1. Periodic Table Trends
Atomic Radius

2. Atomic Radius (Radii)
A trend is a prevailing tendency or a general course.
We commonly discuss weather trends and fashion trends.
Trends also occur in the arrangement of the periodic table.

3. Atomic size is measured according to the radius of the atom, or the atomic radius.
Remember that atoms are spherically shaped and are made up of protons and neutrons within the nucleus and electrons in the electron cloud. The negatively-charged electrons are held within the atom by their attraction to the positively-charged nucleus.

4. An atom doesn’t have a well-defined boundary, and its radius cannot be directly measured.
*Atomic size can be estimated by measuring the distance between the nuclei of two atoms of the same type that are joined together.
Therefore, the atomic radius is defined as one half the distance between the nuclei of two atoms of the same element that are joined.

5. Atomic Radius

6. **LOOK AT THE ATOMIC RADIUS CHART ON THE BACK OF THE NOTESHEET**
Question - What general trends or patterns do you see among the elements?

7. On the periodic table, atomic radii tend to decrease left to right along a period, so group 1 atoms are generally larger than group 18 atoms.
This is because, within a period, the number of principle energy levels (PELs) in each element generally remains constant.
For example, all elements in Period 3 have three energy levels. However, the nucleus gains protons as atomic number increases, so it becomes more attractive to the electrons and pulls them in tighter.
This growing attraction causes the atomic radii to decrease as protons are added.

8. Moving down a group on the periodic table atomic radii tend to increase.
Atomic number increases as protons are added; however, the number of occupied orbitals increases as layers of electrons are added.
As the number of electrons located between the nucleus and the outer shell increases, valence electrons are increasingly shielded from the full attraction of the positive nucleus.
This is called the SHIELDING EFFECT.

9. Electron shielding has a greater influence on the atom than the increased attraction of the nucleus; therefore, atomic radii generally increase moving down the periodic table.
The largest elements are in the lower left corner of the periodic table. The smallest elements are in the upper right corner.

10. IONIC RADIUS
The radius of an ion is different from the radius of a neutral atom.
An ion is an atom with a charge.
A positive ion, called a cation, forms when an atom loses electrons. Metals, such as sodium, tend to lose electrons to form cations. Cations are always smaller than the neutral atom.

11. When an atom loses electrons, the ionic radius decreases for two reasons.
First, lost electrons will almost always be valence electrons, so they can leave an empty outer shell, which makes the radius smaller.
Second, the positively-charged nucleus exerts the same amount of pull but on fewer electrons, so it pulls the remaining electrons closer to the nucleus.

12. A negative ion, called an anion, forms when electrons are gained.
Nonmetals, such as chlorine, tend to gain electrons to form anions.
Negative ions are always larger than the neutral atom.

13. When an atom gains electrons, the ionic radius increases because of the identical charge of all electrons.
Remember that only opposites attract— negatively-charged electrons are attracted to the positively-charged nucleus.
Electrons are not attracted to each other but instead repel and push away from each other.
Adding electrons to the outer shell forces them to push farther apart, which increases the radius.

14. On the periodic table, the trend in ionic radii is similar to the trend in atomic radii.
Ionic radii generally decrease from left to right across a period and increase down a group.
However, groups that form cations will have smaller radii than groups that form anions.

15. Elements in groups 1, 2, 13 and (14) lose electrons to form cations, so the ionic radii are smaller than the neutral atom.
Within these groups, the ionic radii decrease left to right along a period and decrease down the group.

16. Most elements in groups 15, 16, and 17 gain electrons to form anions, so the ionic radii are larger than the neutral atom.
Within these groups, the ionic radii generally increase left to right along a period and increase down the group.
Recall that group 18, the noble gases, do not form ions. Noble gases have a full valence energy level so they do not gain or lose electrons.