The Structure of The Periodic Table

Earlier in the course, we learnt that the periodic table (shown below) is table which lists of all the known elements in order of their atomic number (number of protons).

The periodic table.

Now that we have learnt some more Chemistry concepts, we are ready to examine the periodic table in more detail.

The periodic table lists every element's name, chemical symbol, atomic number and relative atomic mass

Aluminium's box in the periodic table is shown below.

Aluminium’s box in the periodic table.

It contains the following information:

Note that the relative atomic mass has been rounded. The actual value is roughly 26.9815384. However, even this more precise value is only an estimate. The isotope abundances vary slightly from one sample to the next, so scientists have to come up with a value by looking at lots of different samples from around the world and taking an average. Since it is not possible to sample every single atom on Earth, the relative atomic mass values in the periodic table will always be estimates.

In this example, the relative atomic mass is written at the top of the box and the atomic number is written at the bottom. Sometimes they are written the other way round. Every periodic table has a key to tell you which number is which.

The periodic table does not show mass numbers

You may have noticed that while the periodic table shows the atomic number and relative atomic mass of each element, it does not show the mass number (number of protons and neutrons).

There is a very important reasons for this: elements do not have mass numbers.

Mass number is the number of protons and neutrons in the nucleus of an atom or ion. However, different atoms/ions of the same element can have different numbers of neutrons (in other words, there can be different isotopes of an element).

For example, the element carbon has three isotopes: carbon-12, carbon-13, carbon-14. These three isotopes have different numbers of neutrons and therefore different mass numbers. Therefore, the element carbon does not have a mass number.

Individual atoms and ions do have mass numbers, because each atom or ion has a specific number of protons and neutrons. However, the periodic table is not a table of atoms or a table of ions, it is a table of elements. Each element has multiple isotopes and therefore does not have a mass number.

The periodic table is organised into periods and groups

The periodic table is organised as a grid with rows and columns.

The rows (horizontal lines of elements) are called periods.

The periods are numbered from the top down. At the moment, there are 7 periods, however if more elements are discovered then more periods will be added.

The first period contains just hydrogen (H) and helium (He). The second period contains all of the elements from lithium (Li) to neon (Ne). The third period contains all of the elements from sodium (Na) to argon (Ar). The fourth period contains all of the elements from potassium (K) to krypton (Kr). This course will mostly focus on the elements in the first four periods.

The columns (vertical lines of elements) are called groups.

The groups are numbered from left to right. There are 8 groups.

Group 8 is sometimes called group 0.

Between groups 2 and 3 there is region called the d-block, which is not part of any group. It contains a set of elements called the transition elements.

The period and group numbers can be seen on the periodic table below:

The periodic table. Note the period and group numbers.

Flashcards

Flashcards help you memorise information quickly. Copy each question onto its own flashcard and then write the answer on the other side. Testing yourself on these regularly will enable you to learn much more quickly than just reading and making notes.