10 Jul 2024

Covalent Radius Definition and Trend



Covalent radius (see figure below), which is defined as one-half the distance between the nuclei of two identical atoms when they are joined by a covalent bond (this measurement is possible because atoms within molecules still retain much of their atomic identity).



The covalent radius is half the distance between two atoms that share a covalent bond. Usually, you see covalent radius in units of picometers (pm) or angstroms (Å), where 1 Å = 100 pm. For example the average covalent radius for hydrogen is 31 pm and the average neon covalent radius is 58 pm.

Why Are There Different Numbers?

When you look at a table of covalent radius values, its numbers may differ from those found on another table. This is because there are different ways of reporting covalent radius.

In reality, the covalent radius depends on an atom’s hybridization, the nature of the two atoms sharing a covalent bond, and on the chemical environment surrounding the atoms. For example, the covalent radius of carbon is 76 pm for the sp3, 73 pm for the sp2 hybridization, and 69 pm for the sp hybridization.

Also, covalent radius depends on whether the atom forms a single bond, double bond, or triple bond. In general, a single bond is longer than a double bond, which is longer than a triple bond. **

A given table might generalize data or else offer values based on very specific conditions. Tables that cite an average value usually combine data for covalent bonds an atom forms in many different compounds. Some tables list the covalent radius for a homonuclear covalent bond. For example, this is the covalent radius for H2 or O2. Either use the idealized (calculated) or empirical average covalent radius for an atom for maximum transferability.

How Covalent Radius Is Measured

The most common methods of measuring covalent radius are x-ray diffraction and rotational spectroscopy. Neutron diffraction of molecular crystals is another method.

Covalent Radius Trend on the Periodic Table

Covalent radius displays a periodic table trend.

  • Moving left to right across a period, covalent radius decreases.
  • Moving top to bottom down a group, covalent radius increases.

Covalent radius decreases moving from left to right across a row or period because atoms gain more protons in their nucleus and electrons in their outer shells. Adding more protons increases the attractive pull on these electrons, drawing them in more tightly.

Covalent radius increases moving down a column or periodic table group. This is because increasing filled inner electron energy levels shield the outer electrons from the positive nuclear charge. So, the electrons are less attracted to the nucleus and increase their distance to it.

Covalent Radius vs Atomic Radius and Ionic Radius

Covalent radius, atomic radius, and ionic radius are three ways of measuring the sizes of atoms and their sphere of influence. The atomic radius is half the distance between the nuclei of atoms that are just touching each other, where “touching” means their outer electrons shells are in contact. 

The ionic radius is half the distance between two atoms touching each other that share an ionic bond in a crystal lattice.

All three measures of atomic size follow a periodic table trend, where radius generally increases in size moving down an element group and decreases in size moving from left to right across a period. However, the covalent radius and ionic radius often are different sizes from the atomic radius.

The Largest and Small Covalent Radius

The element with the smallest covalent radius is hydrogen (32 pm). The atom with the largest covalent radius is francium (223 pm when it forms a single bond). Basically, this is another way of saying hydrogen is the smallest atom and francium is the largest atom.




 

Group-wise Covalent Radii

Group 1: Alkali Metals

  • Atomic Number 1: H - 37 pm (in H₂)
  • Atomic Number 3: Li - 140 pm
  • Atomic Number 11: Na - 166 pm
  • Atomic Number 19: K - 227 pm
  • Atomic Number 37: Rb - 248 pm
  • Atomic Number 55: Cs - 262 pm

Group 2: Alkaline Earth Metals

  • Atomic Number 4: Be - 112 pm
  • Atomic Number 12: Mg - 160 pm
  • Atomic Number 20: Ca - 197 pm
  • Atomic Number 38: Sr - 215 pm
  • Atomic Number 56: Ba - 222 pm
  • Atomic Number 88: Ra - 247 pm (estimated)

Group 13: Boron Group

  • Atomic Number 5: B - 88 pm
  • Atomic Number 13: Al - 143 pm
  • Atomic Number 31: Ga - 135 pm
  • Atomic Number 49: In - 156 pm
  • Atomic Number 81: Tl - 156 pm

Group 14: Carbon Group

  • Atomic Number 6: C - 77 pm
  • Atomic Number 14: Si - 118 pm
  • Atomic Number 32: Ge - 122 pm
  • Atomic Number 50: Sn - 140 pm
  • Atomic Number 82: Pb - 175 pm

Group 15: Nitrogen Group

  • Atomic Number 7: N - 75 pm
  • Atomic Number 15: P - 110 pm
  • Atomic Number 33: As - 114 pm
  • Atomic Number 51: Sb - 140 pm
  • Atomic Number 83: Bi - 156 pm

Group 16: Chalcogens

  • Atomic Number 8: O - 73 pm
  • Atomic Number 16: S - 104 pm
  • Atomic Number 34: Se - 116 pm
  • Atomic Number 52: Te - 140 pm
  • Atomic Number 86: Po - 150 pm (estimated)

Group 17: Halogens

  • Atomic Number 9: F - 72 pm
  • Atomic Number 17: Cl - 99 pm
  • Atomic Number 35: Br - 114 pm
  • Atomic Number 53: I - 133 pm
  • Atomic Number 85: At - 202 pm (estimated)

Summary

  • Covalent Radii typically decrease across a period from left to right and increase down a group due to the addition of electron shells and the effect of effective nuclear charge.
  • These values can vary slightly depending on the chemical environment and the specific compounds formed.

Atomic NumberElementCovalent Radius (pm)
1Hydrogen37
2Helium-
3Lithium152
4Beryllium112
5Boron87
6Carbon77
7Nitrogen75
8Oxygen73
9Fluorine72
10Neon-
11Sodium186
12Magnesium160
13Aluminum143
14Silicon118
15Phosphorus110
16Sulfur104
17Chlorine99
18Argon-
19Potassium227
20Calcium197
21Scandium162
22Titanium147
23Vanadium141
24Chromium139
25Manganese139
26Iron126
27Cobalt125
28Nickel124
29Copper128
30Zinc139
31Gallium135
32Germanium122
33Arsenic118
34Selenium116
35Bromine114
36Krypton-
37Rubidium248
38Strontium215
39Yttrium180
40Zirconium159
41Niobium146
42Molybdenum139
43Technetium134
44Ruthenium132
45Rhodium134
46Palladium139
47Silver144
48Cadmium148
49Indium145
50Tin139
51Antimony139
52Tellurium139
53Iodine133
54Xenon-
55Cesium262
56Barium211
57Lanthanum208
58Cerium198
59Praseodymium196
60Neodymium185
61Promethium174
62Samarium175
63Europium188
64Gadolinium181
65Terbium182
66Dysprosium180
67Holmium175
68Erbium175
69Thulium173
70Ytterbium172
71Lutetium171
72Hafnium159
73Tantalum146
74Tungsten139
75Rhenium139
76Osmium136
77Iridium136
78Platinum139
79Gold144
80Mercury152
81Thallium156
82Lead175
83Bismuth156
84Polonium150
85Astatine202
86Radon-
87Francium270
88Radium215
89Actinium200
90Thorium175
91Protactinium165
92Uranium156
93Neptunium156
94Plutonium157
95Americium158
96Curium157
97Berkelium160
98Californium163
99Einsteinium166
100Fermium167
101Mendelevium170
102Nobelium171
103Lawrencium172
104Rutherfordium175
105Dubnium175
106Seaborgium176
107Bohrium177
108Hassium178
109Meitnerium179
110Darmstadtium180
111Roentgenium181
112Copernicium182
113Nihonium184
114Flerovium185
115Moscovium186
116Livermorium187
117Tennessine188
118Oganesson189

Atomic Radius

The atomic radius is the distance between the atomic nucleus and the atom’s outermost stable electron orbital. Because rising effective nuclear force on electrons causes the atom to shrink, it tends to decrease during a period from left to right. Because of the inclusion of a new energy level, the atomic radius normally increases as one moves down a group (shell). However, because the quantity of electrons has a greater effect than the sizable nucleus, atomic radii tend to increase diagonally. Lithium, for example, has a smaller atomic radius (145 picometer) than magnesium (150 picometer).

The atomic radius can be classified in four ways:

Covalent radius is half the distance between two singly bound atoms in a diatomic molecule.

Van der Waals radius: half the distance between the nuclei of various molecules in a covalent lattice.

Metallic radius is defined as half the distance between two adjacent atom nuclei in a metallic lattice.

Ionic radius is defined as half the distance between two nuclei of an ionic compound.

THE PERIODIC TABLE



https://www.atomicnumber.net/list-of-elements-by-melting-point

https://www.engineeringtoolbox.com/elements-periodic-table-atomic-weight-melting-boiling-points-density-electronegativity-electron-affinity-configuration-d_999.html



Periods

  • Horizontal rows of the periodic table of elements are the periods.
  • There are seven such periods in the table, each numbered from 1 to 7.
  • All the elements in the period have the same number of shells. The number of electrons in this last shell increase by one across any given period
  • The first period is the shortest having only two elements, namely Hydrogen and Helium. The next two rows or periods, that is the second and the third, are known as short periods. They both contain eight elements each. The next two periods have 18 elements each. And finally, the last (sixth) period is the very long period and has 32 elements.

A period on the periodic table is a row of chemical elements. All elements in a row have the same number of electron shells. 

Each next element in a period has one more proton and is less metallic than its predecessor. 

Arranged this way, elements in the same group (column) have similar chemical and physical properties, reflecting the periodic law.


Groups

  • The vertical columns of the periodic chart are what we call Groups.
  • There are 18 groups in the periodic table.
  • Elements belonging to a particular group make a family and are generally named after the first element in that particular group.
  • If you notice the electronic configuration of all elements in one group you will see they all have the same number of valence electrons. Take Chlorine and Fluorine for example. Both belong to group 17 and both have seven valence electrons.
  • As you go down the group the valence electrons remain same but the number of shell increase.
  • On the extreme left of the table is Group 1 which consists of alkali metals (Li, Na, K, Rb, Cs, Fr). They are highly reactive metals that form strong alkaline hydroxides.
  • The last group is occupied by noble gases (He, Ne, Ar, Kr, Xe, Rn). These are highly nonreactive. The eighteenth group they occupy is also called Group 0.

a column in the periodic table of the chemical elements. In a group, the chemical elements have atoms with

identical valence electron counts and 
identical valence vacancy counts. 

This similarity in both the composition and structure of their 

atomic valence shells implies a corresponding similarity in both their chemical and physical properties. 

Groups are numbered from 1 to 18. From left to right in the periodic table, 

there are two groups (1 and 2) of elements in the s-block, or hydrogen block, of the periodic table; 

ten groups (3 through 12) in the d-block, or transition block; and 

six groups (13 through 18) in the p-block, or main block. 

The elements in the f-block, or inner-transition block, the lanthanoids and actinoids, are not given group numbers.





Inner Transition Elements

  • These are Lanthanides and Actinides.
  • Lanthanides are rare earth metals. They are fifteen in number and found deep in the earth’s crust.
  • These elements are placed below the periodic table of elements and are called the 4f series.
  • Actinides are all radioactive elements. Some of them are not found in nature but are man-made
  • These actinides are also placed below the table in a row after lanthanides.



https://www.researchgate.net/publication/366962442_Pushing_the_Limits_of_the_Periodic_Table_--_A_Review_on_Atomic_Relativistic_Electronic_Structure_Theory_and_Calculations_for_the_Superheavy_Elements



Periodic Trends



Periodic Trends of Properties of Elements In Periodic Table 

Modern periodic law is the base of periodic trends of properties of elements in the modern periodic table. Following properties of elements show a very clear periodic trend in the periodic table –


1. Atomic Radius 

2. Ionisation energy 

3. Electron affinity 

4. Electronegativity 

5. Valence electrons 

6. Valency 

7. Metallic character of the elements 

8. Non – metallic character of the elements

9. Reactivity of elements 

10. Melting and boiling points of elements 

 


Exploring Property Trends

The following properties are available for look up or trend visualization (by selecting Display Property at the top of this page). Additionally, where indicated, the properties have dedicated pages with interactive plots.



11 Mar 2024

Stress

What causes stress?

Many things can cause stress. You might feel stressed because of one big event or situation in your life. Or it might be a build-up of lots of smaller things.

This might make it harder for you to identify what's making you feel stressed, or to explain it to other people.

You may experience stress if you:

Feel under lots of pressure

Face big changes in your life

Are worried about something

Don't have much or any control over the outcome of a situation

Have responsibilities that you find overwhelming

Don't have enough work, activities or change in your life

Experience discrimination, hate or abuse

Are going through a period of uncertainty



*Why do certain things make me feel stressed?*

How stressed you feel in different situations may depend on factors like:

How comfortable you feel in certain types of situation

What else you are going through at the time

Your past experiences, and how these affect the way you feel about yourself

The resources you have available to you, such as time and money

The amount of support you have from other people

Some situations that don't bother you at all might cause someone else a lot of stress. This is because we are all influenced by different experiences. We also have different levels of support and ways of coping.

Certain events might also make you feel stressed sometimes, but not every time. 

For example, if you go shopping for food with enough time and money, you may not feel stressed. But you might feel stressed if you have lots of other things to do, have a tight budget, or need to buy food for a big event.



*What kind of situations can cause stress?*

Many things can cause stress in different areas of our lives. These may include:


*Personal*

Illness or injury

Pregnancy and becoming a parent

Infertility and problems having children

Bereavement

Experiencing abuse

Experiencing crime and the justice system, such as being arrested, going to court or being a witness

Organising a complicated event, like a holiday

Everyday tasks, such as household chores or taking transport


*Friends and family*

Getting married or civil partnered

Going through a break-up or getting divorced

Difficult relationships with parents, siblings, friends or children

Being a carer


*Employment and study*

Losing your job

Long-term unemployment

Retiring

Exams and deadlines

Difficult situations or colleagues at work

Starting a new job


*Housing*

Housing problems, such as poor living conditions, lack of security or homelessness

Moving house

Problems with neighbours


*Money*

Worries about money or benefits

Living in poverty

Managing debt


*Social factors*

Having poor access to services such as medical care, green spaces or transport

Living through a stressful community-wide, national or global event, like the coronavirus pandemic

Experiencing stigma or discrimination, including racism, homophobia, biphobia or transphobia


*Can happy events cause stress?*

Some of these situations are often thought of as happy events. For example, you might feel expected to be happy or excited about getting married or having a baby.

But these events can bring big changes, and you might experience new or unusual demands. So they can still feel very stressful. This can be difficult to deal with, especially if you also feel pressure to be positive.