Everything Science Grade 10

CHAPTER 4. THE ATOM 4.4

A

ZX chemical symbol

number of nucleons

number of protons

FACT

A nuclide is a distinct

kind of atom or nu-

cleus characterised by

the number of protons

and neutrons in the

atom. To be abso-

lutely correct, when

we represent atoms

like we do here, then

we should call them

nuclides.

For example, the iron nucleus which has 26 protons and 30 neutrons, is denoted as:

(^5626) Fe
where the atomic number isZ= 26and the mass numberA= 56. The number of neutrons
is simply the differenceN=A−Z= 30.
Tip
Do not confuse the
notation we have
used here with the
way this informa-
tion appears on the
periodic table. On
the periodic table,
the atomic number
usually appears in
the top left-hand
corner of the block
or immediately above
the element’s symbol.
The number below
the element’s symbol
is itsrelative atomic
mass. This is not ex-
actly the same as the
atomic mass number.
This will be explained
in "Isotopes". The
example of iron is
shown below.
Fe

26

55.85

For a neutral atom the number of electrons is the same as the number of protons, since the
charge on the atom must balance. But what happens if an atom gains or loses electrons?
Does it mean that the atom will still be part of the same element? A change in the number
of electrons of an atom does not change the type of atom that it is. However, the charge
of the atom will change. Theneutralityof the atom has changed. If electrons areadded,
then the atom will become morenegative. If electrons aretaken awaythen the atom will
become morepositive. The atom that is formed in either of these two cases is called an
ion. An ion is a charged atom. For example: a neutral sodium atom can lose one electron
to become a positively charged sodium atom (Na+). A neutral chlorine atom can gain one
electron to become a negatively charged chlorine ion (Cl−). Another example is Li+which
has lost one electron and now has only 2 electrons, instead of 3. Or consider F−which has
gained one electron and now has 10 electrons instead of 9.

Example 1: Standard notation

QUESTION

Use standard notation to represent sodium and give the number of protons, neu-

trons and electrons in the element.

SOLUTION

Step 1:Give the element symbol

Na

Step 2:Find the number of protons

Sodium has 11 protons, so we have: 11 Na

Chemistry: Matter and Materials 73