![Isotopes and Atomic Mass
• Atomic mass is
– the total mass of the atom(s) [protons, neutrons,
and electrons].
– listed in the periodic table in atomic mass units.
• One atomic mass unit is equal to
1.661 × 10–24
gram or 1.661 × 10–27
kg.](https://image.slidesharecdn.com/chapter9atomsandtheperiodictable-150107062120-conversion-gate02/85/Chapter-9-atoms-and-the-periodic-table-15-320.jpg)
Chapter 9 atoms and the periodic table
- 1. Chapter 9 Atoms and the Periodic Table
- 2. This lecture will help you understand: • Atoms Are Ancient and Empty • The Elements • Protons and Neutrons • Isotopes and Atomic Mass • The Periodic Table • Physical and Conceptual Models • Identifying Atoms Using the Spectroscope • The Quantum Hypothesis • Electron Waves • The Shell Model
- 3. Atoms Are Ancient and Empty • Atoms are – ancient. • The origin of most atoms goes back to the birth of the universe. – mostly empty space. • Elements heavier than hydrogen and much of the helium were produced in the interiors of stars.
- 5. The Elements • An element is a material made of only one kind of atom. For example, pure gold is an element because it is made of only gold atoms. • An atom is the fundamental unit of an element. The term "element" is used when referring to macroscopic quantities. The term "atom" is used when discussing the submicroscopic.
- 6. The Elements • Atoms: – Atoms make up all matter around us. – To date, 115 distinct kinds of atoms are known— 90 are found in nature, and the rest are synthesized. • An element is any material that consists of only one type of atom.
- 9. Protons and Neutrons • Protons – carry a positive charge—the same quantity of charge as electrons. – are about 1800 times as massive as electrons. – have the same number of protons in the nucleus as electrons that surround the nucleus of an electrically neutral atom.
- 10. Protons and Neutrons • Electrons – are identical. – repel the electrons of neighboring atoms. – have electrical repulsion that prevents atomic closeness.
- 11. Protons and Neutrons • The 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 they are electrically neutral. • Both protons and neutrons are nucleons.
- 13. Isotopes and Atomic Mass • Isotopes – are atoms of the same element that contain the same number of protons but different numbers of neutrons in the nucleus. – are identified by mass number, which is the total number of protons and neutrons in the nucleus. – differ only in mass, not electric charge; therefore, isotopes share many characteristics. • Total number of neutrons in isotope: Mass number − atomic number
- 15. Isotopes and Atomic Mass • Atomic mass is – the total mass of the atom(s) [protons, neutrons, and electrons]. – listed in the periodic table in atomic mass units. • One atomic mass unit is equal to 1.661 × 10–24 gram or 1.661 × 10–27 kg.
- 17. The Periodic Table • The periodic table is a listing of all the known elements. • It is NOT something to be memorized. • Instead, we learn how to READ the periodic table. • A chemist uses the periodic table much as a writer uses a dictionary. NEITHER needs be memorized!
- 19. The Periodic Table • The elements are highly organized within the periodic table. • Each vertical column is called a group. • Each horizontal row is called a period.
- 24. • A physical model replicates an object at a convenient scale. • A conceptual model describes a system. – An atom is best described by a conceptual model. Physical and Conceptual Models
- 25. • A spectroscope – is an instrument that separates and spreads light into its component frequencies. – allows the analysis of light emitted by elements when they are made to glow. It identifies each element by its characteristic pattern. – Each element emits a distinctive glow when energized and displays a distinctive spectrum. Identifying Atoms Using the Spectroscope
- 26. • An atomic spectrum is an element's fingerprint—a pattern of discrete (distinct) frequencies of light. • Discoveries of the 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. Identifying Atoms Using the Spectroscope
- 27. • Spectral lines of various elements Identifying Atoms Using the Spectroscope
- 28. • Atomic excitation Identifying Atoms Using the Spectroscope
- 29. • There are three transitions in an atom. The sum of the energies (and frequencies) for jumps A and B equals the energy (and frequency) for jump C. Identifying Atoms Using the Spectroscope
- 31. The Quantum Hypothesis • Max Planck, a German physicist, hypothesized that warm bodies emit radiant energy in discrete bundles called quanta. The energy in each energy bundle is proportional to the frequency of the 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, with each quantum called a photon.
- 32. 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 and a particle, depending on the experiment. • The amount of energy in a photon is directly proportional to the frequency of light: E ∼ ƒ
- 33. 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.
- 34. The Quantum Hypothesis • Using the quantum hypothesis (continued): – 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.
- 35. 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.
- 36. 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.
- 37. Electron Waves • An electron's wave nature explains why electrons in an atom are restricted to particular energy levels. The 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 of two wavelengths, and so on.
- 38. Electron Waves • For a fixed circumference, only an integral number of standing waves can occur, and likewise for the paths of electrons about the nucleus.
- 39. The Shell Model • Cutaway view of shells in the shell model of the atom
- 40. The Shell Model • Shell model showing the first three periods of the periodic table