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energies where they are present. Up to two electrons can fit into the closest shell. Before any other shells can
be filled, the one closest to the nucleus is always filled first. Since hydrogen only has one electron, there is only
one empty space in the lowest shell. Since helium has two electrons, its two electrons can completely fill the
lowest shell. The only two elements in the first row of the periodic table are hydrogen and helium, as can be
seen. They only have electrons in their outermost shell, which explains this. The only two elements with only
the lowest shell and no other shells are hydrogen and helium.
Up to eight electrons can be accommodated in the second and third energy levels. Four pairs of the
eight electrons
are formed, and before any pairs are finished, one electron is placed in each position.
You will see that there are seven rows if you look at the periodic table once more (Figure 5). These
rows are arranged according to how many shells each of the row's elements has. As the columns move from
left to right, the elements within a particular row have an increasing number of electrons. Despite the fact that
every element has the same number of shells, not every shell is entirely filled with electrons. Lithium (Li),
beryllium (Be), boron (B), carbon (C), nitrogen (N), oxygen (O), fluorine (F), and neon can be found in the
second row of the periodic table (Ne). All of them only have electrons in the first and second shells. Lithium's
outermost shell
contains just one electron; beryllium's has two; boron's has three; and so on until neon's
outermost shell contains eight electrons altogether.
Although not every element has enough electrons to completely
fill its outermost shell, when this
condition is met, an atom is at its most stable. We observe the formation of chemical bonds—interactions
between two or more of the same or different elements that lead to the formation of molecules—as a result of
these vacancies in the outermost shells. Atoms tend to completely fill their outer
shells in order to achieve
greater stability. To achieve this goal, atoms will bond with other elements and share electrons, accept electrons
from other atoms, or donate electrons to other atoms. This is known as the octet rule because the outermost
shells of the elements with low atomic numbers (up to calcium, with atomic number 20)
can hold eight
electrons. To fill its outer shell and adhere to the octet rule, an element can offer, accept, or share electrons with
other elements.
Ions are atoms that do not have an equal number of protons and electrons. Each ion has a net charge
because the number of protons does not equal the number of electrons. Cations are positive ions that are created
by losing electrons. Anions are the negative ions that are produced when electrons are gained.
For instance, sodium's outermost shell only contains one electron. Sodium can donate that one electron
with less energy than it can accept seven additional electrons to fill the outer shell. Sodium would have 11
protons and only 10 electrons after losing an electron, leaving it with an overall charge of +1. We now refer to
it as a sodium ion.
The outer shell of the chlorine atom contains seven electrons. Again, the gain of one electron is more
energy-efficient for chlorine than the loss of seven. It consequently has a tendency
to pick up an electron,
resulting in an ion with 17 protons and 18 electrons and a net negative (-1) charge. It's now referred to as a
chloride ion. The transfer of electrons between two elements is known as an electron transfer. As shown in
Figure 6, a chlorine atom (Cl) has seven electrons in its outermost shell compared to a sodium atom's (Na)
single electron. A chlorine atom will accept a sodium atom's one electron in order to fill its empty shell,
resulting in the formation of chloride. Now that both ions have finished their outermost shells and satisfy the
octet rule. Each electron has a +1 (sodium) or -1 (chloride) charge because the number of electrons is no longer
equal to the number of protons.