Periodic Trends PreAP Chemistry

According to Bohr’s model of the atom, the nucleus and electrons are very similar to solar systems. The nucleus of an atom acts like the sun of a solar system. The positive electrical charge of the protons provides the “gravity” of the sun.

The electrons of an atom are like the planets of a solar system The electrons of an atom are like the planets of a solar system. They have “orbits” or regions in which they can travel, but they do not travel outside their regions. Atoms are three dimensional. They are not flat. Therefore, the regions in which an electron can travel are not flat circles. We call these three dimensional regions “electron clouds.”

In our solar system, each planet has its own orbit, each is a different distance from the sun. However, multiple electrons can share the same orbit in an atom. In fact, most atoms would like to have eight electrons sharing the outer orbit, known as the valence shell. This would be like eight planets all sharing Neptune’s orbit.

The distance of a planet from the sun depends on the gravity of the sun, the mass of the planet, and the speed the planet is traveling (energy). All electrons have the same mass, so for electrons, mass has no effect on distance from the nucleus. The distance depends only on the “gravity” of the nucleus and the energy of the electron.

If a solar system has a large sun with strong gravity, the planets will be closer to the sun than in a solar system with a small sun with weak gravity.

Stop and think: Which element from each pair represents a solar system with more gravity? A) Li or S B) Ir or Ca C) As or P D) Ba or Pb E) F or Fr Hint: Gravity is affected by mass.

The radius of a solar system is the distance from the center of the sun to the farthest planet. For an atom, the atomic radius is the distance from the center of the nucleus to the farthest electron. Based on the strength of the “gravity” or positive charge of the nucleus of the atom, which of each pair will have the SMALLEST atomic radius? Remember: high gravity pulls planets closer! A) Al vs Cl B) Sn vs Y C) He vs H D) K vs Ca E) N vs Ne

You have discovered that as each element progresses from left to right on the periodic table, it gains protons. Thus, its total positive charge (gravity) increases. When this happens, the pull on the electrons is greater, so the electrons move closer. The atomic radius SHRINKS from left to right on the table.

So what happens when you go DOWN the chart So what happens when you go DOWN the chart? Each period (or row) as you go down the chart represents an additional orbit in your solar system. This is a new shell or energy level for the atom’s electrons. Since you have gained a whole orbit, this will automatically make the radius bigger, in spite of the magnetic pull of the protons.

Now you know the trends for atomic radius: from left to right, the radius decreases due to a greater nuclear magnetic pull on the electrons. From top to bottom, the radius increases due to gaining additional energy levels. The trend looks much like this: Stop and think! Which of these will have the LARGEST atomic radius? A) B, Al, or In? B) V, Cr, or Mn? C) I, Se, or P D) Mg, Ar, or Fe

So, what happens when an atom gains or loses electrons to form an ion? Remember, the octet rule states that atoms want to have 8 electrons in their valence (or outer) shells (orbits). In order to accomplish this, atoms form ions. An ion is an atom with a charge because its electrons and protons are no longer equal. An atom that loses electrons is called a cation. This atom initially had the same number of protons as electrons. Once it loses electrons to follow the octet rule, it will have more protons than electrons. It will then have a positive charge.

Stop and think: If an element such as calcium has two valence electrons, then how does losing electrons satisfy the octet rule? When it loses two electrons, it will also lose its entire valence shell. Thus, instead of four shells, calcium will be left with only three. The third shell is now the valence shell, and it contains eight electrons.

Since a cation loses a shell when losing electrons, its radius as an ion gets smaller. This new radius is called the ionic radius. Since metals act as cations, metals will have a smaller ionic radius than atomic radius.

Unlike metals, nonmetals usually gain electrons in order to fill their current valence shell with eight electrons. Since they will then have more electrons than protons, they have a negative charge. These negatively charged ions are called anions: A Negative ION.

Because the nonmetal atom is gaining electrons without gaining additional protons, the negative electrons will push each other away (like charges repel), but there is no extra force pulling the electrons in toward the nucleus. The electrons take advantage of the extra space outside the atom to push further away, slightly expanding the radius.

Stop and think! List the following atoms in order of increasing size: Rb, Rb+, F, F-, Te. We know that Rb+ is smaller than Rb because it loses an electron and has one less shell. Another reasoning is that since Rb+ has the same number of protons but less electrons, the electrons are held more tightly in Rb+. We know that Te is smaller that Rb because of the trend of decreasing radius to the right within a period. But since Te has one shell more than Rb+ is it bigger. We know that F and F- are both smaller than the rest because they have less shells. Now between F and F-: F- has one more electron, which causes the radius to be bigger. FINAL ANSWER: F, F-, Rb+, Te, Rb

Electronegativity Electronegativity is the tendency of atoms to gain electrons. Since nonmentals want to gain electrons to satisfy the octet rule while metals want to lose electrons, nonmetals have a higher electronegativity than metals. Nonmetals are located in the upper right corner of the periodic table. Thus, electronegativity increases left to right and bottom to top.

Ionization Energy Ionization energy is the energy needed to remove an electron from a neutral atom. Because metals would rather lose electrons to satisfy the octet rule, it is easy to remove electrons. They have a low ionization energy. However, as they gain protons, the magnetic pull on the electrons also increases, making it more difficult to remove them. This is why ionization energy increases from left to right.

Nonmetals want to gain electrons, not lose them Nonmetals want to gain electrons, not lose them! Thus, they have very high ionization energies. These are located at the top right of the periodic table. Thus, the ionization energy, like electronegativity, increases from bottom to top and left to right.

How does it all work together?

Stop and Think! Put the elements C, K, N, Sr, and P in order by: 1) Increasing atomic radius 2) Decreasing electronegativity 3) Decreasing ionic radius 4) Increasing ionization energy Assignment: Write four paragraphs explaining how each of the four trends works and what causes the trend from left to right and top to bottom.