In terms of their electronic configuration and chemical behaviour, alkali metals are characterised by which of the following properties?

Introduction / Context:
Alkali metals, which belong to Group 1 of the periodic table (excluding hydrogen in some contexts), are known for their high reactivity and characteristic chemical behaviour. These metals include lithium, sodium, potassium, and others. Their reactivity is closely related to the arrangement of electrons in their outermost shell. This question asks you to identify the key electronic property that explains why alkali metals form positive ions so readily.

Given Data / Assumptions:

• Alkali metals are Group 1 elements such as Li, Na, K, Rb, Cs, and Fr.
• They have one valence electron in their outermost shell.
• The options consider whether they gain or lose electrons or show unusual stability at room temperature.
• We assume standard conditions and typical chemical behaviour of alkali metals.

Concept / Approach:
Alkali metals have an outer electronic configuration of ns1, meaning there is one electron in the valence shell. To achieve a stable noble gas configuration, they tend to lose this single electron rather than gain additional electrons. Losing one electron is energetically easier than gaining seven. As a result, alkali metals readily form +1 cations (Li+, Na+, K+, etc.) in chemical reactions. They are reactive solids at room temperature, not vapours, and they are not characterised by gaining electrons, which is more typical of non metals such as halogens.

Step-by-Step Solution:
Step 1: Recall the valence electron configuration of alkali metals. Each alkali metal has one electron in its outermost shell (ns1). Step 2: Consider how they achieve a stable configuration. By losing this one electron, they obtain the stable configuration of the preceding noble gas. Step 3: Match this behaviour with the options. The property that describes this is "easily lose electrons". Step 4: Confirm that alkali metals do not usually gain electrons or vaporise at room temperature.

Verification / Alternative check:
In typical reactions such as Na + Cl2 → 2NaCl, sodium (an alkali metal) loses an electron to form Na+ and chlorine gains an electron to form Cl-. Electrochemical series tables also show that alkali metals have low ionisation energies and are strong reducing agents, meaning they readily lose electrons. They are solid metals at room temperature and only melt or vaporise at higher temperatures. These consistent observations across many reactions and properties confirm that their defining behaviour is easy loss of electrons to form positive ions.

Why Other Options Are Wrong:
Option A: Alkali metals are reactive, but they are not described as "highly unstable" at room temperature; they can be stored and used, often under oil to prevent reaction with air. Option B: Alkali metals do not vaporise at room temperature; they are soft solids and require heating to melt or vaporise. Option C: Easily gaining electrons is characteristic of non metals like halogens, not of alkali metals.

Common Pitfalls:
Students sometimes misinterpret the term "reactive" and think it means a substance must be unstable or gaseous at room temperature. Another confusion is mixing up electron gain and loss, especially when studying redox reactions. A simple way to remember is that metals, especially alkali metals, tend to lose electrons and form cations, while non metals tend to gain electrons and form anions. This rule holds well across many basic inorganic chemistry questions.

Final Answer:
Alkali metals are characterised by the fact that they easily lose electrons to form +1 cations.