Periodic Table & Atoms (SQA National 5 Chemistry): Revision Note

Using the Periodic Table

• The Periodic Table is a way of organising all the known chemical elements

• You can find it on page 4 of the SQA Data Booklet

How the Periodic Table is arranged

• Elements are arranged in order of increasing atomic number

• The atomic number is the number of protons in the nucleus of an atom

• As you move from left to right across the table, each element has one more proton than the one before it

• For example:

  • Hydrogen has atomic number 1

  • Helium has atomic number 2

  • Lithium has atomic number 3

Finding metals and non-metals

• The Periodic Table has a heavy "staircase" or "zig-zag" line that divides the elements into two main categories:

  1. Metals

  2. Non-metals

• Metals are found to the left of the staircase line

  • The vast majority of elements are metals

• Non-metals are found to the right of the staircase line

  • Hydrogen (H) is an exception

  • It is a non-metal, even though it is located on the far left of the table

The Periodic Table

The Periodic Table is arranged in groups (columns) and periods (rows)

Groups of the Periodic Table

• Groups are the vertical columns in the Periodic Table.

• All the elements within the same group share important chemical characteristics

• The group number tells you the number of electrons in the outer energy level (shell) of an atom

*Exception: Helium is in Group 0 but has only 2 outer electrons

Why are groups important?

• The number of outer electrons determines how an element reacts

• All elements in a group have the same number of outer electrons

• This means that they have:

  • Similar chemical properties:

    • So, they react in a very similar way

    • For example:

      • All Group 1 metals are soft, reactive metals that react vigorously with water

      • All Group 7 elements are reactive, coloured non-metals

  • The same valency:

    • So, they form the same number of chemical bonds

    • For example, elements in Group 2 have a valency of 2

      • This is why the chemical formulae of their compounds are often similar, such as MgCl₂ and CaCl₂

Electron arrangement

• Electrons are not positioned randomly around the nucleus

  • They are found in specific energy levels called electron shells

• An electron arrangement is a numerical code that shows how many electrons are in each shell of an atom

The rules for filling shells

• For the first 20 elements, there are simple rules for working out the electron arrangement:

  1. Electrons always fill the shells closest to the nucleus first

  2. Each shell has a maximum number of electrons it can hold

A simplified model showing the electron shells

How to write an electron arrangement

• There are two ways to work out an electron arrangement, using the Periodic Table (Data Booklet page 4)

Method 1: Counting from the start

1. Find the element's atomic number from the Periodic Table

   • This gives the total number of electrons

2. Fill the shells with electrons according to the rules above

Method 2: Using the Periodic Table position

• This is a faster method that shows a deeper understanding of the Periodic Table's structure

1. Find the Group number

   • This gives the number of outer electrons

2. Find the Period (Row) number:

   • This gives the total number of shells the atom has

3. Fill the shells:

   • Place the outer electrons in the outermost shell

   • Then, then fill the inner shells from the nucleus outwards according to the rules above

How to draw an electron arrangement

• To draw an electron arrangement:

  • Show the shells as circles around the nucleus

  • Draw the electrons on these circles

Chlorine electron arrangement

• The electron arrangement of chlorine is 2,8,7

  • This can be deduced from the methods above or found on page 6 of the Data Booklet

• This means that it has 3 electron shells

  • The first shell has 2 electrons

  • The second shell has 8 electrons

  • The third (outer) shell has 7 electrons