An atom is composed of protons, neutrons, and electrons. Protons and neutrons are located in the nucleus, while electrons move around the nucleus. The number of protons determines the element, while the number of neutrons can vary, creating isotopes of that element. Isotopes are identified by their isotope notation, which includes the element symbol, mass number (protons + neutrons), and atomic number (protons).

2. •Atom is regarded as the smallest, basic unit of
matter. Even if it is the basic unit of matter, it is still
composed of parts.
•The parts are the electrons (-), protons (+), and
neutrons (0).
•An atom has equal number of protons and
electrons. This makes the atom neutral.
•Protons and neutrons are relatively heavier than
electrons. They compose the nucleus and
collectively called as nucleons. The mass of an
atom is mainly determined by the mass of the
nucleus.

3. •Several models have tried to show how the
subatomic particles are arranged in an atom. Thus far,
the accepted model places the protons and neutrons
in the center of the atom; or the so-called, nucleus.
Moving rapidly around the nucleus are the electrons.
•Atomic number, or the number of protons of an atom
distinguishes an element from the others. This number
is the same for all atoms of a particular element.
•While the number of protons is fixed for an atom of
an element, the number of neutrons may vary. Atoms
having the same number of protons but different
number of neutrons are referred as isotopes. The
isotopes are identified through their mass number
which is the sum of the number of protons and the
number of neutrons in an atom.

4. Isotopes
All uncharged or neutral atoms of an
element in its stable state have the same
number of electrons and number of protons
but the number of their neutrons can be
different.
The atoms of an element that differ in the
number of neutrons in their nuclei are
called ISOTOPES of the element.

5. Isotope notation
One way to identify the different isotopes of an
element is through a symbol called isotope
notation. It has 4 parts:
Element symbol (X), mass number (A), atomic
number (Z) and charge of an atom (e).
ᴬꓫᵉ
z
A = mass number (protons + neutrons)
Z = atomic number (number of protons)
e = atomic charge (protons – electrons)
U
238
92
U - 238
or
+1

6. Try solving
1. What is the isotope notation for an
atom with 11 protons, 13 neutrons, and
10 electrons?
2. What is the isotope notation of an atom
with 6 protons, 6 electrons and 8
neutrons?
3. What is the isotope notation of the
element W?

7. ISOTOPES ELEMENT
NAME
# of p ⁺ # of e ⁻ # of n  charged
As-75 Arsenic 33 30 45 +3
Xe-131 Xenon 54 54 77 0
At-210 Astatine 85 86 124 -1

9. ISOTOPES ELEMENT NAME # of p ⁺ # of e ⁻ # of n  charged
B-6 5 1 0
N-14 7
9 10 -1
Neon 10 10
Mg-24 10
Al-27 +3
Si-28 14
S-32 0
K-35 +1
Strontium 48 +3
24 28 -2
Be -10 +2
Ga - 70 28
Krypton 47 0
U–238 87