Phosphorus-31: Protons, Neutrons, and Electrons

  • Isotope notation basics
    • In isotope notation, the mass number A is placed on top and the atomic number Z on the bottom, with the element symbol to the right: ZAX^{A}_{Z}\mathrm{X}
    • For phosphorus with mass number 31 and atomic number 15, the symbol is 1531P^{31}_{15}\mathrm{P}
  • Phosphorus facts used here
    • Phosphorus atomic number: Z=15Z = 15
    • Phosphorus mass number given: A=31A = 31
    • The goal: find numbers of protons, neutrons, and electrons for a neutral phosphorus atom with A = 31
  • Key relationships (foundational formulas)
    • Total nucleons (protons + neutrons): A=Z+NA = Z + N
    • Neutron count: N=A−ZN = A - Z
    • For a neutral atom, electrons equal protons: E=ZE = Z
  • Step-by-step calculation for ^{31}_{15}P
    • Protons (p): Z=15Z = 15
    • Neutrons (n): N=A−Z=31−15=16N = A - Z = 31 - 15 = 16
    • Electrons in a neutral atom (e): E=Z=15E = Z = 15
  • Isotope notation confirmation
    • The complete isotope symbol for this phosphorus atom is: 1531P^{31}_{15}\mathrm{P}
  • Summary of counts for neutral phosphorus-31
    • Protons: 1515
    • Neutrons: 1616
    • Electrons: 1515
  • Special cases
    • If the phosphorus atom is ionized with charge +1, electrons would be E=Z−1=14E = Z - 1 = 14
    • If ionized with charge -1, electrons would be E=Z+1=16E = Z + 1 = 16
  • Quick cross-check
    • Total nucleons check: p+n=15+16=31=Ap + n = 15 + 16 = 31 = A
    • Charge balance for neutral atom: net charge = 0 since protons and electrons are equal in number
  • Context and significance
    • Phosphorus-31 is the only stable isotope of phosphorus, with 15 protons and 16 neutrons
    • The 31P nucleus geometry and neutron count influence nuclear stability and nuclear magnetic resonance properties
    • In biology and chemistry, phosphorus is essential (e.g., ATP, DNA backbones); isotope 31P is often used in 31P NMR spectroscopy for studying biochemical processes
  • Practical takeaway
    • To determine subatomic counts from A and Z: use p=Zp = Z, n=A−Zn = A - Z, e=Ze = Z (for neutral atoms)
    • To write the isotope symbol: top = A, bottom = Z, symbol = element
  • Connections to broader concepts
    • Distinction between mass number A and atomic mass: A is an integer count of nucleons; atomic mass is a weighted average of isotopes
    • Isotopes share chemical properties but differ in mass and nuclear properties
  • Example recap (for memory)
    • Given: A=31,Z=15A = 31, Z = 15
    • Protons: p=Z=15p = Z = 15
    • Neutrons: n=A−Z=16n = A - Z = 16
    • Electrons (neutral): e=Z=15e = Z = 15
  • Key formulas to memorize
    • A=Z+NA = Z + N
    • N=A−ZN = A - Z
    • E=ZE = Z (neutral atom)
  • Final answer for the transcript prompt
    • Protons = 15, Neutrons = 16, Electrons (neutral) = 15
  • Note on notation used in the transcript
    • Writing the symbol with mass number on top and atomic number on bottom is a standard isotope notation approach, corroborated here by the example 1531P^{31}_{15}\mathrm{P}