<< Chapter < Page Chapter >> Page >
By the end of this section, you will be able to:
  • Describe and explain the observed trends in atomic size, ionization energy, and electron affinity of the elements

The elements in groups (vertical columns) of the periodic table exhibit similar chemical behavior. This similarity occurs because the members of a group have the same number and distribution of electrons in their valence shells. However, there are also other patterns in chemical properties on the periodic table. For example, as we move down a group, the metallic character of the atoms increases. Oxygen, at the top of group 16 (6A), is a colorless gas; in the middle of the group, selenium is a semiconducting solid; and, toward the bottom, polonium is a silver-grey solid that conducts electricity.

As we go across a period from left to right, we add a proton to the nucleus and an electron to the valence shell with each successive element. As we go down the elements in a group, the number of electrons in the valence shell remains constant, but the principal quantum number increases by one each time. An understanding of the electronic structure of the elements allows us to examine some of the properties that govern their chemical behavior. These properties vary periodically as the electronic structure of the elements changes. They are (1) size (radius) of atoms and ions, (2) ionization energies, and (3) electron affinities.

Variation in covalent radius

The quantum mechanical picture makes it difficult to establish a definite size of an atom. However, there are several practical ways to define the radius of atoms and, thus, to determine their relative sizes that give roughly similar values. We will use the covalent radius    ( [link] ), 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). We know that as we scan down a group, the principal quantum number, n , increases by one for each element. Thus, the electrons are being added to a region of space that is increasingly distant from the nucleus. Consequently, the size of the atom (and its covalent radius) must increase as we increase the distance of the outermost electrons from the nucleus. This trend is illustrated for the covalent radii of the halogens in [link] and [link] . The trends for the entire periodic table can be seen in [link] .

Covalent Radii of the Halogen Group Elements
Atom Covalent radius (pm) Nuclear charge
F 64 +9
Cl 99 +17
Br 114 +35
I 133 +53
At 148 +85
This figure has two parts: a and b. In figure a, 4 diatomic molecules are shown to illustrate the method of determining the atomic radius of an atom. The first model, in light green, is used to find the F atom radius. Two spheres are pushed very tightly together. The distance between the centers of the two atoms is indicated above the diagram with a double headed arrow labeled, “128 p m.” The endpoints of this arrow connect to line segments that extend to the atomic radii below. Beneath the molecule is the label, “F radius equals 128 p m divided by 2 equals 64 p m.” The next three models are similarly used to show the atomic radii of additional atoms. The second diatomic molecule is in a darker shade of green. The distance between the radii is 198 p m. Beneath the molecule is the label, “C l radius equals 198 p m divided by 2 equals 99 pm.” The third diatomic molecule is in red. The distance between the radii is 228 p m. Beneath the molecule is the label, “B r radius equals 228 p m divided by 2 equals 114 pm.” The fourth diatomic molecule is in purple. The distance between the radii is 266 p m. Beneath the molecule is the label, “I radius equals 266 p m divided by 2 equals 133 p m.” In figure b, a periodic table layout is used to compare relative sizes of atoms using green spheres. No spheres are provided for the noble or inert gas, group 18 elements. General trends noted are increasing circle size moving from top to bottom in a group, with a general tendency toward increasing atomic radii toward the lower left corner of the periodic table.
(a) The radius of an atom is defined as one-half the distance between the nuclei in a molecule consisting of two identical atoms joined by a covalent bond. The atomic radius for the halogens increases down the group as n increases. (b) Covalent radii of the elements are shown to scale. The general trend is that radii increase down a group and decrease across a period.

Questions & Answers

who is the father of chemistry
Naomi Reply
what is hybridization and bonding
Simon Reply
please who is the father of chemistry
Antoine Lavoisier
I recommend reading on Google.
Also, god has created everything
(Allah, the creator)
how to determine the empirical formula
Vickie Reply
what is neutralisation reaction
Ugbaje Reply
It is a reaction where an acid (aq) reacts with an alkali (aq) to form salt AND water.
What is the ratio of the average kinetic energy of a SO 2 molecule to that of an O 2 molecule in a mixture of two gases? What is the ratio of the root mean square speeds, u rms , of the two gases?
xMah_Bx Reply
what is methyl orange
Wisdom Reply
its an indicator
It is an organic molecule that reacts with acid/base medium and shows change in colour (due to formed products). It can be used to test for alcohols which are prohibited to drink in Islam as it is bad for brain and liver and immunity.
it is an indicator used to determine the end point in an acid-base titration
explain more about this topic
Ayomide Reply
Which topic please?
covalent bond explanation
God has lictured these rules that the electrons move with them and the space time curve. So we prwy to God in Islam or we suffer. Please learn about Islam and science and mention God and thank him.
yes, it is very true. God is the one who inspires science. Then we, as his children, have the privilege to learn about what he has created. I'm still a novis at chemistry. I still have a lot to learn.
Beautiful, however, what I learned from the Quran is that god has created human and has a mission like managing the ecology, building, learning, mentioning god (saying glory to Almighty for instance few times and periodic). And the moat important prayer lile prophet Muhammed the last meassenger.
Also God forgives all sins except assossiating any one with him like sons and daughters or stone sculpture. Beautifully, if some one stops from saying this God can switch their sins to virtueness. And God all merciful doesnt get bored forgiving people who ask for forgiveness.
Also, there is a versw in the Quran that sates: { corruption has appeared in the land and the sea he will taste them from what their hands have gained} this reminds me of covid from stupid politicians who will be throne to hell.
nature of bond in N2 molecule is
Ahmad Reply
Triple covalent bond, and that is why it is hardly reactive because in order to react you must break three strong covalent bonds.
what is electromagnetic energy
Onyekwu Reply
in a school of 120 students, 41studied mathematics, 48studied chemistry and 42 studied physics, 16 studied both chemistry and mathematics, 14 studied mathematics and physics, 18 studied chemistry and physics and 9 studied all the three subjects. how many of them studied exactly one subject?
Kafayat Reply
Does Chromium oxidize?
Jag Reply
@Alaa It doesn't.
yes. it's a very strong oxidizing agent
what is compound
Queen Reply
what are the types of hydrocarbon
Dolapo Reply
homologous series is under what
Don't get your Question.
OK Aliphatic and Aromatic.
What is an acid
Progress Reply
What is hydro carbon
A hydro carbon is an organic compund formed of carbon and hydrogen only that comes from plant derivatives. It is mercy from God as it has lots of benifits. In Islam, we know from the Quran that God has created for every illness a cure. We sometimes find such compunds in plants and we use it in pills
what are the types of hydrocarbon
we have two types of hydrocarbons Aliphatic and Aromatic
under Aliphatic we have Alkane, Alkene,Alkyl Aromatic we have Benzene
Practice Key Terms 5

Get Jobilize Job Search Mobile App in your pocket Now!

Get it on Google Play Download on the App Store Now

Source:  OpenStax, Chemistry. OpenStax CNX. May 20, 2015 Download for free at http://legacy.cnx.org/content/col11760/1.9
Google Play and the Google Play logo are trademarks of Google Inc.

Notification Switch

Would you like to follow the 'Chemistry' conversation and receive update notifications?