A large planet such as Jupiter has enough gravity to hold on to most of its hydrogen and helium, which is why these elements dominate the atmospheres of gas giants. But the gravity of Earth isn't strong enough, so Earth's early atmosphere of helium and free hydrogen evaporated into space.
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And it does so very quickly. Why are hydrogen and helium not present in the Earth’s atmosphere? Hydrogen and Helium are the most abundant elements in the known Universe. However their scarcity in our atmosphere is due to, simply put, floating away into space.
You do find vast quantities of hydrogen in the atmospheres of the gas giants — however, those planets are more massive than Earth, which means they have a stronger gravitational pull and a greater escape velocity.
Earth; Mars; Venus. Mercury is the only terrestrial planet that still has hydrogen and helium in its atmosphere.
Terms in this set (10) Any hydrogen and helium that ___, ___, and ___ held in their initial atmospheres readily escaped. Earth; Mars; Venus Mercury is the only terrestrial planet that still has hydrogen and helium in its atmosphere. Why could Mars not hold onto its atmosphere?
There are no hydrogen atoms in the air because hydrogen is highly reactive as a sole atom and requires another hydrogen atom to stabilise itself. Furthermore hydrogen is of low frequency in the Earth’s atmosphere because the H2 has very low molecular weight and therefore escapes the Earth’s gravity- rising into space.
Sorry to disagree with the other answers but the main reason hydrogen gas does not exist in any significant quantity in the Earth’s atmosphere is that it is pretty volatile stuff. It reacts with the other chemicals in the atmosphere (primarily oxygen to produce H2O molecules).
Because helium is lighter than air, a helium balloon rises. Hydrogen is another gas lighter than air; it is even lighter than helium. Hydrogen is not used in balloons and this demonstration shows why. Helium is a special gas called a Noble Gas, which means it doesn’t burn.
This is also the reason that helium atoms escape (very slowly) the earth’s gravitational pull. Helium is completely inert in the atmosphere and would accumulate if it did not slowly diffuse into outer space.
It’s not a perfect analogy, but in the early solar system, the part of the disk closest to the sun (the part where Earth and the other rocky planets formed) was simply too hot for hydrogen to be able to grab onto the chunks of rock that would eventually become the planets.
The atmosphere of Earth is the layer of gases, called air, that surrounds our planet and is retained by its gravity.
Up there they encounter the sun’s full spectrum of radiation (visible, UV and infrared). It heats them up. “Heat” in a gas is a measure of the energy of the molecules, and usually ends up as the simple speed of their motion through the atmosphere (or space).
However, helium is formed by the radioactive decay of certain heavier elements within Earth’s crust. In fact, since the Earth’s beginning, it appears large underground reserves may have built up. Most of that has been separated from certain natural gas deposits.
An atom of helium consists of an atomic center or nucleus containing 2 protons and 2 neutrons, surrounded by two orbiting electrons. The element variety is not determined by the electrons, however, so an He atom stripped of its two electrons is still considered to be He.
Its weight is 4.003. So helium gas is roughly twice the weight of hydrogen gas. Odd as it may sound, this is relatively insignificant, since the weight of an average molecule of air or atmosphere is about 28. This is 7 times as heavy as helium and 14 times as heavy as hydrogen.
Ninety of the elements occur in nature. The smallest and lightest of the elements is hydrogen. Yet, it is abundant in Earth. Almost no hydrogen gas escapes Earth’s atmosphere. This is partly because hydrogen is reactive and exists almost exclusively in combination with other, heavier elements. Helium, too, is a gas.
Both gases, this would suggest, should soon have been depleted. Once again, though, hydrogen exists almost completely in combined form, not in its elemental gaseous form.
It assumes, not monatomic form, H, but diatomic form, H₂. The weight of that is twice the weight of an isolated atom, that is 1.008 x 2 = 2.016. Helium, on the other hand, being non-reactive, exists solely in its monatomic form, He. Its weight is 4.003. So helium gas is roughly twice the weight of hydrogen gas.
Mostly, terrestrial planets (which are mainly made up of rocks) are usually smaller in size and therefore have lower mass, in comparison to gas planets. An example, in our solar system, is Saturn and Jupiter are bigger than Earth, Mercury and Venus.
The answer is B. Hydrogen and Helium can't be retained because the planet masses are too small.
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