Download Protons, Neutrons, Electrons

Protons, Neutrons, Electrons
Name
We know that atoms are made of protons, electrons, and neutrons. This sheet will discuss how we show the
composition of atoms: How many p, e, and n a particular atom has. We will also relate the makeup of each
element’s atoms to the periodic table.
Key terms to know, covered on this sheet: Atomic number, mass number, ion, isotope,
The atomic number of an element is defined as the number of protons an atom of that element has in its nucleus.
For a neutral atom, this will also be equal to the number of electrons the atom has. However, since atoms can gain or
lose electrons (becoming ions in the process), we define each element by its protons. The atomic number is always a
whole number; note that the periodic table is ordered by increasing atomic number.
2 vocab words often confused, so here’s your warning: Ion and Isotope. Both have something the same, something
different. Both start with the letter “i". Don’t confuse them!
Isotopes are atoms of the same element that have identical numbers of protons (by definition!!) but different
numbers of neutrons. Ions are made when an atom gains or loses electrons.
The mass number of an atom is the number of protons plus the number of neutrons in the atoms’s nucleus. The
mass number also must be a whole number. The mass number is not found on the periodic table; it must be either
given or be calculated for any atom we’re interested in. The mass number is used to identify the different isotopes
of an element. For example, there are two different isotopes that of the element vanadium, Vanadium-50 and
Vanadium-51. We can write this in different ways:
Vanadium - 50 or 5023V or 50V
51
Vanadium - 51 or 51
V
23V or
Question: Why is the number 23 “optional” when we identify the isotopes of vanadium?
Here is a completely-written identification
for an atom of Vanadium-51. Note the location of the mass number, the

atomic number, and where the ion charge (if any) are written. You must remember what each of these numbers
means, so that you can fully describe the isotope!
Mass number
Atomic number
51
23
V
Ion charge (if none, it’s
an atom, not an ion!
Finding protons, neutrons, electrons, element symbol, ionic charge:
If we have identified an isotope with a symbol like above, we can determine the amount of each of the particles.
• Protons: By definition, we can tell the # of protons by the identity of the element using a periodic table. Given “V”
or “vanadium,” look it up on the periodic table to find its atomic number, which must equal the number of protons.
• Neutrons: The number of neutrons is never written directly in a isotopic notation as in the box above; you must
calculate it! Since mass # = protons + neutrons, and atomic # = protons, by subtracting the two, we get the number
of neutrons. Here, 51-23 = 28 neutrons. In general you’ll have to be given either the # of neutrons or the mass
number to identify the isotope fully.

• Electrons: The charge of the ion identifies the number of electrons compared to the number of protons. In a neutral
atom, electrons = protons. When an atom gains electrons, it becomes negatively charged (surplus of negative
electrons) When an atom loses electrons, it becomes positively charged (more protons than electrons). You may find
this equation helpful: Charge = (number of protons) – (number of electrons)
Directions: Complete the table on the other side of the sheet. The information above is enough to help you do
everything you need to do. KEY THOUGHT: Unless you are shown it’s an ion (written charge on isotope symbol
or protons ≠ electrons), you may assume that each line represents a neutral atom.
Practice – Determining PNE for Atoms and Ions
a)
Name:____________________
Number of
Protons
Number of
Neutrons
Number of
Electrons
Atomic
number
Mass
number
Isotope name
23
28
23
23
51
Vanadium-51
c)
23
d)
47
e)
38
28
20
109
V

50
110
48
g)
35
h)

Bromine-79
36
22
Cd +2
81
18
46
j)
Radium-223
74
108
m) 8
72
8
l)
n)
V

f)
k)
51
23
50
23
b)
i)
Isotope
symbol
16
9
Oxygen-18
127 –1
I
o)
