Hewitt/Lyons/Suchocki/Yeh Conceptual Integrated Science

Hewitt/Lyons/Suchocki/YehConceptual Integrated Science Chapter 9 THE ATOM

This lecture will help you understand: • The Elements • The Periodic Table • Atoms Are Ancient, Tiny, and Empty • Protons and Neutrons • Isotopes and Atomic Mass • Atomic Spectra • The Quantum Hypothesis • Electron Waves • Probability Clouds and Atomic Orbitals

The Elements Atoms: • make up all matter around us • to date, 115 distinct kinds of atoms— 90 found in nature, remainder synthesized Element any material consisting of only one type of atom

The Periodic Table Periodic table: • list of chemical elements • designates each element by its atomic symbol—first letter is capitalized

Atoms Are Ancient, Tiny, and Empty Atoms are • ancient • origin of most atoms goes back to birth of universe • tiny • first and lightest atom making up 90% of the universe is hydrogen, H, followed by He • in perpetual motion • so small that when you inhale, you breathe atoms that were once part of every person who ever lived

Atoms Are Ancient, Tiny, and Empty Atoms are • tiny • can’t be seen with visible light—smaller than the wavelength of visible light • made up of subatomic particles, protons and neutrons, in a central nucleus surrounded by electrons • mostly empty space Elements heavier than hydrogen and much of the helium were produced in the interiors of stars.

Atoms Are Ancient, Tiny, and Empty CHECK YOUR NEIGHBOR Which of the following are incorrect statements about the atom? A. Atoms are smaller than the wavelength of visible light. • Atoms are mostly empty space, just as the solar system is mostly empty space. • Atoms are perpetually moving. • Atoms are manufactured in plants, and in humans during pregnancy.

Atoms Are Ancient, Tiny, and Empty CHECK YOUR ANSWER Which of the following are incorrect statements about the atom? A. Atoms are smaller than the wavelength of visible light. • Atoms are mostly empty space, just as the solar system is mostly empty space. • Atoms are perpetually moving. • Atoms are manufactured in plants, and in humans during pregnancy.

Protons and Neutrons Protons: • carry a positive charge—same quantity of charge as electrons • are about 1800 times as massive as an electron • have the same number of protons in the nucleus as electrons surrounding the nucleus of an electrically neutral atom

Protons and Neutrons Electrons: • are identical • repel electrons of neighboring atoms • have electrical repulsion that prevents atomic closeness

Protons and Neutrons Atomic number is the number of protons in each element listed in the periodic table. Neutrons: • accompany protons in the nucleus • have about the same mass as protons but no charge, so are electrically neutral Both protons and neutrons are nucleons.

Isotopes and Atomic Mass Isotopes: • refers to atoms of the same element that contain the same number of protons but different numbers of neutrons in the nucleus • identified by mass number, which is the total number of protons and neutrons in the nucleus • differ only in mass and not by electric charge; therefore, isotopes share many characteristics Total number of neutrons in isotope = mass number – atomic number

Isotopes and Atomic Mass Atomic mass: • total mass of the atom(s) [protons, neutrons, and electrons] • listed in periodic table as atomic mass unit One atomic mass unit is equal to 1.661  10–24 gram or 1.661  10–27 kg

Isotopes and Atomic Mass CHECK YOUR NEIGHBOR The atomic number of an element matches the number of A. protons in the nucleus of an atom. • electrons in a neutral atom. • both of the above. • none of the above.

Isotopes and Atomic Mass CHECK YOUR ANSWER The atomic number of an element matches the number of A. protons in the nucleus of an atom. • electrons in a neutral atom. • both of the above. • none of the above. Comment: When the atomic number doesn’t match the number of electrons, the atom is an ion.

Isotopes and Atomic Mass CHECK YOUR NEIGHBOR A nucleus with an atomic number of 44 and a mass number of 100 must have A. 44 neutrons. • 56 neutrons. • 100 neutrons. • none of the above.

Isotopes and Atomic Mass CHECK YOUR ANSWER A nucleus with an atomic number of 44 and a mass number of 100 must have A. 44 neutrons. • 56 neutrons. • 100 neutrons. • none of the above. Comment: Be sure to distinguish between neutron and nucleon. Of the 100 nucleons in the nucleus, 56 are neutrons. A neutron is a nucleon, as is a proton.

Atomic Spectra Spectroscope: • an instrument that separates and spreads light into its component frequencies • allows analysis of light emitted by elements when they are made to glow—identifies each element by its characteristic pattern Each element emits a distinctive glow when energized and displays a distinctive spectrum.

Atomic Spectra Atomic spectrum is an element’s fingerprint—a pattern of discrete (distinct) frequencies of light. Discoveries of atomic spectrum of hydrogen: • A researcher in the 1800s noted that hydrogen has a more orderly atomic spectrum than others. • Johann Balmer expressed line positions by a mathematical formula. • Johannes Rydberg noted that the sum of the frequencies of two lines often equals the frequency of a third line.

Atomic Spectra Spectral Lines of Various Elements

Atomic Spectra Atomic Excitation

Atomic Spectra Three transitions in an atom. The sum of the energies (and frequencies) for jumps A and B equals the energy (and frequency) of jump C.

Atomic Spectra CHECK YOUR NEIGHBOR Each spectral line in an atomic spectrum represents A. a specific frequency of light emitted by an element. • one of the many colors of an element. • a pattern characteristic of the element. • all of the above.

Atomic Spectra CHECK YOUR ANSWER Each spectral line in an atomic spectrum represents A. a specific frequency of light emitted by an element. • one of the many colors of an element. • a pattern characteristic of the element. • all of the above. Explanation: Many lines make up a pattern that is characteristic of the element, so choice C doesn’t fly. Interestingly, the line shape of each spectral line is an image of a thin slit in the spectroscope.

Atomic Spectra CHECK YOUR NEIGHBOR The hydrogen spectrum consists of many spectral lines. How can this simple element have so many lines? A. One electron can be boosted to many different energy levels. • The electron can move at a variety of speeds. • The electron can vibrate at a variety of frequencies. • Many standing electron waves can fit in the shell of the hydrogen atom.

Atomic Spectra CHECK YOUR ANSWER The hydrogen spectrum consists of many spectral lines. How can this simple element have so many lines? A. One electron can be boosted to many different energy levels. • The electron can move at a variety of speeds. • The electron can vibrate at a variety of frequencies. • Many standing electron waves can fit in the shell of the hydrogen atom.

Atomic Spectra CHECK YOUR NEIGHBOR When an atom is excited, its A. electrons are boosted to higher energy levels. • atoms are charged with light energy. • atoms are made to shake, rattle, and roll. • none of the above.

Atomic Spectra CHECK YOUR ANSWER When an atom is excited, its A. electrons are boosted to higher energy levels. • atoms are charged with light energy. • atoms are made to shake, rattle, and roll. • none of the above.

Atomic Spectra CHECK YOUR NEIGHBOR The frequencies of light emitted by an atom often add up to equal A. a higher frequency of light emitted by the same atom. • a lower frequency of light emitted by the same atom. • both of the above. • none of the above.

Atomic Spectra CHECK YOUR ANSWER The frequencies of light emitted by an atom often add up to equal A. a higher frequency of light emitted by the same atom. • a lower frequency of light emitted by the same atom. • both of the above. • none of the above. Explanation:This follows from two energy transitions in an atom summing to equal another energy transition. See Figure 9.20.

The Quantum Hypothesis Quantum Hypothesis Max Planck, German physicist, hypothesized— warm bodies emit radiant energy in discrete bundles called quanta. Energy in each energy bundle is proportional to the frequency of radiation. Einstein stated that light itself is quantized. A beam of light is not a continuous stream of energy but consists of countless small discrete quanta of energy, each quantum called a photon.

The Quantum Hypothesis Is light a wave, or a stream of particles? Light can be described by both models—it exhibits properties of both a wave or a particle, depending on the experiment. The amount of energy in a photon is directly proportional to the frequency of light: E 

The Quantum Hypothesis CHECK YOUR NEIGHBOR In the relationship E , the symbol  stands for the frequency of emitted light, and E stands for the A. potential energy of the electron emitting the light. • energy of the photon. • kinetic energy of the photon. • all of the above.

The Quantum Hypothesis CHECK YOUR ANSWER In the relationship E , the symbol  stands for the frequency of emitted light, and E stands for the A. potential energy of the electron emitting the light. • energy of the photon. • kinetic energy of the photon. • all of the above. Explanation: For those answering choice A, note that the energy of the photon is equal to the difference in energy levels for the electron emitting the photon—not its value at one energy level.

The Quantum Hypothesis CHECK YOUR NEIGHBOR Which of these has the greatest energy per photon? A. Red light. • Green light. • Blue light. • All have the same.

The Quantum Hypothesis CHECK YOUR ANSWER Which of these has the greatest energy per photon? A. Red light. • Green light. • Blue light. • All have the same. Explanation: In accord with E , the highest frequency light has the greatest energy per photon.

The Quantum Hypothesis CHECK YOUR NEIGHBOR Which of these photons has the smallest energy? A. Infrared. • Visible. • Ultraviolet. • All have the same.

The Quantum Hypothesis CHECK YOUR ANSWER Which of these photons has the smallest energy? A. Infrared. • Visible. • Ultraviolet. • All have the same. Explanation: In accord with E , the lowest frequency radiation has the smallest energy per photon.

The Quantum Hypothesis Using the quantum hypothesis: • Danish physicist Niels Bohr explained the formation of atomic spectra as follows: • The potential energy of an electron depends on its distance from the nucleus. • When an atom absorbs a photon of light, it absorbs energy. Then a low-potential-energy electron is boosted to become a high-potential-energy electron.

The Quantum Hypothesis Using quantum hypothesis: • When an electron in any energy level drops closer to the nucleus, it emits a photon of light. • Bohr reasoned that there must be a number of distinct energy levels within the atom. Each energy level has a principal quantum number n, where n is always an integer. The lowest level is n = 1 and is closest to the nucleus. Electrons release energy in discrete amounts that form discrete lines in the atom’s spectrum.

The Quantum Hypothesis CHECK YOUR NEIGHBOR Which of the following is a quantum number? A. 0.02 • 0.2 • 2 • 2.5

The Quantum Hypothesis CHECK YOUR ANSWER Which of the following is a quantum number? A. 0.02 • 0.2 • 2 • 2.5 Explanation: Quantum numbers are integers only.

The Quantum Hypothesis Bohr’s model explains why atoms don’t collapse: • Electrons can lose only specific amounts of energy equivalent to transitions between levels. • An atom reaches the lowest energy level called the ground state, where the electron can’t lose more energy and can’t move closer to the nucleus.

The Quantum Hypothesis Planetary model of the atom: Photons are emitted by atoms as electrons move from higher-energy outer levels to lower-energy inner levels. The energy of an emitted photon is equal to the difference in energy between the two levels. Because an electron is restricted to discrete levels, only lights of distinct frequencies are emitted.

Electron Waves An electron’s wave nature explains why electrons in an atom are restricted to particular energy levels. Permitted energy levels are a natural consequence of standing electron waves closing in on themselves in a synchronized manner. The orbit for n = 1 consists of a single wavelength, n = 2 is of two wavelengths, and so on.

Electron Waves For a fixed circumference, only an integral number of standing waves can occur, and likewise in the paths of electrons about the nucleus.

Probability Clouds and Atomic Orbitals Three-dimensional electron waves: • They comprise a probability cloud. • They are more intense in some regions than in others. • Erwin Schrödinger, Austrian scientist, formulated an equation from which intensities of electron waves in an atom can be calculated. • The Schrödinger wave equation describes the probability of finding the electron at various locations in the atom.

Probability Clouds and Atomic Orbitals The densest regions correspond to where the electron’s wave intensity is greatest. The probability cloud is a close approximation to the actual shape of an electron’s three-dimensional wave.

Probability Clouds and Atomic Orbitals Atomic orbitals: • They are simply a volume of space within which an electron may reside. • Each orbital represents a different region in which an electron of a given energy is most likely to be found. • They are classified by letters s, p, d, and f and come in a variety of shapes. • Electron energies are quantized, and the sizes of atomic orbitals are quantized.

Hewitt/Lyons/Suchocki/Yeh Conceptual Integrated Science