Diagonal Relationship

Stop what you are doing and think about everything that you know about the Periodic Table. You have probably learned that atomic radius of elements decreases across a period and increases down a group, and also that ionization energy and Electronegativity tend to increase across a period and up a group. 

Diagonal Relationship Diagonal Relationship

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    You might also remember that elements in the same group have a vertical relationship and show similar physical and chemical characteristics. However, did you know that certain pairs of diagonally adjacent elements possess a diagonal relationship? Let's explore the world of diagonal relationship!

    • First, we will talk about the definition of diagonal relationship.
    • Then, we will look at the causes of diagonal relationship.
    • After, we will look at the different diagonal relationships in the Periodic Table.

    Diagonal Relationship Definition

    Before looking at diagonal relationships let's review the basics of the periodic table. The modern periodic table is composed of 118 elements arranged in order of increasing atomic number. For example, magnesium (Mg) has an atomic number of 12, and an atomic mass of 24.30 atomic mass units (amu). The rows in the periodic table are known as groups, whereas the columns are called periods.

    Atomic number is the number of protons found in the nucleus of an element.

    An element's atomic mass is the average mass of a single atom, measured in atomic mass unit (amu). It also represents the average mass of one mole of the atom.

    Elements within groups have similar chemical and Physical Properties. For example, the elements in Group 1 (except for hydrogen) are called Alkali Metals. These metals are characterized by having low melting points, low density, being shiny, soft, and also good conductors of heat and electricity!

    In terms of chemical properties, elements in Group 1 are very reactive (francium being the most reactive), and they tend to react with:

    • Nonmetals to form ionic compounds
    • Air to form metal oxides
    • Water to form metal hydroxides

    Need a refresher on the different trends related to the periodic table? Check out "Periodic Trends"!

    Now, let's look at the definition of diagonal relationship.

    The diagonal relationship is referred to as the similarity in chemical properties that sometimes occur between two immediately adjacent elements found in periods 2 and 3 of the periodic table.

    Causes of Diagonal Relationship

    There are two main causes of diagonal relationships: electropositivity and Electronegativity. Let's start with electropositivity. Since electropositivity is mainly exhibited by metals, it is also known as metallic character.

    Electropositivity describes the tendency of an atom to donate/lose electrons and form positively charged ions (cations).

    Alkali Metals are considered to be the most highly positive elements. The general trend on electropositivity is that electropositivity decreases from left to right in a period and increases down a group.

    Electronegativity is basically the opposite of electropositivity, and its general trend is that electronegativity increases across a period (going from left to right) and decreases going down a group.

    Electronegativity describes an element's tendency to accept electrons and form negatively charged ions (anion).

    Another cause of diagonal relationship is having similar ionic radii. For example, Mg2+ has an ionic radius of 0.72 A, similar to the ionic radius of Li+ (0.76 A).

    Diagonal Relationship in Periodic Table

    In the periodic table, diagonal relationships happen between lithium (Li) and magnesium (Mg), beryllium (Be) and aluminum (Al), and between boron (B) and silicon (Si).

    Diagonal Relationship between Boron and Silicon

    Let's start by looking at the diagonal relationship between boron (B) and silicon (Si).

    Boron is a metalloid that is not naturally found in the environment, but it is a part of mineral compounds. When combined with other elements, boron can be used in the armor of military tanks (boron carbide) and also in the manufacture of household cleaners (borax salt). Boron (B) has an atomic number of 5, and it is found on group 13. It has a melting point of 2075 °C, and a boiling point of 4000 °C.

    Silicon (Si) is also a metalloid. It has an atomic number of 14 and part of group 14. Silicon has a melting point of 1410 °C, and a boiling point of 2355 °C. It is the second-most abundant element in the Earth's crust, after oxygen. Silicon is greatly used in microchips to regulate the flow of electricity, and it is also used to make silicone, a household sealant!

    Boron and Silicon possess similar properties. For starters, they are both semiconductors of heat and electricity. They are also both metalloids with high melting, boiling points, and similar densities (2.33 g/cm3 for silicon and 2.34 g/cm3 for boron).

    Both elements form covalent compounds due to their high ionization energies and small sizes.

    Covalent compounds are compounds formed due to covalent Bonding. In covalent bonding, two atoms share a pair of electrons to acquire a full outer shell.

    Now, When allowed to react with oxygen, both boron and silicon form oxides that are strongly acidic in character.

    $$ \underset{\text{Boron}}{\text{4 B}}\text{ + }\underset{\text{Oxygen gas}}{\text{3 O}_{2}} \text{ } \longrightarrow \text{ }\underset{\text{Boron Oxide}}{\text{2 B}_{2}\text{O}_{3}} $$

    $$ \underset{\text{Silicon}}{\text{Si}}\text{ + }\underset{\text{Oxygen gas}}{\text{O}_{2}} \text{ } \longrightarrow \text{ }\underset{\text{Silicon dioxide}}{\text{Si}\text{O}_{2}} $$

    Moreover, if both boron and silicon oxides are dissolved in water, they form an oxyacid.

    Oxyacids are acids that possess H+, oxygen and a nonmetal.

    $$ \underset{\text{Boron Oxide}}{\text{B}_{2}\text{O}_{3}} \text{ + } \underset{\text{Water}}{\text{3 H}_{2}\text{O }} \longrightarrow \text{ } \underset{\text{Boric Acid}}{\text{2 B(OH)}_{3}} \text{ } $$

    $$ \underset{\text{Silicon dioxide }}{\text{ Si}\text{O}_{2}} \text{ + } \underset{\text{Water}}{\text{ H}_{2}\text{O }} \longrightarrow \text{ } \text{ + }\underset{\text{Metasilicic Acid}}{\text{ H}_{2}\text{O}_{3}\text{Si}} \text{ } $$

    They also react with aqueous bases to form borates and silicates. For example, if boron reacts with sodium hydroxide (NaOH), it forms sodium borate, whereas if silicon reacts with NaOH, it will form sodium silicate.

    $$ \underset{\text{Boron }}{\text{ 2 B}} \text{ + } \underset{\text{Sodium Hydroxide}}{\text{ 6 Na}\text{OH }} \longrightarrow \text{ } \text{ + }\underset{\text{Sodium Borate}}{\text{ 2 Na}_{3}\text{BO}_{3}} \text{ + } \underset{\text{ Hydrogen gas}}{\text{3 H}_{2}} $$

    $$ \underset{\text{Silicon }}{\text{ Si}} \text{ + } \underset{\text{Sodium Hydroxide}}{\text{ 4 Na}\text{OH }} \longrightarrow \text{ } \text{ + }\underset{\text{Sodium Silicate}}{\text{ Na}_{4}\text{SiO}_{4}} \text{ + } \underset{\text{ Hydrogen gas}}{\text{2 H}_{2}} $$

    But, what happens when boron and silicon react with metals? Well, they form borides and silicides! The Chemical Equations below show the reaction of boron and silicon with magnesium metal (Mg).

    $$ \underset{\text{Magnesium }}{\text{Mg}} \text{ + } \underset{\text{Boron}}{\text{2 B}\text{}} \longrightarrow \text{ } \text{}\underset{\text{Magnesium diboride}}{\text{ Mg}\text{B}_{2}} $$

    $$ \underset{\text{Magnesium }}{\text{2 Mg}} \text{ + } \underset{\text{Silicon}}{\text{Si}\text{}} \longrightarrow \text{ } \text{}\underset{\text{Magnesium silicide}}{\text{ Mg}_{2}\text{Si}} $$

    Diagonal Relationship between Beryllium and Aluminum

    Beryllium and Aluminum also have a diagonal relationship. Beryllium (Be) is an alkaline earth metal found in Group 2, period 2, whereas aluminum (Al) is a post-transition metal found in group 13, period 3. Beryllium (Be) is a very intriguing metal as it can only be generated by a supernova. Beryllium has a small density and atomic weight, and its strength and high melting point make it a great metal for making spacecraft!

    Aluminum (Al) is a post-transition metal that is soft and malleable, and a good conductor of electricity, having a wide range of uses from food containers all the way to electrical cables!

    Let's take a look at some similarities between Be and Al. First, they both have similar electronegativity values: beryllium has an EN value of 1.57 and aluminum has an EN value of 1.61.

    When allowed to react with acids such as nitric acid, they are both considered unreactive. However, they both react with a base (Ex. NaOH) in water to form hydrogen gas.

    $$ \underset{\text{Beryllium }}{\text{Be}} \text{ + } \underset{\text{Soidum hydroxide}}{\text{2 NaOH}\text{}} \longrightarrow \text{ } \text{}\underset{\text{Sodium beryllate}}{\text{ Na}_{2}\text{BeO}_{2}}\text{ +}\underset{\text{Hydrogen}}{\text{H}_{2}} $$

    $$ \underset{\text{Aluminum }}{\text{2 Al}} \text{ + } \underset{\text{Soidum hydroxide}}{\text{2 NaOH}}\text{+}\underset{\text{ Water}}{\text{2 H}_{2}{O}}\longrightarrow \text{ } \text{}\underset{\text{}}{\text{ 2 Na}\text{AlO}_{2}}\text{ +}\underset{\text{Hydrogen}}{\text{ 3 H}_{2}} $$

    The oxides and hydroxides of beryllium and magnesium are considered amphoteric, meaning that they can react both as acids or bases.

    Lastly, both the carbides of beryllium and aluminum give out methane on hydrolysis.

    $$ \text{Be}_{2}\text{C} (s) \text{ + 4 H}_{2}\text{O}(l) \longrightarrow 2 \text{ Be(OH)}_{2}(s) \text{ + CH}_{4}(g) $$

    $$ \text{2 Al}_{4}\text{C}_{3} (s) \text{ + 12 H}_{2}\text{O}(l) \longrightarrow 4 \text{ Al(OH)}_{3}(s) \text{ + 3 CH}_{4}(g) $$

    Diagonal Relationship between Lithium and Magnesium

    To finish off, let's talk about the diagonal relationship between lithium and magnesium. Lithium (Li) is a group 1 (alkali metal), period 2 element considered the lightest metal in the periodic table. Magnesium (Mg), on the other hand, is a Group 2, period 3 alkaline earth metal.

    Both lithium and magnesium form normal oxides when they burn in oxygen (O2).

    $$ \text{4 Li} (s) \text{ + O}_{2}(g) \longrightarrow 2 \text{ Li}_{2}\text{O}(s) $$

    $$ \text{2 Mg} (s) \text{ + O}_{2}(g) \longrightarrow 2 \text{ Mg}\text{O}(s) $$

    Also, they both combine with Nitrogen to form nitrides.

    $$ \text{3 Mg} (s) \text{ + N}_{2}(g) \longrightarrow \text{(Mg)}^{2+}_{3}\text{(N)}^{3-}_{2} (s) $$

    $$ \text{6 Li} (s) \text{ + N}_{2}(g) \longrightarrow \text{2 Li}_{3}\text{N} (s) $$

    When it comes to their carbonates, both lithium and magnesium carbonates decompose to oxides on heating.

    $$ \text{Li}_{2}\text{CO}_{3 } (s) \longrightarrow \text{ Li}_{2}\text{O}(s)\text{ + CO}_{2} (g) $$

    $$ \text{Mg}\text{CO}_{3 } (s) \longrightarrow \text{Mg}\text{O}(s)\text{ + CO}_{2} (g) $$

    Now, I hope that you were to understand diagonal relationship a bit more!

    Diagonal Relationship - Key takeaways

    • The diagonal relationship is referred to as the similarities in chemical properties that sometimes occur between two immediately adjacent elements found on periods 2 and 3 of the periodic table.
    • The three causes of diagonal relationship are electropositivity, electronegativity, and ionic radius.
    • In the periodic table, diagonal relationship happens between lithium (Li) and magnesium (Mg), beryllium (Be) and aluminum (Al), and also between boron (B) and silicon (Si).


    1. A Simple Note on Similarities Between Beryllium And Aluminium. (n.d.). Unacademy. Retrieved November 2, 2022, from https://unacademy.com/content/upsc/study-material/chemistry/a-simple-note-on-similarities-between-beryllium-and-aluminium/
    2. Mg, N., & Si, A. (n.d.). Diagonal relationship. Retrieved November 2, 2022, from https://digitalteachers.co.ug/wp-content/uploads/2020/08/Diagonal-relationship.pdf
    3. Zumdahl, S. S., Zumdahl, S. A., & Decoste, D. J. (2019). Chemistry. Cengage Learning Asia Pte Ltd.
    Frequently Asked Questions about Diagonal Relationship

    What is a diagonal relationship? 

    The diagonal relationship is referred to as the similarities in chemical properties that sometimes occur between two immediately adjacent elements found on periods 2 and 3 of the periodic table. 

    How many diagonal relationships are in the periodic table? 

    There are three diagonal relationships in the periodic table. 

    Does boron and silicon have a diagonal relationship? 

    Yes, B and Si have a diagonal relationship. 

    Why diagonal relationship takes place between beryllium and aluminium? 

    Some relationships between Be and Al involve similar electronegativities, unreactivivity in acids, and they both react with an aqueous base to form hydrogen gas. 

    What is the diagonal relationship between lithium and magnesium? 

    The relationships between lithium and magnesium involve the formation of normal oxides when reacting with oxygen gas, and forming nitrites when combined with nitrogen. 

    Test your knowledge with multiple choice flashcards

    Which of the following elements have a diagonal relationship with Beryllium (Be)? 

    Which of the following elements have a diagonal relationship with Silicon (Si)? 

    Which of the following elements have a diagonal relationship with lithium (Li)? 

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