Introduction

The origins of the elements, nuclei, & isotopes

Nuclear Chemistry

1. Nucleogenesis (Big Bang) {Photocopy of chart of nuclides}

a. Energy captured in formation of hydrogen and helium

b. E=mc² – stable atomic nuclei (low E) have low mass (low m).

i. ¹H – 1.00782503 amu (1 amu ≈ 9x10¹⁰ KJ/mole)

ii. ¹n – 1.00866491 amu (a free neutron will decay to ¹H!)

iii. ⁴He – 4.00260323

1. (4x¹H = 4.0313 < ⁴He – ¹H spontaneously reacts to form ⁴He, but very slow unless Temp ~10⁷K or more)

2. Hot dense H “burns” to He

iv. ⁸Be – 8.0053051 > ⁴He+⁴He – difficult to burn ⁴He, so the short nucleogenesis stops at He + Li.

2. Nucleosynthesis (Stars)

a. Main process is H -> He burning.

b. Long timescale, high pressure, high temperatures allow build-up of ⁴He, eventually creation of ¹²C (12.0 amu < 3x⁴He = 12.00781 amu)

c. Once ¹²C is produced, heavier elements produced by successive ⁴He capture and binary fusion (¹²C + ⁴He -> ¹⁶O, … ¹²C + ¹²C -> ²⁴Mg)

d. ⁵⁶Fe is the most stable nucleus, end product of fusion

e. All heavier elements formed by successive addition of ¹n or ¹p⁺ to existing heavy nuclei.

i. S-process: “slow” addition of neutrons, unstable n-rich nuclei usually decay before next neutron is added.

1. tends to form relatively n-rich nuclei connected to the line of stability (the “neutron drip line”).

2. does not readily form extra-heavy nuclei or p-rich nuclei.

3. dominant mechanism for forming heavy nuclei through ²⁰⁹Bi

a. most common nuclei of heavy elements are s-process nuclei

ii. R-process: “rapid” addition of neutrons – unstable n-rich nuclei may react with additional neutrons before they can decay.

1. during R-process unstable n-rich nuclei form, afterwards they decay (usually via b- emission) back to line of stability.

2. may form extra-heavy nuclei like 100Mo, 96Zr, and U+Th+Pu.

3. r-only nuclei tend to be rarer than s-process nuclei.

iii. P-process: addition of protons

1. forms n-poor nuclei, on the upper-left edge of the line of stability (e.g., ⁷⁴Se).

2. p-process nuclei tend to be most rare (p-process did not produce many nuclei that were incorporated into the solar system).

3. Relevance to solar system geochemistry:

a. H, He dominate solar system and universe.

b. Even-numbered elements C -> Fe next most abundant

c. Odd-numbered elements (N, P) less abundant

d. Elements heavier than Fe progressively rarer

4. Formation of the Earth

a. Early Earth too small, hot to retain full solar system abundance of gasses (H₂, He, CO, CH₄, N₂, NH₃)

b. Refractory (high vaporization temperature) elements concentrated (Fe, MgO, CaO, Al₂O₃, SiO₂).

c. Formation of the core removed most Fe, Ni, Zn, other alloy-loving elements.

Formation of the Earth’s crust by melting & crystallization of lavas concentrated low-melting temperature elements (Na, K, P) at the Earth’s surface – re-concentrates gasses at surface as well.