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Group 1 – The alkali metals

How did Mendeleev know that there must be undiscovered elements and how did he take this into account when he designed his periodic table? (2 marks)

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Group 1 – The alkali metals

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  1. How did Mendeleev know that there must be undiscovered elements and how did he take this into account when he designed his periodic table? (2 marks) 2) By the early 20th century protons and electrons had been discovered .Describe how this discovery allowed chemists to place elements in their correct order and correct group. (3 marks)

  2. How did Mendeleev know that there must be undiscovered elements and how did he take this into account when he designed his periodic table? (2 marks) • left gaps (1) • if placed consecutively, then elements would be in wrong group / have wrong properties (1)

  3. 2) By the early 20th century protons and electrons had been discovered .Describe how this discovery allowed chemists to place elements in their correct order and correct group. (3 marks) • (elements placed in) atomic number order (1) • (elements in ) same group have same number of outer electrons. (1) • reactions (chemical) properties depend on the (outer) electrons (1)

  4. Group 1 – The alkali metals • L.O.: • Describe the reaction of group 1 element with water . • Predict the properties of group 1 elements according to their position in the group.

  5. Group I and Group VII elements

  6. Fr Cs Rb K Na Li Where are the alkali metals? The elements in group 1, on the left of the periodic table, are called the alkali metals. lithium sodium potassium rubidium caesium francium These metals are all very reactive and are rarely found in nature in their elemental form.

  7. Write down the balanced equation of sodium with water. 2 Na (s) + 2H2O (l) → 2NaOH (aq)+ H2 (g) 2. What can you say about the density of Na 3. Which metal is more reactive Na or K?

  8. Braniac clip

  9. Electron structure All alkali metals have 1 electron in their outer shell. This means that: lithium 2,1 • They can easily obtain a full outer shell by losing 1 electron. sodium 2,8,1 • They all lose their outer shell electron in reactions to form positive ions with a +1 charge. • They have similar physical and chemical properties. potassium 2,8,8,1

  10. Explain the trends in reactivity as we go down a group in terms of the number of energy levels in the atoms. As we go down a group the number of occupied energy levels increases, and atoms get bigger. The larger atoms lose electrons more easily. The outer electrons are further away from the nucleus. They are also more shielded by inner electrons.

  11. Why are they called the ‘alkali metals’? The alkali metals are so reactive that, as elements, they have to be stored in oil. This stops them reacting with oxygen in the air. Alkali metals are soft enough to be cut with a knife, and the most common alkali metals, lithium, sodium and potassium, all float on water. The elements in group 1 also react with water and form alkaline compounds. This is why they are called alkali metals.

  12. What are the properties of the alkali metals? The characteristic properties of the alkali metals are: • They are soft and can be cut by a knife. Softness increases going down the group. • They have a low density. Lithium, sodium and potassium float on water. • They have low melting and boiling points. These properties mean that the alkali metals are different to typical metals. However, alkali metals do also share some properties with typical metals: • They are good conductors of heat and electricity. • They are shiny. This is only seen when alkali metals are freshly cut.

  13. What are the trends in melting point?

  14. What are the trends in boiling point?

  15. What is the trend in melting and boiling points? The melting points and boiling points of alkali metals decrease going down the group. Boiling point (°C) Element Melting point (°C) 1342 lithium 181 883 sodium 98 760 potassium 64 686 rubidium 39 caesium 28 671 The melting and boiling points decrease going down group 1 because the atoms get larger. Melting points are lower than for typical, transition, metals, because alkali metals only have 1 electron in their outer shell. Not much heat energy is needed for this electron to be lost.

  16. Li Na K increase in reactivity Rb Cs Electron structure and reactivity The reactivity of alkali metals increases down the group.What is the reason for this? • The size of the element’s atoms, and the number of full electron shells, increases down the group. • This means that, down the group, the electron in the outer shell gets further away from the nucleus and is shielded by more electron shells. • The further away an electron is from the positive attraction of the nucleus, the easier it can be lost in reactions. • This means that reactivity increases with the size of the atom.

  17. Task 1) Workbook page 107 2) Cs worksheet

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