Question

atoms of different elements contain different numbers of protons and electrons - this is what makes the elements different.
two numbers can be assigned to any atom, the atomic number and the mass number.

atomic number

• atomic number (or proton number) is the number of protons in the nucleus of one atom of an element.
• since the number of electrons in an atom is always equal to the number of protons, the number of electrons in an atom is equal to the atomic number.
• each element has its own unique atomic number.

mass number

• mass number (or nucleon number) is the total number of protons and neutrons in the nucleus of one atom of an element.
• the number of neutrons in an atom can be calculated by subtracting the atomic number from the mass number.
• more than one element can have the same mass number, so the number is not unique to a particular element.

nuclear notation

• an atom (or ion) of an element can be represented using the nuclear notation given below:

$_{z}^{a}x$

where: $x$ = atomic symbol
$a$ = mass number
$z$ = atomic number

at this point, pause and watch the khan academy video: elements and atomic number

part 4: determining the number of protons, neutrons, and electrons

worked example
using the nuclear notation, the number of protons, neutrons and electrons in an atom can be determined. the nuclear notation for copper is cu.

from this, the following can be deduced about a copper atom:
mass number = 63
atomic number = 29
number of protons = 29
number of neutrons = 63 - 29 = 34
number of electrons = 29

example 2

Explanation:

Step1: Recall atomic number concept

The atomic number is the number of protons. For an atom, number of electrons = number of protons.

Step2: Recall mass - number concept

Mass number = number of protons + number of neutrons. So, number of neutrons = mass number - atomic number.

Answer:

For a general atom with mass number $A$ and atomic number $Z$:

• Number of protons = $Z$
• Number of neutrons = $A - Z$
• Number of electrons = $Z$