Trends in the Periodic Table (WJEC GCSE Chemistry: Combined Science)

• Elements in the same group have similar chemical and physical properties 
• We can observe these for Group 1 and Group 7 elements

Group 1 elements

• The Group 1 metals are located in the first column of the Periodic Table and are known as the alkali metals
  • They form alkaline solutions when they react with water
• Group 1 metals all share the following properties: 
  • They are all soft metals which can easily be cut with a knife
  • They have relatively low densities and low melting points
  • They are very reactive (they only need to lose one electron to become highly stable)
• The alkali metals share similar chemical properties because they each have one electron in their outermost shell

The Periodic Table showing the location of Group 1 

The alkali metals lie on the far left of the Periodic Table, in the very first group 

Trends in Group 1 properties

• The alkali metals are soft and easy to cut, getting softer as you move down the group
  • Potassium is the exception; it has a lower density than sodium
• The first three alkali metals are less dense than water
• They all have relatively low melting points
  • These decrease as you move down the group, due to decreasing attractive forces between outer electrons and positive ions

Diagram to show the trend in melting points going down Group 1 

The melting point of the Group 1 metals decreases as you descend the group

Group 7 elements

• The elements in Group 7 are known as the halogens
• These are fluorine, chlorine, bromine, iodine and astatine
• These elements are non-metals that are poisonous
• Halogens are diatomic, meaning they form molecules made of pairs of atoms sharing electrons (forming a single covalent bond between the two halogen atoms) such as F₂, Cl₂, etc

Trends in Group 7 properties 

• At room temperature (20 °C), the physical state of the halogens changes as you go down the group
  • Chlorine is a pale yellow-green gas, bromine is a red-brown liquid and iodine is a grey-black solid
  • This demonstrates that the density of the halogens increases as you go down the group
• The colours of the halogens also change as you descend the group - they become darker

Diagram to show the change in state, density and colour of the halogens

The halogens get denser and change colour moving down the group

• The melting and boiling points of the halogens increase as you go down the group
• This is due to increasing intermolecular forces as the atoms become larger, so more energy is required to overcome these forces

Diagram to show the trend in melting/boiling points going down Group 7

Melting and boiling points increase going down Group 7 

• Elements react in order to obtain a full outer shell of electrons and become chemically stable 
  • To do this, they either lose or gain electrons
• Noble gases are chemically unreactive due to already having a full outer shell of electrons
• Therefore the reactions of Group 1 and Group 7 elements involves the loss or gain of electrons
• Group 1 atoms lose one electron from their outer shell to form positively charged ions 
  
  • `The ions formed will have a 1+ charge 

Diagram to show how sodium loses an electron to form an ion

Group 1 elements form ions with a 1+ charge 

• Group 7 atoms gain an electron to form negatively charged ions
  • The ions formed will have a 1- charge

Diagram to show how chlorine loses an electron to form an ion

Group 7 elements form ions with a 1- charge 

Higher Tier

• The reactivity of an element is determined by its readiness to lose or gain electrons and form ions 
• The more readily an atom loses or gains an electron, the more reactive it is 
• This explains the trend in reactivity down Groups 1 and 7

Why does reactivity increase going down Group 1?

• When a Group 1 element reacts, its atoms only need to lose one electron, as there is only one electron in the outer shell
  • When this happens, 1+ ions are formed
• The next shell down automatically becomes the outermost shell and since it is already full, a Group 1 ion obtains noble gas configuration
• As you go down Group 1 reactivity increases because:
  • The number of electron shells increases 
  • This means that the outermost electron gets further away from the nucleus, so there are weaker forces of attraction between the outermost electron and the nucleus
  • Less energy is required to overcome the force of attraction as it gets weaker
  • The outer electron is lost more easily 

Diagram to show the electronic structure of Group 1 elements

Going down Group 1, the outer electron is further away from the nucleus

Why does reactivity decrease going down Group 7?

• When a Group 7 element reacts its atoms need to gain one electron as they have seven electrons in their outer shell 
  • When this happens, ions with a 1- charge are formed 
• As you go down Group 7 reactivity decreases because:
  • The number of electron shells increases 
  • The distance between the electron being gained and the nucleus increases so the attraction between the electron being gained and the nucleus decreases
  • It is harder for the atom to gain an electron 

Diagram to show the electronic structure of Group 1 elements

Going down Group 7, the electron being gained is less attracted to the positive nucleus