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atomicnumber

The atomic number, denoted Z, is the number of protons in the nucleus of an atom. In neutral atoms, Z also equals the number of electrons. It uniquely identifies a chemical element and serves as the primary coordinate in the periodic table.

Historically, the concept was established by Henry Moseley in the early 20th century. He showed that the

The atomic number is distinct from the mass number, A, which is the total number of protons

In the periodic table, elements are arranged in order of increasing Z, with periods reflecting successive filling

frequencies
of
characteristic
X-rays
emitted
by
elements
increase
with
nuclear
charge,
providing
a
reliable
measure
of
the
number
of
protons
and
helping
to
resolve
anomalies
in
atomic
weights
that
had
complicated
early
element
classifications.
and
neutrons
in
the
nucleus.
Isotopes
are
variants
of
the
same
element
with
the
same
Z
but
different
numbers
of
neutrons,
hence
different
A.
Z
largely
governs
chemical
properties
through
electron
configuration;
it
determines
the
arrangement
of
electrons
in
shells
and
subshells,
and
thus
an
element’s
placement
in
the
periodic
table
and
its
typical
bonding
behavior.
of
electron
shells
and
groups
exhibiting
similar
valence
electron
arrangements.
As
of
now,
118
elements
have
been
observed,
with
atomic
numbers
from
1
to
118.
Ongoing
research
continues
to
explore
heavier
elements
and
the
limits
of
the
periodic
table.