8 (a) An atom of potassium has atomic number 19 and mass number 39 - Edexcel - GCSE Chemistry - Question 8 - 2020 - Paper 1

The ion K⁺ is formed by the loss of one electron from the neutral potassium atom. Therefore, it has:

• Number of protons: 19 (stable for potassium)
• Number of neutrons: 20 (calculated as mass number 39 - number of protons 19)

Thus, the correct row is: B: 19 protons and 20 neutrons.

Potassium and caesium belong to the same group (Group 1) of the periodic table because they both have the same number of electrons in their outermost shell. Potassium has one electron in its outer shell (3s¹), while caesium has one electron in its outer shell (6s¹). This similar electron configuration in the outer shell results in similar chemical properties, including their tendency to lose that outer electron easily in reactions.

The low boiling point of fluorine can be attributed to the weak intermolecular forces present between fluorine molecules. Fluorine molecules (F₂) are held together primarily by van der Waals forces, which are relatively weak compared to covalent bonds. As a result, only a small amount of energy is required to overcome these forces, leading to a low boiling point of -188 °C.

The balanced equation for the reaction between potassium and fluorine to form potassium fluoride is:

$2K (s) + F_2 (g) \rightarrow 2KF (s)$

Here, potassium is a solid (s), and fluorine is a gas (g), while potassium fluoride is formed as a solid (s).

The elements in group 1 of the periodic table are called alkali metals.

From Figure 14, the boiling point of bromine is approximately 59 °C.

The two elements from Figure 14 that are solids at room temperature are iodine and astatine.