Why does helium fusion require a higher temperature than hydrogen fusion?

Answer 1

They are both in period 1, so have their valence electrons in the first energy level. However, helium has 2 protons in the nucleus holding on to those electrons. Hydrogen has only 1 proton holding the electrons. Thus it takes more energy to remove an electron from helium than from hydrogen.

Answer 2

This one will be easy to get your head around if we back up a bit. So let's. Ready? Jump with me. Atoms are formed of a nucleus and an electron or electrons, the electron(s) forming up in orbitals around that nucleus. At elevated temperatures, the electrons start to leave. At really elevated temperatures, all the electrons go on vacation because there is soooo much energy around that the electrons cannot stay in their orbits - or anywhere else around the neighborhood. Recall that there are three basic states of matter, and you know them as solids, liquids and gases. When things are superheated, a fourth state of matter forms - plasma. Guess what the sun is. Plasma! It's a big soup of plasma! And that's 'cause it's so very, very hot. Let's jump again. One way to look at the "stuff" on the sun is as a bunch of atomic nuclei. Make sense? All the electrons are on vacation and the nuclei are floating around in the plasma soup. Heck, they are the plasma soup. It's mostly hydrogen nuclei, which are mostly single protons. There's also some helium nuclei floating around. They're mostly two protons and two neutrons. Still with us? Good. Not far to go. Jump again. Fusion is the "smooshing together" of atomic nuclei and the "gluing together" of the joined bits to keep them together. Fusion. Simple and easy. When we "smoosh" hydrogen nuclei together to make helium, we smoosh two protons together (and a couple of neutrons). Protons are positively charged. And they don't like each other! Like charges repel, remember? But at high enough energies (high enough temperatures), we can force them together and fuse them, or the stars can. Now picture two helium atoms, each with its two protons and its neutrons. Which is harder: smooshing together two hydrogen nuclei, each with a positive charge, or smooshing together two helium atoms, each with twoprotons and some neutrons? Bingo! Helium smooshing is harder, and it can only occur under conditions of higher energy - higher temperatures. The threshold temperature of hydrogen fusion (sometimes called a proton-proton reaction) is on the order of 10,000,000 K to 14,000,000 K or 10 to 14 million degrees Kelvin. Blazing hot! But the threshold temperature for helium fusion is on the order of 100,000,000 K or 100 million degrees Kelvin! Wow! Get out your Raybans and the sunblock! Need links? You got 'em.

Answer 3

Because the helium nuclei needs to be moving faster than hydrogen nuclei to fuse. This requires higher temperatures and densities.

Hydrogen has a small positive charge compared to helium and thus will have a weaker repulsion. A higher temperature is required to "push" the helium close enough to fuse. About 100 million degrees K compared to about 10 million degrees K for hydrogen.

The values get even higher for oxygen fusion, about 1.5 billion degrees K.

See related link for more information.

Answer 4

The helium atom has a greater nuclear charge, against which the electron must be pulled away.