23. Atomic nucleus. Isotopes.

Question 1

Describe the structure of an atom, elaborate on the structure of the nucleus

Answer 1

Atom:

• size: 10^-10 m
• consists of e^- and protons (= nucleus)
• e^- are responsible for chemical behavior
• nucleus is responsible for radioactivity

Structure of the nucleus:

• size = 10^-15 m
• mass (number) (A) = no. of protons + neutrons (1 amu each)
• atomic number (Z) = no. of protons (N)
• N/Z ratio:** **the stronger the repulsion force, the more neutrons are needed to stabilize the nuclei
  
  • ​for Z < 20 = N/Z 1:1
  • for Z 20-83 = N/Z 1.5:1
  • for Z > 83 = radioactive

Question 2

Which **nuclear forces **are responsible for the stability of the nucleus?

Answer 2

**Coulomb forces **exerted by protons cause repulsion and destabilization.

BUT: nuclear forces stabilize the nucleus:

• long-range repulsion forces overcome by short-range attraction forces → +/- charged regions of protons align (very strong)
• independent of charges (no net-charge, BUT: down(-), up(+) quarks cause regional charges)
• only short range (~fm = 10^-15 m)
• can be explained by inverse-squared electrical force described by Coulomb’s law
• stabilized by neutrons
• quantized E-levels for the nucleus
• binding energy in MeV range

Question 3

What are isotopes? Give a definition and classify them according to their N/Z ratio

Answer 3

Isotopes are variants of a chemical element which differ in N, although all isotopes of a given element have the same no. of protons (and e^-) in each atom.

→ same chemical properties

Can be classified:

• unstable (= radioactive) isotopes
• stable isotopes

→ determined by N/Z ratio: the stronger the repulsion force, the more neutrons are needed to stabilize the nuclei

• ​for Z < 20 = N/Z 1:1
• for Z 20-83 = N/Z 1.5:1
• for Z > 83 = radioactive