AP Chemistry Big Idea 1 Coulomb’s Law & Ionization Energy

Coulomb’s Law Holding it all together

When you have two charged objects, they will either attract or repel

Opposite charges (positive and negative) attract + -

Like charges (positive /positive or negative/negative) repel ++ --

The degree to which two objects attract or repel is driven by two factors: The magnitude of the charge (how many charged particles are present) The distance between the charged particles

The force of attraction between two oppositely charged particles is directly proportional; therefore, the more charged particles, the greater the force of attraction

The smaller the distance between two charged particles, the greater the force of attraction. The larger the distance between two charged particles, the weaker the force of attraction. This is an inverse relationship m 2 m

The combined effect of these two factors on the force of attraction/repulsion between charged particles is called Coulomb’s Law: F= force (attraction/repulsion) q 1, q 2 = magnitude of charged particles r = distance between charged particles

Let’s Practice #1 Which of the following situations would you expect to have the strongest force of attraction? a. b cm ✔

Let’s Practice #2 Which of the following situations would you expect to have the weakest force of attraction? a. b cm cm ✔

In order to separate oppositely charged particles, you must input energy to overcome the force of attraction. The larger the force of attraction, the more energy is needed to separate the charges.

Let’s Practice #3 a. b distance in cm Which situation would require a larger input of energy to separate the charges? ✔

Let’s Practice #4 a. c distance in cm Which situation would require a larger input of energy to separate the charges? + - ✔

Let’s Practice #5 510 distance in cm Which negative particle (electron) would be the easiest to remove (require the least energy)?

Let’s Practice #6 a. b. 510 distance in cm Would it be easier to remove the furthest electron in a or b? ✔

Let’s Practice #7 a. b. 510 distance in cm How would the energy needed to remove the furthest electron compare in situation a versus b?

Atoms contain both positively charged particles (protons) and negatively charged particles (electrons). All of the behavior of atoms and molecules can be attributed to the forces of attraction and repulsion of these particles.

Ionization Energy

The energy required to remove an electron from an atom or ion. Example: Hydrogen ProtonsElectrons The force of attraction between the positive proton and the negative electron hold the atom together. To remove an electron, there needs to be an input of energy that matches or exceeds the force of attraction.

Ionization Energy Atoms with more than one electron have more than one ionization energy. Example: Helium ProtonsElectrons st ionization energy 2 nd ionization energy

Ionization Energy Atoms with more than one electron have more than one ionization energy. Example: Helium ProtonsElectrons Which of helium’s ionization energies would you expect to be larger? 1 st IE 24.6 eV 2 nd IE 54.4 eV Explain with Coulomb’s Law

Ionization Energy Example: Lithium ProtonsElectrons st IE 5.4 eV 2 nd IE 3 rd IE 1 st IE 5.4 eV 2 nd IE 75.6 eV 3 rd IE 1 st IE 5.4 eV 2 nd IE 75.6 eV 3 rd IE 122 eV Explain with Coulomb’s Law The first ionization energy is always the energy to remove the outermost electron.

Let’s Practice #8 How would the first ionization energy of Hydrogen compare to the first ionization energy of Helium? Explain your reasoning using Coulomb’s Law

Factors affecting ionization energy Nuclear Charge: How many protons are in the nucleus? “Shielding”: Are there inner electrons that are repelling the outermost electrons? Distance (radius): How far is the electron from the nucleus?

Compare H and Li Both atoms have a single electron in their outermost shell. Lithium, however, has two core electrons that are repelling the outermost electron, and “shielding” the electron from the full charge of the nucleus Effectively, the outermost electron in lithium only “feels” the effect of one proton. Effective Nuclear Charge = # of protons - # of core electrons

What is the effective nuclear charge for each electron in Lithium? Z eff = +1 Z eff = +3

Comparing 1 st Ionization Energies

First Ionization energy Atomic number Helium (He) has… Greater nuclear charge No shielding Higher IE H He n H e - 2e - Comparing 1 st Ionization Energies

First Ionization energy Atomic number Li has… Greater nuclear charge Shielding (2 core electrons) Election is on an outer shell. According to Coulomb’s Law, the distance from nucleus outweighs magnitude of charge H He Li n

First Ionization energy Atomic number H He Li Be B C N O F Ne n Why is Ne lower than He? Ne has a Higher nuclear charge, but There are 8 electrons shielding the nucleus (repelling the outermost electrons) There is a greater distance from nucleus

Let’s Practice #9 Which of the following atoms would you expect to have the lowest first ionization energy? a. Na b. K c. Rb ✔

Let’s Practice #10 Which of the following atoms would you expect to have the highest first ionization energy? a. K b. Ca c. Rb ✔