15 Mendeleev’s Periodic Table
121
quantum mechanics and other aspects of atomic theories, such as electron
spin, exclusion principle, etc., this periodicity of the periodic table resulted
as a simple consequence of quantum theory.
A key feature of the periodic table as we go horizontally is, each element has
one more proton in the nucleus than the previous element. We will denote
the number of protons in a nucleus by Z (atomic number) and number of
neutrons by the letter N—so as we go higher, the elements go from Z →
Z + 1 → Z + 2 . . .. There can be many different nuclei with a single Znumber but different neutron numbers N—they are called isotopes of each
other and are “lumped” in at one spot in the periodic table. To understand the
origin of the periodic table, we have to understand how in the evolution of the
universe, the nuclei with one value of Z generated the next nuclei with a value
of Z + 1, and so on. This is called heavy element nucleosynthesis. Currently
there are 118 elements in the periodic table, out of which 92 occur naturally
and the rest were created in accelerators.
Little did Mendeleev know (or could have known) that what he created is
intimately related to a tiny elusive particle called the neutrino. But that is what
it was. The cosmic soup of quarks and leptons got turned into the elements
in the periodic table due to neutrinos constantly bombarding against quarks
in the first few minutes of the early universe. Neutrinos converted the protons
and neutrons to helium, lithium, and beryllium, which are the ancestors of
all the elements we know today and owe our existence to. There are two clues
to understanding the heavy element nucleosynthesis; one is that it is easier
for nuclei to absorb a neutron than a proton, since the proton is repelled by
the electric repulsion from other protons in the nuclei, thus the proton has
difficulty getting inside the nucleus. Second is that in beta decay reaction, a
neutron emits a neutrino and an electron and converts the neutron to a proton
i.e. n → p +e
−
+ ¯
ν e and thereby it changes the nucleus with Z → Z +1. But
how did it all start at the beginning of time, since in those early moments the
universe was so hot that all protons and neutrons “melted” to three quarks?
Somehow the quarks must have united form protons and neutrons as the
universe expanded and the protons and neutrons must have united to form
various nuclei. Finally, the nuclei had to combine with the electrons to form
an atom. This is a complex and long drawn process that has lasted from the
time when the universe was a minute old to “almost now” when the stars
formed and evolved. This is a fascinating story. To get to this discussion,
we need to establish some background about the role of neutrinos, creating
even the first few elements of the periodic table, such as helium, lithium, etc.
from which things eventually grow to more complex nuclei. The neutrons and
neutrinos hold the key to this story, for at least the 92 known elements that
occur naturally. We describe this process in the next chapter.
121
quantum mechanics and other aspects of atomic theories, such as electron
spin, exclusion principle, etc., this periodicity of the periodic table resulted
as a simple consequence of quantum theory.
A key feature of the periodic table as we go horizontally is, each element has
one more proton in the nucleus than the previous element. We will denote
the number of protons in a nucleus by Z (atomic number) and number of
neutrons by the letter N—so as we go higher, the elements go from Z →
Z + 1 → Z + 2 . . .. There can be many different nuclei with a single Znumber but different neutron numbers N—they are called isotopes of each
other and are “lumped” in at one spot in the periodic table. To understand the
origin of the periodic table, we have to understand how in the evolution of the
universe, the nuclei with one value of Z generated the next nuclei with a value
of Z + 1, and so on. This is called heavy element nucleosynthesis. Currently
there are 118 elements in the periodic table, out of which 92 occur naturally
and the rest were created in accelerators.
Little did Mendeleev know (or could have known) that what he created is
intimately related to a tiny elusive particle called the neutrino. But that is what
it was. The cosmic soup of quarks and leptons got turned into the elements
in the periodic table due to neutrinos constantly bombarding against quarks
in the first few minutes of the early universe. Neutrinos converted the protons
and neutrons to helium, lithium, and beryllium, which are the ancestors of
all the elements we know today and owe our existence to. There are two clues
to understanding the heavy element nucleosynthesis; one is that it is easier
for nuclei to absorb a neutron than a proton, since the proton is repelled by
the electric repulsion from other protons in the nuclei, thus the proton has
difficulty getting inside the nucleus. Second is that in beta decay reaction, a
neutron emits a neutrino and an electron and converts the neutron to a proton
i.e. n → p +e
−
+ ¯
ν e and thereby it changes the nucleus with Z → Z +1. But
how did it all start at the beginning of time, since in those early moments the
universe was so hot that all protons and neutrons “melted” to three quarks?
Somehow the quarks must have united form protons and neutrons as the
universe expanded and the protons and neutrons must have united to form
various nuclei. Finally, the nuclei had to combine with the electrons to form
an atom. This is a complex and long drawn process that has lasted from the
time when the universe was a minute old to “almost now” when the stars
formed and evolved. This is a fascinating story. To get to this discussion,
we need to establish some background about the role of neutrinos, creating
even the first few elements of the periodic table, such as helium, lithium, etc.
from which things eventually grow to more complex nuclei. The neutrons and
neutrinos hold the key to this story, for at least the 92 known elements that
occur naturally. We describe this process in the next chapter.