Predicting and identifying reactions and products Study Guide

The chemical properties of elements are largely determined by the number of electrons in their outermost electron shell. Elements in the same group of the Periodic Table have the same number of outer shell electrons, which explains why they exhibit similar chemical behaviours. Group 1 elements, known as alkali metals, all have one electron in their outer shell. This single electron is easily lost to form a positive ion with a +1 charge. As you move down Group 1, the atomic radius increases because there are more electron shells. The attraction between the positively charged nucleus and the outermost electron decreases due to both increased distance and increased shielding from inner electron shells. This makes it even easier to lose the outer electron, resulting in increased reactivity down the group. Therefore, melting points and boiling points decrease down Group 1 as the metallic bonding forces weaken due to increased atomic size and less effective nuclear attraction for the delocalised electrons.

Group 7 elements, the halogens, all have seven electrons in their outer shell. They tend to gain one electron to achieve a full outer shell, forming a negative ion with a -1 charge, or share electrons covalently. As you move down Group 7, the atomic radius increases. The incoming electron would be further from the nucleus and experience greater shielding from inner electron shells. This reduces the nuclear attraction for an added electron, making it less likely for larger halogen atoms to gain an electron. Consequently, reactivity decreases down Group 7. Their melting points and boiling points increase down the group because the size of the halogen molecules (which exist as diatomic molecules, e.g., F2, Cl2) increases, leading to stronger intermolecular forces that require more energy to overcome.

Group 0 elements, the noble gases, have a full outer shell of electrons (2 for helium, 8 for others). This stable electron configuration makes them very unreactive as they have no tendency to lose, gain, or share electrons. Due to their full outer shells, they are monatomic and exist as single atoms. They have extremely low melting and boiling points, and these points increase slightly down the group due to the increasing number of electrons leading to stronger, albeit still weak, temporary intermolecular forces (London dispersion forces).

The periodic table organises elements based on their atomic number and recurring chemical properties. This organisation places elements with similar properties into vertical columns called Groups. Understanding the simple properties of elements in Group 1 (Alkali Metals), Group 7 (Halogens), and Group 0 (Noble Gases) is fundamental to predicting their behaviour and reactivity.

Group 1 elements are highly reactive metals. They have one electron in their outermost shell, which they readily lose to form a positive ion with a +1 charge. This tendency to lose electrons makes them strong reducing agents. Their reactivity generally increases as you go down the group because the outer electron is further from the nucleus and shielded by more electron shells, making it easier to remove. They are soft metals with low melting and boiling points, which decrease down the group. They react vigorously with water to produce hydrogen gas and an alkaline solution (a metal hydroxide).

Group 7 elements are known as the halogens. They are reactive non-metals with seven electrons in their outermost shell. They tend to gain one electron to form a negative ion with a -1 charge, making them strong oxidising agents. Their reactivity decreases down the group because the incoming electron is further from the nucleus and experiences more shielding, making it harder to attract. At room temperature, fluorine and chlorine are gases, bromine is a liquid, and iodine is a solid. Their melting and boiling points increase down the group.

Group 0 elements are the noble gases. They are unreactive non-metals with a full outer electron shell (except helium, which has two electrons, filling its first shell). This full outer shell makes them very stable and reluctant to gain, lose, or share electrons. Due to their lack of reactivity, they are monatomic (exist as single atoms rather than molecules) and have very low melting and boiling points, which increase down the group. They are colourless gases at room temperature.