The Modern Periodic Table

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The modern periodic table

 

Image: Pixabay

 

• Elements are arranged in order of atomic number.
• The columns show the groups.
• The group number shows the number of electrons in the outer shell.
• The rows are called periods – the period shows how many shells of electrons there are e.g. Hydrogen and Helium are in period 1 and only have electrons in the first shell.
• The periodic table can be used to predict reactivity.
• Group 13 is generally known as group 3, group 14 is known as group 4 and so on.
• The middle section between group 2 and 3 is known as the transition metals.

The development of the periodic table

Some elements have been known of since ancient times. Scientists were always looking for patterns between the known elements but there were many elements that had not been found and some elements were put into the wrong groups.

• In 1829 Dobereiner had noticed that some groups of three elements had similar properties. These groups were:
  • Lithium, Sodium and potassium,
  • Calcium, strontium and barium
  • Chlorine, Bromine and iodine

The groups were named the Dobereiner triads.

• In 1860 a list of accurate atomic weights was created.
• In 1865 John Newlands observed that if the elements were put in order of atomic weight then there was a pattern of similar properties every 8 elements. This was called the “law of octaves’.
• In 1869 Dimitri Mendeleev left gaps in the periodic table (arranged by atomic mass). He believed there was some undiscovered elements. He used patterns of chemical behaviour to decide where the gaps were.

• In 1932 the discovery of the neutron allowed scientists find evidence of isotopes. The periodic table was then arranged by the number of protons.

 

Metals and Non-metals

Metals are found on the left-hand side of the periodic table and non-metals are found on the right. The line of elements separating the metals from the non-metals are called the metalloids. (Boron, Silicon, Germanium, Antimony).

Metals

Metals have a tendency to lose electrons from the outer shell to form a positive ion. This is because they only have a ‘few’ outer electrons.

Properties of metals

• Strong
• Malleable
• Ductile
• Good conductors of electricity
• Good conductors of heat

 

The transition metals

• The transition metals have all the usual properties of metals.
• They make very good catalysts e.g. Iron is used as a catalyst during the Haber process when making ammonia.
• Compounds containing transition metal ions are often coloured.
• Transition metals can lose different numbers of electrons. E.g. copper can take the form of Cu⁺ and Cu²⁺. Iron can be as Fe²⁺ and Fe³⁺.

 

Non-metals

Non-metals have spaces in their outer shell in which they can gain (become a negative ion) or share electrons with other atoms.

Properties of non-metals

• Dull
• Brittle
• Some can be liquids and gases at room temperature

 

Group 0

• The noble gases are helium, neon, argon, kypton and xenon
• They are all unreactive as they have full outer shells.
• They all exist as single atoms
• They are colourless gases at room temperature
• The boiling points all increase as they go down the group – they have greater intermolecular forces between the atoms because of the increase in the number of electrons

Group 7

• The halogens are fluorine, chlorine, bromine and iodine.
• They are non-metals.
• They exist as pairs of atoms (diatomic) g. Cl₂
• They become less reactive as you go down the group as it is harder to gain an extra electron due to the outer shell being further away from the nucleus.
• The melting and boiling points become higher as you go down the group.
• They react vigorously with metals such as magnesium, sodium and potassium.

Group 7 trends

•  Fluorine and Chlorine are gases, bromine is a liquid and iodine is a solid at room temperature
• The reactivity decreases as you go down the group.
• If a more reactive halogen is bubbled through a solution of a metal halide containing a less reactive halogen then a displacement reaction will take place.

 

e.g. chlorine + potassium bromide   ——–>   bromine + potassium chloride

 

Group 1

• The alkali metals are lithium, sodium, potassium, rubidium, caesium and francium.
• The metals are all soft.
• They all have one electron in their outer shell, making them very reactive.
• They are low density.
• They form ionic compounds with non-metals.

 

Group 1 trends

• The reactivity increases as you go down the group as the atoms get larger and it is easier to lose an electron that is further away from the nucleus.
• They react with oxygen to form metal oxides.
• They react with water to produce hydrogen and an alkali.

e.g.  lithium + water   ——>   lithium hydroxide + hydrogen

2Na + 2H₂O    ——>   2NaOH + H₂

• They react with chlorine to produce a metal salt.

e.g. lithium + chlorine   ——>   lithium chloride

2Li + Cl_{2   ——>}   2LiCl

 

 

Reaction Trends

1.Reactivity increases as you move down group 1

• The outer electron is further away from the nucleus in a potassium atom compared to a sodium atom so there is less attraction between the positive nucleus and the negative electron. The electron is lost more easily and therefore potassium is more reactive.

Source: Yenka.com

 

2. Reactivity decreases as you move down group 7

• The larger the atom the less the “pull” from the nucleus on the electron that is trying to transfer into the atom. So bromine is less reactive than fluorine.

 

Questions

1. Explain why an atom is neutral overall. Use the idea of sub-atomic particles in your explanation. (2)

 

2. Why are fluorine and chlorine in the same group of the periodic table. (2)

 

3. In the early version of the periodic table how did Newlands and Mendeleev arrange the elements? (1)

 

4. Which particle allowed the elements to be arranged in order of their atomic number in the modern periodic table. (1)

 

5. Which one of the elements shown in the diagram below has the lowest boiling point? (1)

 

6.Explain why the reactivity of the elements increases going down Group 1 from lithium to rubidium (3).

 

Answers

 

1. The relative electrical charges are −(1) for an electron and +(1) for a proton. The number of electrons is equal to the number of protons.

 

2. The electronic structure of fluorine is 2,7. The electronic structure of chlorine is 2,8,7. Fluorine and chlorine are in the same group as they have 7 electrons in their highest energy level or outer shell.

 

3. Atomic weight/mass

 

4. Proton

 

5. Fluorine

 

6. As you move down the group there are more energy levels and the atom is larger. There is less attraction between the nucleus and the electron (more shielding of the nucleus) and so the outer electron is lost more easily.