envirochem test prep - cosmochemistry

Temperature

Temperature

________ and pressure are high enough to allow fusion of elements.

Solar Neutrinos

________- Produced during conversion of H to He.

Density

________ Gives us a clue on the chemical composition of the planet, but is a rough approximation.

generation star

A first ________ is a star made from the elements of the big bang- primarily hydrogen and helium.

nuclear fusion

In the cores of stars, ________ occurs due to very high temperatures and pressures So as clouds of He and H are pulled together, the gravitational energy is converted into heat.

A first generation star is a star made from the elements of the big bang

primarily hydrogen and helium

This neutron capture process can happen in two ways

Slow (s) process and rapid (r) process

What is corrected density and why is it required = the density of the planet without the influence of gravity

important because larger planets have a larger gravitational pull so we want to correct those factors out to know the density without the effect of gravity

define an isotope

Isotopes are atoms with the same number of protons, but a different number of neutrons - e.g 16O

define an isobar

Isobars are different elements with the same mass number - e.g 40Ar, 40K, 40Ca

define isotone

Isotones are atoms with the same number of neutrons, but a different number of protons - e.g 37Cl

Describe the changes that occur during beta decay.

An unstable atomic nucleus has too many neutrons and so undergoes decay as it does not like to be radioactive, it prefers to be stable. In doing so, a new element is formed. b-decay of a neutron transforms into a proton by emission of an electron (neutron -> proton + electron) •

What applications use beta capture?

It is often used for dating of minerals

What evidence is there that the universe is expanding?

Evidence for the expansion of the universe is from the observed red shift in the spectra of light reaching us from the stars in very distant galaxies.

Explain what is meant by ‘red shift’.

The displacement of spectral lines towards longer wavelengths (the red end of the spectrum)

Explain why Li, Be and B have very low abundances.

Because there are no stable nucleides of 5 or 8 so as soon as they form they decay. Because they don’t hang around, this is why we don’t have much Li, Be, or B. Instead, 3 particles are needed to combine - e.g two 1H with 4He to form 6Li This is tricky to do because This required three particles to be in the same place at the same time in the correct orientation

describe the features of a red giant

Red giants are extremely large (radius is very large, hence high luminosity) but lower temperature stars (because star's energy spreads across a larger area, so temperature is cooler)

Discuss the chemical composition of the solar system.

The most abundant elements in the solar system are hydrogen and helium. As the atomic number increases, there is a general trend downwards in abundance that creates a saw-tooth like plot. This sawtooth pattern is because elements with even numbers of protons are more abundant than elements with odd atomic numbers (protons).

Explain why iron has a much higher abundance in the solar system than expected.

Most stable element and is the end point of fusion in the stars. Additionally, it is a low volatile element and was captured so is abundant today

Describe how elements are produced in stars by nucleosynthesis.

Protons and neutrons combined, rapid expansion occurred, temperature and density fell, and this left us with light elements. The vast abundance of these elements were hydrogen and helium. In the cores of stars, nuclear fusion occurs due to very high temperatures and pressures So as clouds of He and H are pulled together, the gravitational energy is converted into heat. This is important because the hotter an atom is, the faster it moves, and nuclei need to move fast in order to interact with each other. the Positively charged nuclei are forced together (overcoming Coulomb barrier, such that the strong nuclear force overcomes electrostatic repulsion force) Most stars are converting hydrogen into helium, but in order for other elements to be formed, other processes must also be occurring.

Explain what a first generation star is and how it is formed

A first-generation star is a star made from the elements of the big bang - primarily hydrogen and helium. Two hydrogen atoms have enough energy that come together to form deuterium (heavy hydrogen) Deuterium can then form with another hydrogen to form free helium This process is happening all around So then two heliums can come together to give the stable 4He. During this reaction, two hydrogens are released, and these guys can then go back to the beginning of the cycle to create more helium This process occurs at high temperature. When the central temperature of a star reaches about 10^7 K, protons in the H/He mixture are in sufficiently rapid motion for fusion to occur.

How is the collapse of a star prevented?

by the internal pressure created by the escaping heat produced by the formation of helium from hydrogen - He nuclei build up in the core of the star, but do not interact with each other at 107 K (it is not hot enough or dense enough), causing a build up of He.

How are elements redistributed in space?

The elements synthesised in the core of a star may be distributed back into space as interstellar dust and gas by: • mass loss from giant stars (solar flare, eruption) • supernova explosions

how do second generation stars form

mass loss from giant stars (solar flare, eruption) • supernova explosions This allows second (and subsequent) generation stars to start with more than H. They have formed with the heavier nuclei that have formed at the end of the first generation star's life. The important consequence of this is that subsequent stars begin their lives containing small amounts of the heavier elements N, C, and elements up to Fe.

Describe the s and r processes of neutron capture

S-process: Slow neutron capture by stable isotopes. It occurs in the core of stars during the late stages of their evolution. It produces heavier isotopes by capturing neutrons one by one. it gradually increases the number of protons in the nucleus until we hit an unstable isotope. Then it will DECAY (so remember the neutron converts into a proton) which converts into another element. This process continues until the last stable atom.

R-process: Rapid neutron capture by unstable isotopes. It occurs during supernova explosions. It produces very heavy isotopes by capturing many neutrons in a short time. In a supernova explosion, an atomic nucleus is hit with neutrons until the nucleus cannot absorb any more. At this point, the neutrons begin to pass through.

The elements cannot decay to a stable isotope during this time because a neutron will constantly jump in when it undergoes decay. This process allows the formation of radioactive isotopes. Once the bombardment stops, The neutrons disappear and the neutron-rich isotopes can undergo radioactive decay until the element reaches a stable neutron-to-proton ratio.

compare the s and r process

s process is steady and slow, r process is more rapid

What evidence is there for stellar nucleosynthesis

1. Energy - Only nuclear fusion would provide sufficient energy to keep stars burning. Temperature and pressure are high enough to allow fusion of elements

2. Solar Neutrinos - Produced during conversion of H to He. A burst of neutrinos was observed during formation of SN1987A They interact weakly with matter

3. We have observed stars going supernova and the spectral lines observed during this event gave the spectral lines for Tc, and there is none of this on earth. Therefore, the only way we could have got this element is from stellar nucleosynthesis

Discuss the evidence that suggests that the planets were formed from the sun.

1.All planets are spinning in the same direction as the sun 2.The orbit of each planet around the sun is almost circular and they all lie in nearly the same plane (corresponding to the sun’s equator). 3.Even spacing between planets (increase is 1.6 x). This works fine until we get to the space between mars and jupiter but then we have the asteroid belt and think that there was a failed planet in that space.

What is ‘corrected density’ and why is it used?

the density of the planet without the influence of gravity - important because larger planets have a larger gravitational pull so we want to correct those factors out to know the density without the effect of gravity

Why do the inner planets have a higher density than outer planets?

Early in the formation of the solar system, ions produced by the Sun removed Highly volatile elements from the inner solar system and were mainly lost as gases to the outer solar system, leaving dust particles and heavy elements (less volatile) behind, which subsequently condensed to form the terrestrial planets

What information can density give us about planets? What assumptions do we have to make?

Density gives us a clue about the chemical composition of the planet but is a rough approximation. One of the main assumptions we make is the heavier the element, the greater the density

beta decay vs beta capture vs alpha decay

Beta decay: Unstable atomic nucleus that has too many neutrons and so undergoes decay as it does not like to be radioactive, it prefers to be stable.

b-decay of a neutron transforms into a proton by emission of an electron (neutron -> proton + electron). therefore YOU GAIN A PROTON

Beta capture: a type of radioactive decay process in which a nucleus absorbs an electron and LOSES a proton

Alpha decay: radioactive decay in which an alpha particle is emitted from the nucleus of an atom, reducing its atomic number by 2 and its neutron number by 4