Atomic structure
An atom is the smallest part of an element that can exist.
Atoms consist of three basic particles:
 Protons
 Neutrons
 Electrons
The nucleus (center) of the atoms contains the protons (positively
charged) and the neutrons (uncharged)
The outermost regions of the atom are called electron shells and contain
The electrons (negatively charged).
Each element has a certain number of protons and neutrons, which makes
it easy to differentiate them. This is how atoms are arranged in the periodic
table.
Why do atoms have no electric charge? (Electrically neutral)
Atoms contain charged particles but atoms have no overall electric charge. They
are Neutral. This is because in any atom the number of protons is the same as the
number of electrons.
Examples:
 A Helium atom is made up of:
2 protons (positively charged) 2 electrons (negatively charged) 2 neutrons
 Hydrogen atom
1 proton (positively charged), 1 electron (negatively charged) and no
neutrons.
Arranging the electrons
 Electrons occupy shells (energy levels)
 Each shell can hold a maximum number of electrons:
1st shell can hold up to 2 electrons
2nd shell can hold up to 8 electrons
3rd shell can hold up to 8 electrons
 The outer most shell can never carry more than 8 electrons
 Electrons fill the shells that are closest to the nucleus first
 The maximum number of shells is 7
Mass number(Nucleon number): is the number of neutrons and protons
inside the atom.
.
Atomic number: is the number of protons . this is also the same as the number
of electrons), each element have a specific number of protons in fact the number
of protons determine who the element is.
number of neutrons = mass number – atomic number
N.B The periodic table is arranged in increasing atomic number not mass
number.
Be
Protons:
Electrons:
Neutrons:
…….
…….
…….
N
…….
…….
…….
O
..….
…….
……..
Ne
……..
……..
…….
Li
…….
……..
……..