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# SQA National 5 Chemistry Periodic table structure

In 1869, Russian scientist Mendeleev designed the periodic table that is still used today. The elements were ordered by their atomic number and grouped by their physiochemical properties.

Periodic table allowed scientists to predict how theoretical undiscovered elements would act and over the years gaps that previously existed in the periodic table have been filled in. The periodic table got its name from the periods (rows) in the table which show common trends in atomic structure and properties that are similar for the elements in the same rows, called groups.

The columns of the periodic table are called groups. There are specific names used for some groups of the periodic table:
group 0/8 (18) → noble gases
group 1 → alkali metals
group 7 (17) → halogens

Elements in the same group have similar properties since they have the same number of electrons in their outer shell. For groups 1-2 and 3-8 it is the same as the group number. For example, phosphorus has 5 electrons in its outer shell. If you use a periodic table with 1-18 group numbers, you should convert groups 13-18 to 3-8 respectively as the aforementioned rule doesn’t apply for the groups 3-12.

As you go down a group, the elements get bigger as they gain an extra shell of electrons. For example, oxygen has 2 shells of electrons, while iodine has 5 shells of electrons.

Rows in the periodic table are called periods, they are numbered and represent how many shells of electrons an element has. As you move towards the right across a period, the atomic number and nuclear charge gets bigger, and thus, the size of an atom gets smaller. The number of shells would be more important than the position within a period while considering the size of an atom.

Each period of elements has elements with the same number of electron shells. For example, period 1 elements have 1 shell of electrons and period 5 elements have 5 shells of electrons.

Where an element is found in the periodic table can tell you a lot about its physical and chemical properties. Most elements are metals except for the top right corner of the periodic table. In between metals and non-metals we can find metaloids that have properties similar to both metals and non-metals.

Since the group an element is in indicates how many electrons are in the outer shell, it can be used to determine the ionic charge. Metals usually form positive ions with the charge of their group number and a plus (+). Non-metals usually form negative charges with their charge being their group number minus 8.

Each period consists of sublevels that can be filled with a certain number of electrons. To illustrate this, the periodic table can be split into blocks, s, p, d and f. The block and period an element is found in shows which sublevel the valence electrons are stored in.

Aluminium (Al) would have valence electrons in sublevel 3p, with an electron configuration of 1s² 2s² 2p⁶ 3s² 3p¹. A shorthand way to write the electron configuration is starting from the noble gas before, e.g. [Ne] 3s² 3p¹ for aluminium.

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