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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

what is hydrocarbon
Sensible Reply
this is an organic compound that consist of hydrogen and ox
why is isotonic solution the as blood
Hawa Reply
Elaborate more pls
could you rephrase the question please?
what is hydrocarbon? pls
Hydrocarbon are organic compound consisting of hydrogen and carbon atoms
why is electrolytes really active in water
relationship between an orbit and a shell
Hawa Reply
relationship between orbit and a shell
What a homogenous mixture
Allison Reply
these are mixture of same substance or particles
they are mixture of same particles or substance
Define homologous series
Chizoba Reply
it is an organic compound having the same chemical properties which differs in a successive member of CH4
when iron is exposed to moisture and it rusts,the value of deltaG for the reaction is
Mary Reply
what is chemistry
Siyanbola Reply
it is a branch of science that deals with the mixture of chemicals and properties and uses of matter
define aci according to Lewis
Ryhanna Reply
an acid is an electron pair acceptor according to lewis
What is a homogenous mixture
it is d mixture of d same substances or particle or compound
homogenous is the combination or mixture of the same particle or compound
the third ionization energy of aluminium
sanmi Reply
name the allotrope of carbon used in gas masks
Moses Reply
Discuss the synthesis of sucrose
Ojiya Reply
What is a glycoside and discuss the formation
explain the transitioning of glucose molecules from Fischer projection of glucose to the Haworth projection
Rosey Reply
what is quantum
hezekiah Reply
what are metalliod
Metalloids are substances having the characteristics of both metals and non-metals
what is acid
define acid according to lewis
Acid is an electron acceptor
Wat z quantum
what is valency
odukoya Reply
The number of chemical bonds the atoms of a certain element can form.
what is ionization energy?
Hussniz Reply
ionization energy is qualitatively defined as the minimum amount of energy required to remove the most loosely bound electron, the valence electron, of an isolated neutral gaseous atom, molecule or io
Wow. Thumb 👍 up
Very explanatory. Tanx
Practice Key Terms 5

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Source:  OpenStax, Chemistry. OpenStax CNX. May 20, 2015 Download for free at http://legacy.cnx.org/content/col11760/1.9
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