The Periodic TableAQA GCSE Chemistry: Revision notes

Newlands' Law of Octaves (1865) was one of the first attempts to organise elements. He arranged elements in order of atomic weight and noticed that every eighth element had similar properties, like the musical notes in an octave. However, his table was limited and didn't allow for undiscovered elements.

Mendeleev's approach (1869) also arranged elements by atomic weight, but he made a crucial innovation: he left gaps in his table for elements that hadn't been discovered yet. He also reordered some elements where their chemical properties didn't match their atomic weight order, predicting that atomic weight measurements must be wrong for those elements. He even predicted the properties of undiscovered elements, which were later found to match closely.

Mendeleev's table was more successful because it was flexible enough to accommodate new discoveries and prioritised chemical properties over strictly following atomic weight order.

The modern periodic table is arranged in order of increasing atomic (proton) number, not atomic weight. This was a key discovery that explained why Mendeleev had to reorder some elements.

Atomic number is the number of protons in the nucleus of an atom and is now the definitive way to identify and order elements. This ordering works perfectly because:

• It reflects the electronic structure of atoms
• It explains why elements in the same group have similar chemical properties
• It correctly places all elements without any inconsistencies

This discovery came from the work of Moseley and others studying X-rays, which revealed the true identity of elements. By using atomic number instead of atomic weight, all apparent inconsistencies in Mendeleev's table disappeared, and the periodic table became a powerful tool for predicting element behaviour.

The periodic table is organised into periods (rows) and groups (columns):

Periods (horizontal rows):

• There are 7 periods in the periodic table
• Each period represents a complete set of electron shells
• As you move left to right across a period, the atomic number increases by 1
• Each new element has one more proton and one more electron than the previous one

Groups (vertical columns):

• There are 8 main groups (Groups 1, 2, and 13–18)
• Elements in the same group have the same number of outer (valence) electrons
• This is why elements in the same group have similar chemical properties
• For example, all elements in Group 1 (alkali metals) have 1 outer electron and react in similar ways

The position of an element tells you:

• Group number = number of outer electrons (for main group elements)
• Period number = number of electron shells

For example, sodium (Na) is in Group 1, Period 3: it has 1 outer electron and 3 electron shells.

The periodic table is divided into metals and non-metals based on their positions:

Metals (majority of elements):

• Located on the left side and centre of the periodic table
• Include all of Groups 1, 2, and the transition metals (between Groups 2 and 13)
• Generally found to the left of a diagonal line running from boron to astatine
• Have properties such as: good conductors of electricity and heat, malleable, ductile, shiny

Non-metals:

• Located on the right side of the periodic table (upper right)
• Found in Groups 13–18, with the majority in Groups 14–18
• Include gases like oxygen and nitrogen, and solids like carbon and sulfur
• Have properties such as: poor conductors, brittle (if solid), dull appearance

Semi-metals (Metalloids):

• Found along the diagonal boundary between metals and non-metals
• Include elements like silicon, arsenic, and germanium
• Have properties intermediate between metals and non-metals

The periodic table layout reflects the trend in atomic structure: as you move from left to right, atoms gain electrons, and chemical properties change from metallic to non-metallic. This positioning makes the periodic table an excellent tool for predicting element behaviour.

Elements in the same group have the same number of outer electrons (valence electrons). This is the key reason they have similar chemical properties.

Chemical properties depend on outer electrons because:

• Outer electrons are involved in all chemical bonding and reactions
• The number of outer electrons determines how an atom will react
• Atoms in the same group need to gain, lose, or share the same number of electrons to achieve a stable electron configuration

Example trends in groups:

The number of electron shells increases as you go down a group (so atoms get larger), but the number of outer electrons stays the same. This means elements down a group show a gradual change in properties while maintaining the same general pattern of reactivity.