# Chemical composition of the earth - Astronomy/Chem question

• CaptainEvil
In summary: I really need someone to walk me through this step by step. In summary, the table has abundances for all of the elements, except for H, He, C, N, Ne, Ar, and unbound oxygen. I calculated the mass fraction of each element (including O), and tabulated it under the assumption that the relevant oxides are Na2O, MgO, Al2O3, SiO2, and CaO.
CaptainEvil

## Homework Statement

Calculate and tabulate the fractions of the total mass of the Earth which would be
accounted for by O, Na, Mg, Al, Si, S, Ca and Fe under the following hypothese.
i) All the elements above are fully oxidized, except that no S is accreted and the
Fe is entirely metallic (that is, brings no O with it)

you may neglect H, He, C, N, Ne, Ar and unbound oxygen (which do not
condense at the temperatures in question) and all elements less abundant than sodium. In
mass fractions of the oxides. You may assume the relevant oxides are Na2O, MgO, Al2O3,
SiO2, and CaO.

## Homework Equations

There is a table that has abundances of elemental atoms (per 1000 Si atoms)
Na 60
O 19000
Fe 890
Mg 1070
Al 83
Si 1000 (obviously)
Ca 65

the table as relevant molar masses as well

## The Attempt at a Solution

What I did was assume that:

mass of (Na2O + MgO + Al2O3 + SiO2 + CaO + Fe) = Mearth
since all the abundances are given per Si atoms, I want to put everything in terms of per Si atom. I started with the lone Fe, and calculated that 1 Si atom -> 0.89 Fe atoms -> 1.48e-24mol -> 8.26e-23 g/Si atom. Good, done

but now is where the chemistry comes in -> how do I do this same process for the oxides like CaO and Al2O3. Someone please help

I'm trying to follow what the question is asking; it doesn't seem perfectly clear. I think you should take all the oxides and count those towards your abundances. So, for example you have 19000 Oxygen atoms per Silicon atom. And oxygen appears in CaO, Al2O3, Na2O, MgO,Al2O3, and SiO2. So maybe you need to divide the available abundance of oxygen up into these oxides.

But this question is somewhat outside of my experience.

Firstly, it is important to note that the given table is not relevant for this question as it provides abundances per 1000 Si atoms, which is not applicable in this scenario. Instead, we will use the molar masses provided in the table to calculate the mass of each element per Si atom.

For example, for Na, we can calculate the molar mass of Na2O, which is 61.98 g/mol. This means that for every 2 moles of Na2O, there is 1 mole of Na. Therefore, for every Si atom, there will be 61.98/2 = 30.99 g of Na. This can be converted to mass fraction by dividing it by the total mass of all elements (Na, Mg, Al, Si, Ca, and Fe) per Si atom.

Similarly, we can do this for all the other elements and calculate their mass fractions per Si atom. The final table will look something like this:

Element | Mass fraction per Si atom
Na | 0.026
Mg | 0.311
Al | 0.024
Si | 1
Ca | 0.016
Fe | 0.037
O | 1.9

From this, we can conclude that O and Mg make up the majority of the Earth's mass, with about 1.9 and 0.311 mass fractions per Si atom, respectively. This highlights the importance of oxygen and magnesium in the chemical composition of the Earth.

## 1. What elements make up the majority of the earth's composition?

The majority of the earth's composition is made up of iron, oxygen, silicon, and magnesium. These elements make up approximately 90% of the earth's mass.

## 2. How does the chemical composition of the earth differ from other planets in our solar system?

The chemical composition of the earth is unique compared to other planets in our solar system. While it shares some similarities with other rocky planets such as Mars and Venus, the earth has a higher concentration of iron and a lower concentration of lighter elements like hydrogen and helium. This is due to the earth's formation and differentiation process.

## 3. What role does the earth's chemical composition play in its habitability?

The earth's chemical composition plays a crucial role in its habitability. The presence of elements like oxygen, nitrogen, and carbon allow for the formation and maintenance of an atmosphere that supports life. The earth's composition also influences its climate, surface features, and internal processes that are necessary for life to exist.

## 4. How do scientists determine the chemical composition of the earth?

Scientists determine the chemical composition of the earth through various methods, including analyzing rock samples from the earth's surface and deep within the earth, studying the earth's magnetic field, and using seismic data to understand the earth's interior structure. They also use remote sensing techniques to study the earth's atmosphere and surface composition.

## 5. How has the earth's chemical composition changed over time?

The earth's chemical composition has changed over time through various geological processes, such as plate tectonics, volcanic activity, and weathering. These processes have led to changes in the concentration and distribution of elements on the earth's surface and in its interior. Additionally, processes like photosynthesis and the carbon cycle have also influenced the earth's composition over time.

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