Page 14 - The Periodic Table Book

Inside an atom

Introduction An atom is the smallest unit of an element. Atoms are too small to see
(even with the most powerful microscopes) but they are everywhere.
They consist of smaller particles called protons, neutrons, and
electrons. Every element has a unique number of protons.

What’s the atomic number?
The number of protons in an atom of an element is
called the atomic number. The atomic number of an
atom identifies the element it belongs to. Every atom
also has an equal number of electrons. For elements
found naturally on Earth, hydrogen has the smallest
atomic number (1), while uranium atoms have the
highest atomic number (92).

Electron ❯ The tiny,
1 Atomic This shell is the space negatively charged
number
H in the hydrogen atom particles in an atom are
where one electron
called electrons. They
circles the proton at
the centre of the atom. are involved in the way
the atoms of an element
Hydrogen atom
react and form bonds
with the atoms of
other elements.

3 In a lithium atom,
Li two shells house
three electrons, which
circle the protons and
neutrons at the centre.

Lithium atom

92
U

Seven shells house
the 92 electrons in
a uranium atom.

Shell ❯ The electrons in an atom move
around the nucleus. They are arranged
in layers called shells. When reacting
with each other, atoms tend to fill up
their outer shells to become more stable.
12 Uranium atom

012-013_Inside_an_atom.indd 12 02/12/16 7:47 pm