Question

Atomic numbers of few elements are given below. Which of the pairs belomgs to s-block? (a) 7,14 (b) 3,20 (c) 8,15 (d) 9,17

Short answer

Pair (b) 3,20 belongs to the s-block of the periodic table.

Step-by-step solution

Understanding S-block Elements

Elements categorized under the s-block in the periodic table have their outermost electrons in the s subshell. This includes Group 1 (alkali metals) and Group 2 (alkaline earth metals), as well as hydrogen and helium. An element with its last electron entering the s subshell belongs to the s-block.

Identifying Atomic Numbers in S-block

Elements in the s-block have atomic numbers 1, 2 (for helium) and then 3 to 4 (lithium and beryllium), continuing in a similar pattern at the start of each period, specifically elements with atomic numbers 11, 12 (sodium and magnesium), 19, 20 (potassium and calcium), and so on. The pattern is based on the way electrons fill the available atomic orbitals.

Analyzing the Given Pairs

We will check each pair to see if both atomic numbers correspond to s-block elements. (a) 7,14: 7 (Nitrogen) and 14 (Silicon) do not belong to s-block as they are p-block elements.(b) 3,20: 3 (Lithium) and 20 (Calcium) are both s-block elements.(c) 8,15: 8 (Oxygen) and 15 (Phosphorus) are p-block elements.(d) 9,17: 9 (Fluorine) and 17 (Chlorine) are also p-block elements. Thus, the only pair where both elements are in the s-block is pair (b) 3,20.

Key concepts

Periodic Table

The periodic table is a systematic arrangement of the chemical elements, organized on the basis of their atomic numbers, electron configurations, and recurring chemical properties. Elements are arranged in order of increasing atomic number in rows called periods and columns known as groups.

Within the periodic table, the s-block elements are located on the leftmost side and consist of Group 1 and Group 2 elements. These blocks are named after the characteristic orbital where the outermost electrons reside, in this case, the s orbital. The progressive filling of the s orbital corresponds to the recurrent pattern seen across the periods, which reflects the periodic nature of the element's properties. Such a structured layout aids in predicting a range of elemental properties and their chemical behavior.

Atomic Number

The atomic number of an element is a fundamental property that indicates the number of protons found in the nucleus of an atom. This number, denoted by the letter 'Z', uniquely identifies a chemical element. The atomic number also determines the position of an element in the periodic table.

In addition, the atomic number relates directly to the element's electronic structure since the number of protons equals the number of electrons in a neutral atom. Hence, the atomic number is critical in determining how an element reacts chemically. For instance, in the context of s-block elements, those with atomic numbers 1, 2, and then 3 to 12 belong to this category, reflecting their outermost electrons being in an s orbital.

Alkali Metals

Alkali metals comprise the first group of the s-block elements and include lithium (Li), sodium (Na), potassium (K), rubidium (Rb), cesium (Cs), and francium (Fr). These elements have a single electron in their outermost s-orbital, which makes them highly reactive, particularly with water.

Alkali metals are characterized by their softness, low densities, and low melting points relative to most other metals. Due to their strong tendency to donate this lone electron, they are extraordinarily reactive and thus are not found in nature as free elements, but rather as compounds. Their reactivity increases down the group, cesium being the most reactive of them.

Alkaline Earth Metals

Following the alkali metals in the s-block are the alkaline earth metals, which make up Group 2 of the periodic table. This group includes beryllium (Be), magnesium (Mg), calcium (Ca), strontium (Sr), barium (Ba), and radium (Ra). Unlike alkali metals, these elements have two electrons in their outermost s-orbital.

The presence of two valence electrons contributes to their higher melting points and densities compared to alkali metals. While still reactive, particularly with halogens to form ionic bonds, alkaline earth metals are less reactive than alkali metals. They are commonly found in the earth's crust in various mineral compounds.