Everything Science Grade 10

4.4 CHAPTER 4. THE ATOM

proton neutron electron

Mass (kg) 1 , 6726 × 10 −^271 , 6749 × 10 −^279 , 11 × 10 −^31

Units of charge +1 0 − 1

Charge (C) 1 , 6 × 10 −^190 − 1 , 6 × 10 −^19

Table 4.2: Summary of the particles inside the atom

Atomic number and atomic mass number ESABC

The chemical properties of an element are determined by the charge of its nucleus, i.e. by

thenumber of protons. This number is called theatomic numberand is denoted by the

letterZ.

DEFINITION: Atomic number (Z)

The number of protons in an atom

You can find the atomic number on the periodic table (see periodic table at front of book).

The atomic number is an integer and ranges from 1 to about 118.

The mass of an atom depends on how many nucleons its nucleus contains. The number of

nucleons, i.e. the total number of protonsplusneutrons, is called theatomic mass number

and is denoted by the letterA.

FACT

Currently element
118 is the highest
atomic number for an
element. Elements of
high atomic numbers
(from about 93 to
118) do not exist for
long as they break
apart within seconds
of being formed.
Scientists believe
that after element
118 there may be an
“island of stability”
in which elements
of higher atomic
number occur that
do not break apart
within seconds.

DEFINITION: Atomic mass number (A)

The number of protons and neutrons in the nucleus of an atom

The atomic number (Z) and the mass number (A) are indicated using a standard notation,

for example carbon will look like this:^126 C

Standard notation shows the chemical symbol, the atomic mass number and the atomic

number of an element as follows:

72 Chemistry: Matter and Materials