post. I was really tired and got kind of lazy and skipped some details.

Appreciate the help provided to me so far guys. Thank you. Have a great day.

[mikasaur]

I will try my best to rephrase the questions again.
Question 1
• This is the definition of an Isotope correct ? "Isotopes are atoms of the same elements with different numbers of neutrons." Yes.
Question 2
• Due to the difference in neutrons between isotopes , different isotopes will have different mass numbers correct ? Yes.
Question 3
• Basically there is this statement / example in my guide book that goes " Isotopes have the same chemical properties but different physical properties."My question is , what this statement is trying to say or refer to is that if we have two identical elements / samples of the same element made out of different isotopes / atoms of same element with different neutron numbers , the physical properties of the two elements will be different from one another due to the difference in the nucleon number / mass number of the atoms making up the elements correct ? You're using the word "elements" incorrectly and are confusing yourself and others. I would refresh yourself on its meaning. If you have a sample made entirely of C-12 it will have different physical properties from a sample made entirely of C-13.
Question 4
• How exactly does the mass number of an isotope in an element affect the physical properties of that element  ? If you start adding or removing neutrons you're changing the atom. There are many ways this affects the properties of a sample made up of those atoms, e.g. https://www.wikiwand.com/en/Deuterium#/Physical_properties
Question 5
• Lets say if I have an element that consists of 50 atoms. Each atom has 6 protons and neutrons. If I were to let's say change one of the 50 atoms to the point where that one atom has 6 protons and 7 neutrons , will there be a change in the physical properties of that element ? Will the change be drastic ? Yes. No, probably not.

I sincerely apologise for the previous poorly written post. I was really tired and got kind of lazy and skipped some details.

Appreciate the help provided to me so far guys. Thank you. Have a great day.

[RGraham9]

Okay I got it. Thanks guys. Appreciate it. Have a great day ahead.

[Corribus]

Chemistry relates primarily to the physics of the interactions between positive and negative charges - predominantly electrons and protons. Therefore changing the nuclear mass with the addition/subtraction of a neutral particle (neutron) does not usually change the chemical properties of an atom or molecules containing the atom. However neutrons have other physical properties of significance, one of which (magnetism) has already been mentioned here*. So, it can be possible to distinguish magnetically between different isotopes. Heavier isotopes are also, well, heavier, which can impact other physical properties of atoms and molecules - such as the speed at which they vibrate or rotate. These differences have a lot of implications and uses in molecular spectroscopy and analytical chemistry.

You should be aware that there are exceptions to the statement that isotopic substitution does not affect the chemical properties of atoms. A heavier isotope, for example, may be radioactive, and radioactive decay can impact the chemical identify of an atom. Likewise, substitution of isotopes can impact the rates of chemical processes through something called the kinetic isotope effect. To understand why is probably a bit above your level, but I mention it to show that statements like "isotopes have the exact same chemical properties" is a generalization that is in most cases true, but not always.

*Magnetic differences can also impact chemical reactivity in some cases, e.g., reactions and photoreactions involving radicals.

[RGraham9]

@Corribus : I see , appreciate the info , will definitely keep aware of the generalizations. Thank you.

[thetada]

You should be aware that there are exceptions to the statement that isotopic substitution does not affect the chemical properties of atoms.

Corribus, I should have known that was an oversimplification!

Can you explain why heavy water has stronger OH bonds than regular water (as mentioned in the above-refed wiki page)? It seems counter intuitive to me. Assuming that the atoms vibrate relative to each other, surely the vibrations will be higher energy for the heavier isotope? If that were true, could the bonding orbital be closer in energy to the corresponding atomic orbitals? I feel like I'm talking rubbish....

[AWK]

Comment to question 5.
In carbon we have approximately 1 atom of C-13 in 100 atoms of C-12. And we can see just this 1 atom C-13 in mass spectrometer (but also 100 atoms of C-12) or in nuclear magnetic resonance (only 1 atom of C-13). And we will notice practically no change, eg.  in melting or boiling points).
So all answers given by Mikasaur (yes, no, probably not) are correct.

[Corribus]

In general, bonds between involving heavier isotopes are slightly stronger than bonds involving lighter isotopes. Consider D₂ and H₂ - which is a stronger bond? Because D is heavier than H, D₂ vibrates more slowly than H₂. In the quantum mechanical harmonic oscillator approximation, molecules vibrate even at zero temperature, a feature called the "zero point energy", which is given by 0.5 ħω. Because D₂ has a lower vibrational frequency than H₂, it also has a lower energy. Another way of stating this is that it is a more stable bond, because it takes more energy to dissociate. Experimental bond dissociation energies of D₂, HD, and H₂ at 298 K are 443.3, 439.32, and 436.0 kJ/mol (Source: http://www.nist.gov/data/nsrds/NSRDS-NBS31.pdf). Conclusion, the D₂ bond is stronger than the H₂ bond. This logic extends to other bonds involving heavy isotopes, which all have lower ZPE than their lighter analogs - and, by extension, intermolecular interactions. This is evident when comparing physical properties of deuterated versus nondeuterated substances. E.g., at 100 K, the enthalpy of vaporization of tetradeuterated methane CD₄ exceeds that of hydrogenated methane CH₄ by 65.6 J mol^-1 (Source: J Phys Chem 1989, 93, 3355) - meaning it is harder to boil, and hence intermolecular forces are stronger. The effect here is pretty small, because the intermolecular forces here are weak to begin with. The effect is more pronounced with stronger intermolecular forces like H-bonds (D-bonds): for H₂O and D₂O, the boiling points are at 101.325 kPa are 99.974, 101.40 °C, respectively (Source: CRC 96th Ed, p 6-9).

The stronger bonds formed by heavier isotopes are also responsible for the aforementioned Kinetic Isotope Effect - bonds to deuterium are harder to break than those to hydrogen, meaning reactions involving deuterium tend to be slower.

These effects are true not only of hydrogen/deuterium but other isotope substitutions as well. However the effects are greatest in the H/D pair because the relative increase in mass between H/D is far larger than between other isotopes. E.g., the relative mass difference between H and D is huge compared to that between ¹²C and ¹³C.

[thetada]

Awesome, thanks!

[Enthalpy]

In addition to the delocalization of the protium versus deuterium:
- The electron's reduced mass differs between protium and deuterium. This changes the electron's energy levels and the strength of intramolecular bonds too.
- Different hydrogen mass changes the molecule's possible rotation states for the liquid and gas, which must influence the melting point. Though, I haven't seen data about that up to now. At 20K with liquid hydrogen it changes everything. At 273K for water it must be incidental.
- If some reaction need a proton to tunnel, the difference with a deuteron must be huge.