Video Transcript
Complete the following
sentence. The elements that formed in the
early universe are blank than the ones that formed later. Is it (A) lighter or (B)
heavier?
To answer this, let’s first recall
that the “heaviness” of an element refers to its atomic mass, which depends on how
many protons and neutrons are in its nucleus. In the first moments of the
universe, very shortly after the big bang, the universe was too hot and dense for
particles to even exist. Then, as the universe expanded
rapidly, it cooled rapidly. So less than a second after the big
bang, conditions allowed for individual protons and neutrons to form.
Recall that the nucleus of
hydrogen, the simplest and lightest element, contains a single proton. Thus, hydrogen was the first
element to exist, and it was the most plentiful element in the universe. Consequently, hydrogen is still the
most abundant element in the universe today.
In the first few minutes after the
big bang, nuclear fusion took place, meaning protons and neutrons combined to create
other elements heavier than hydrogen. Still though, these new elements
were very light, with their nuclei containing only a couple of protons and
neutrons.
Helium, whose nuclei contains two
protons, was the first element other than hydrogen to form. Lithium nuclei, which contain three
protons, also formed, but lithium was much less abundant than hydrogen and
helium. Lithium is believed to be the
heaviest stable element that formed in the early universe.
Within the first hour after the big
bang, the universe cooled and expanded so much that it was no longer hot and dense
enough for nuclear fusion to occur. Thus, it would be another several
hundred million years until any heavier elements were formed due to nuclear fusion
occurring in the first-ever stars. The life cycle of stars is
responsible for creating the heavier elements in the universe. So as time passed, increasingly
heavier elements were able to be formed.
Knowing this, we can see that the
correct answer is option (A). Filling in the blank, we have that
elements that formed in the early universe are lighter than the ones formed
later.
