Atomic Shorthand
Introduction

We have already seen how we can show how electrons are arranged in atoms using a type of shorthand. The electronic structures can be shown without drawing out all the energy levels (shells) around the nucleus. We simply show the numbers of electrons in each energy level, starting with the first. So the electrons in a potassium atom are arranged 2, 8, 8, 1.

In this unit we will look at other abbreviations we can use to write down information about an atom quickly and efficiently without having to draw all those circles!

Atomic number
Each chemical
element
An element is a substance made from only one type of atom. An element cannot be broken down into any simpler substances.
element
has its own unique atomic number. The
atomic number
The atomic number is the number of protons in an atom (which equals the number of electrons).
atomic number
tells us the number of protons in an
atom
An atom is the smallest particle of an element that can still be defined as that element.
atom
, hence it is sometimes referred to as the proton number.

As you know, all atoms are neutral. They carry no overall charge. The positive protons are balanced out by the negative electrons. Therefore, the atomic number of an element also tells us the number of electrons in an atom.

We can say that:

Atomic number of an element  =  the number of protons (which equals the number of electrons) in each atom of the element.


By convention we show the atomic numbers of different elements by putting the number as a subscript before the chemical symbol for the element. This is shown below:

1H    2He    3Li    4Be    5B    etc.

So, for example, given the atomic number of lithium (Li), we have the information shown below:

Figure 1.   Information we can deduce from the atomic number.
Display as fullscreen


You are not expected to learn atomic numbers by heart. You will always be given these to work from in a question.

An atom of uranium has 92 protons. What is its atomic number?
  • Click here to mark the question
The atomic number of carbon is 6. What information about a carbon atom does this give us?
  • It has six protons.
    It has six neutrons.
    It has six electrons.
  • Click here to mark the question
A chlorine atom has 17 electrons. What is its atomic number?
  • Click here to mark the question
Mass number
Can you remember which are the heavy particles inside an atom? And where do we find them within the atom? The mass number of an atom tells us how many heavy particles – protons and neutrons – there are in the
nucleus
The nucleus is the centre of an atom, containing protons and neutrons.
nucleus
of an atom.

Mass number  =  the number of protons  +  the number of neutrons in an atom.


(Some people refer to the
mass number
The mass number is the number of protons plus neutrons in the nucleus of an atom.
mass number
as the 'nucleon number' because 'nucleon' means a proton or a neutron.)

We show the mass number by writing it as a superscript before the symbol for the element as shown below:

1H    4He    7Li    9Be    11B    etc.

Taking lithium (Li) as an example, we know from its mass number that its nucleus must contain a total of seven protons and neutrons. But how many of each? To answer this, we need to be told the atomic number as well.

Given that the atomic number of Li is 3, we now know that Li has three protons. Therefore, it must have four neutrons, to make up its mass number of 7.

We can summarize this reasoning in the following equation:

Number of neutrons  =  mass number  −  atomic number.


In the case of lithium (Li):

Number of neutrons  =  7  −  3  =  4

Figure 2.   Information we can deduce about the nucleus of an atom given the mass number and atomic number.
Display as fullscreen
An atom of potassium has 19 protons and 20 neutrons. What is its mass number?
  • Click here to mark the question
An atom of beryllium has an atomic number of 4 and a mass number of 9. How many neutrons does it contain?
  • Click here to mark the question
An atom of uranium has an atomic number of 92 and a mass number of 235. How many neutrons does it contain?
  • Click here to mark the question
Summary


The atomic number tells us the number of protons (which equals the number of electrons) in an atom.

The mass number tells us the number of protons plus the number of neutrons in the nucleus of an atom.

These are shown like this:

Figure 3.   Atomic shorthand.
Display as fullscreen
Exercises
Figure 4.  
Display as fullscreen
1. What is the atomic number for the element shown in Fig.4?
  • Click here to mark the question
2. What is the mass number (nucleon number) for the element shown in Fig.4?
  • Click here to mark the question
3. An atom of sulfur has 16 protons and 16 neutrons. What is its mass number?
  • Click here to mark the question
4. An atom of iron has an atomic number of 26 and a mass number of 56. How many neutrons does it contain?
  • Click here to mark the question
5. An atom of radon has an atomic number of 86 and a mass number of 222. How many neutrons does it contain?
  • Click here to mark the question
6. An atom of magnesium has an atomic number of 12 and a mass number of 24. How many protons, neutrons, and electrons does it contain?
  • Click here to mark the question
7. An atom of argon has an atomic number of 18 and a mass number of 40. How many protons, neutrons and electrons does it contain?
  • Click here to mark the question
8. Here is an atom of gold: . How many protons, neutrons, and electrons does it contain?
  • Click here to mark the question
9. Here is an atom of nickel: . How many protons, neutrons, and electrons does it contain?
  • Click here to mark the question
10. Here is an atom of hydrogen: . How many protons, neutrons, and electrons does it contain?
  • Click here to mark the question
Well done!
Try again!