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.

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

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.

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.

Exercises

Figure 4.

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