Chapter 5 “Electrons in Atoms”

Section 5.1 – Models of the Atom OBJECTIVES: Identify the inadequacies in the Rutherford atomic model. Identify the new proposal in the Bohr model of the atom. Describe the energies and positions of electrons according to the quantum mechanical model. Describe how the shapes of orbitals related to different sublevels differ.

Ernest Rutherford’s Model Discovered dense positive piece at the center of the atom- “nucleus” Electrons would surround and move around it, like planets around the sun Atom is mostly empty space

Ernest Rutherford’s Model These were all significant discoveries, but a few questions still went unanswered… Why is there so much empty space? Why don’t the electrons just fall into the nucleus, after all, Electrons are negative & Protons are positive?

Niels Bohr’s Model He said electrons around the nucleus: Neils Bohr helped answer this question: He said electrons around the nucleus: Move like planets around the sun. Move in specific circular paths, or orbits, at different levels. An amount of fixed energy separates one level from another.

Who has more gravitational potential energy? Niels Bohr’s model We call these levels where electrons reside Energy levels of an atom They work something like a ladder: Who has more gravitational potential energy?

The Ladder Analogy the similarities… An atom: Electrons can’t exist between energy levels. When electrons absorb energy they can move to higher energy levels. When an electron moves down to a lower energy level it emits energy. A ladder: You can’t stand in between the rungs, only on them. When you step up to the next rung on the ladder you gain potential energy. When you step down you loose potential energy.

The Ladder Analogy the similarities… An atom: An electron requires a specific and definite amount of energy in order to move to a higher energy level. That amount is determined by the energy level it is moving from and moving to. When an electron drops to a lower energy level it emits a specific and definite amount of energy A ladder: When I move between rungs I experience a change in energy that is a specific and definite amount. That amount is determined mainly by my height from the ground.

The Ladder Analogy the differences… An atom: The energy an electron has is electromagnetic energy. An electron must have the energy prior to moving up to higher energy level, it is this additional energy that allows it to move. A ladder: The energy I posses on the ladder is gravitational potential energy. I acquire greater potential energy after moving up the ladder.

The Ladder Analogy the differences… An atom: If an electron were to jump down to a lower energy level, its energy would decrease because some of its energy would be released as light. A ladder: If I were to jump down to a lower rung, my energy would decrease because some of my energy would be converted to kinetic energy (energy associate with motion).

The Ladder Analogy the differences… A ladder: Rungs of a ladder are usually evenly spaced. An atom: Energy levels in an electron are NOT separated by equal amounts of energy

A Diagram of Energy Levels High energy Low energy Each level has a certain amount of energy associated with it

Let’s see how this works: http://www.visionlearning.com/library/module_viewer.php?mid=51

Niels Bohr’s model A Quantum of energy is the amount of energy (a packet or chunck) required to move an electron from one energy level to another Since the energy of an atom is NEVER “in between” there must be a quantum leap in energy.

And then came… The Quantum Mechanical Model Good Grief! Another Model of the atom !*#@? We know now that Bohr’s model, although very close, is not completely accurate.

The Quantum Mechanical Model The quantum mechanical model is the currently accepted model of the atom. In the next section of this chapter when we look at how electrons for specific elements are arranged in the atom, we will use the Bohr model. It is accurate and sufficient for the work we’ll be doing. However, you are required to know some characteristics of the currently accepted model and so we will review them now.

The Quantum Mechanical Model Energy levels are NOT circular paths. Energy levels are areas where there is a high probability of finding an electron that we say is “in” that energy level. We draw: In reality: ● http://universe-review.ca/I15-53-quantum.jpg http://www.iun.edu/~cpanhd/C101webnotes/modern-atomic-theory/images/orbitals.jpg

The Quantum Mechanical Model The nucleus is found inside a blurry “Electron Cloud” Think of a fan blades spinning

The Quantum Mechanical Model Each energy level is made up of orbitals. (sub-levels) If an energy level has 8 electrons in it, all 8 electrons don’t have the same probability of being found in the same place. 2 of those electrons may spend most of their time… Another 2 here… Another 2 here… Another 2 here…

The Quantum Mechanical Model There are more orbitals with different & more complicated shapes, but if we tried to illustrate what an atom with many electrons in many orbitals would look like… This would be a simplified version

Before we go into any more detail, lets do a quick review of the historical development of the atom

Section 5.2 – Electron Arrangement in Atoms OBJECTIVES: Describe how to write the electron configuration for an atom. Explain why the actual electron configurations for some elements seem ‘out of order’ Describe what type of electron configuration makes an atom stable.

Principal Quantum Number Generally symbolized by “n”, it denotes the energy level in which the electron is located.

2n2 Maximum number of electrons that can fit in an energy level: 2(12) = 2 … energy level 1 holds 2 e- n=2: 2(22) = 2(4) = 8 … energy level 2 holds 8 e- n=3: 2(32) = 2(9) = 18 … energy level 3 holds 18 e- n=4: 2(42) = 2(16) = 32 … energy level 4 holds 32 e- n=5: 2(52) = 2(25) = 50 … energy level 5 holds 50 e- 2n2

Electron Configuration Energy levels with the lowest energy are filled first. An electron entering an atom will move into the energy level with the lowest energy (as long as it is not already filled.) What energy level gets filled first? Energy level 1, also written n=1

Electron Configuration The closer an energy level is to its maximum number of electrons, the more stable the atom is. Example: Energy level 1 can hold 2 electrons. Which is more stable? Explain why – use space in notes. Helium Atom Hydrogen Atom

Electron Configuration Helium is a very stable gas – not reactive under normal circumstances Hydrogen: a violently explosive gas The Hindenberg & Hydrogen? Hydrogen Balloon

An Example of Electron Configuration - Phosphorous Phosphorous (P) has 15 electrons Draw a nucleus Fill up level 1 2 electrons Fill up level 2 8 electrons Fill up level 3 5 electrons nucleus

An Example of Electron Configuration - Chlorine Cl has 17 electrons Draw a nucleus Fill up level 1 2 electrons Fill up level 2 8 electrons Fill up level 3 7 electrons 7 8 2 nucleus

An Example of Electron Configuration - Copper Cu has 29 electrons Draw a nucleus Fill up level 1 2 electrons Fill up level 2 8 electrons Fill up level 3 19 electrons? No…Why? Only 18 e- fit Fill up level 4 1 electrons 1 19? 18 8 2 nucleus

An Example of Electron Configuration - Calcium Ca has 20 electrons Draw a nucleus Fill up level 1 2 electrons Fill up level 2 8 electrons Fill up level 3 10 electrons? No…Why? Up to 18 e- can fit… 10? 8 2 nucleus

Overlapping Amounts of Energy High energy Low energy Energy level: 1 2 3 4 5 6 7

Energy levels that are farther from the nucleus are closer together

Back to Calcium Cu has 20 electrons Draw a nucleus Fill up level 1 2 8 8 2 nucleus

Energy Level Diagrams Draw energy level diagrams for: Li Na K Rb Be B C N O Nucleus: 1cm radius colored red #p+ #no Energy levels: #e- 1 = 2cm radius 2 = 4cm radius 3 = 5cm radius 4 = 6cm radius 5 = 7cm radius

Questions What do all the elements in the 1st column of the periodic table have in common? What do all the elements in the 2nd row of the periodic table have in common?

Section 5.3:Physics and the Quantum Mechanical Model OBJECTIVES: Identify the source of atomic emission spectra. Describe the relationship between the wavelength and frequency of light.

Light The light that we see with our eyes is just a small part of a large spectrum of electromagnetic radiation

“R O Y G B I V” High Frequency Low Frequency Wavelength Longer

Wavelength, Frequency and Energy Frequency (ƒ ) and Wavelength (λ) have an inverse relationship As one goes up the other goes down. Frequency (ƒ ) and Energy have a direct relationship As one goes up, so does the other. ƒ _____ ; λ _____; E ______

Wavelength, Frequency and Energy Different frequencies of light are different colors, the whole range is called a spectrum. The color of the light indicates its wavelength, frequency and Energy

Atomic Spectra White light is made up of all the colors of the visible spectrum. Passing light through a prism bends it. The λ determines how much it is bent. Each color, with its own wavelength bends at its own angle. Result: the light separates into its component colors

If the light is not white Recall from earlier in chapter: when electrons drop to a lower energy they release energy as light Each element gives off its own characteristic colors When we pass the light given off by an atom through a prism, we see only the component colors present

Atomic Emission Spectrum The characteristic color sequence given off by an element is called its Atomic Emission Spectrum Can be used to identify the atom. This is how we know what stars are made of.

Why does each element have its own spectrum? How does this Happen?

Ground State When we write electron configurations, as in the previous section, we are writing the configuration for the lowest energy. In this configuration, the atom is said to be in its ground state The ground state is stable Atoms tend toward ground state configurations.

Ground State Let’s look at a hydrogen atom, with only one electron, and in the first energy level This is the electron configuration with the lowest energy for hydrogen, we call it _____________________

Changing the energy Heat, electricity, or light can move the electron up to different energy levels. The electron is now said to be “excited”

Changing the energy The excited state is higher energy, less stable. The atom will tend toward its ground state. As the electron falls back to the ground state, it gives the energy back as light

Changing the energy They may fall down in specific steps Each step has a different energy The further they fall, more energy is released and the higher the frequency

Changing the Energy Remember, every amount of energy corresponds to a certain frequency, wavelength and therefore color

These are called the atomic emission spectrum Unique to each element, like fingerprints! Very useful for identifying elements