So far, we have learned that the elements are arranged in the periodic table according to their atomic number and that there is a rough correlation between the arrangement of the elements and their electron configuration. We will further explore the trends of the periodic table in this section.
Why do elements in the same family (or group) generally have similar properties?

"Main group" elements are represented by groups 1 and 2, 13-18. We will refer to "main group" elements throughout this unit.
What is the first element in group 17?

In chemistry, a valence electron is an outer shell electron that is associated with an atom, and that can participate in the formation of a chemical bond. Similar to an electron in an inner shell, a valence electron has the ability to absorb or release energy in the form of a photon. An energy gain can trigger an electron to move (jump) to an outer shell; this is known as atomic excitation.
Which electrons are involved in chemical bonding?

Valence electrons have the ability to absorb or release electrons in the formation of __________.

The electrons that determine how an atom reacts chemically are those whose average distance from the nucleus is greatest; that is, those with the highest energy.
These are the valence electrons-electrons available to be lost, gained, or shared in the formation of chemical compounds.
Valence electrons are those electrons in the outer energy level. Elements have a maximum of 8 valence electrons.
For example:
The electron configuration for C is 1s²2s²2p²  So the outer energy level "2" has 4 electrons. If you notice C is in "main group" 14. So carbon has 4 valence electrons.
How many valence electrons does Li have?

How many valence electrons does the element with the electron configuration 1s² 2s² have?

How many valence electrons does boron have?

How many valence electrons does an element with the electron configuration 1s²2s²2p⁴ have?

How many valence electrons does Xe have?

The elements in "main group" 18 have 8 valence electrons. They all have the maximum number (8) electrons in their outer shell.
In other words the "p" orbital is full. This generally means that the noble gases of group 18 are non-reactive.
Helium is in group 18. However, He has an electron configuration 1s^2. Helium has no other energy levels therefore it has 2 valence electrons and not 8.
How many valence electrons does Helium have?

The atomic radius of a chemical element is  measured as one-half the distance between the nuclei of identical atoms that are bonded together. Electrons do not have definite orbits, or sharply defined ranges. Rather, their positions must be described as probable locations that taper off gradually as one moves away from the nucleus. This size is typically measured in Angstroms (or 10^-10 meters)
Our first trend is a measure of the size of an atom or its _________  _____________.

The radii generally decrease along each period (row) of the table, from the alkali metals to the noble gases; and increase down each group (column).
What is the trend in atomic radius down a group?

Which of the following elements has the smallest atomic radius: Li, O, C, F?

Of the elements Ca, Be, Ba, and Sr, which has the largest atomic radius?

Of the elements Ca, Be, Ba, and Sr, which has the smallest atomic radius?

Of the elements Al, Mg, Si, Na, which has the smallest atomic radius?

Rank the following elements by increasing atomic radius:
carbon, aluminum, oxygen, phosphorus.
Not sure? check out page 151 in your textbook