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Elements, Atoms & Ions

Elements, Atoms & Ions. Elements. Over 112 known 88 found in nature others man-made Abundance is the percentage found in nature oxygen most abundant element (by mass) on earth and in the human body abundance and form varies in different parts of the environment

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Elements, Atoms & Ions

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  1. Elements, Atoms & Ions

  2. Elements • Over 112 known • 88 found in nature • others man-made • Abundance is the percentage found in nature • oxygen most abundant element (by mass) on earth and in the human body • abundance and form varies in different parts of the environment • Each element has a unique symbol • The symbol of an element may be one letter or two • if two letters, the second is lower case

  3. Dalton’s Atomic Theory Elements are composed of atoms • tiny, hard, unbreakable, spheres All atoms of a given element are identical • all carbon atoms have the same chemical and physical properties Atoms of a given element are different from those of any other element • carbon atoms have different chemical and physical properties than sulfur atoms

  4. Dalton’s Atomic Theory • Atoms of one element combine with atoms of other elements to form compounds. • Law of Constant Composition • all samples of a compound contain the same proportions (by mass) of the elements • Chemical Formulas

  5. Dalton’s Atomic Theory Atoms are indivisible in a chemical process. • all atoms present at beginning are present at the end • atoms are not created or destroyed, just rearranged • atoms of one element cannot change into atoms of another element • cannot turn Lead into Gold by a chemical reaction

  6. Formulas Describe Compounds • a compound is a distinct substance that is composed of atoms of two or more elements • describe the compound by describing the number and type of each atom in the simplest unit of the compound • molecules or ions • each element represented by its letter symbol • the number of atoms of each element is written to the right of the element as a subscript • if there is only one atom, the 1 subscript is not written • polyatomic groups are placed in parentheses • if more than one

  7. Are Atoms Really Unbreakable? • J.J. Thomson investigated a beam called a cathode ray • he determined that the ray was made of tiny negatively charged particles we call electrons • his measurements led him to conclude that these electrons were smaller than a hydrogen atom • if electrons are smaller than atoms, they must be pieces of atoms • if atoms have pieces, they must be breakable • Thomson also found that atoms of different elements all produced these same electrons

  8. The Electron • Tiny, negatively charged particle • Very light compared to mass of atom • 1/1836th the mass of a H atom • Move very rapidly within the atom

  9. Thomson’s Plum Pudding Model Atom breakable!! Atom has structure Electrons suspended in a positively charged electric field • must have positive charge to balance negative charge of electrons and make the atom neutral mass of atom due to electrons atom mostly “empty” space • compared size of electron to size of atom

  10. Rutherford’s Gold Foil Expt • How can you prove something is empty? • put something through it • use large target atoms • use very thin sheets of target so do not absorb “bullet” • use very small particle as bullet with very high energy • but not so small that electrons will affect it • bullet = alpha particles, target atoms = gold foil •  particles have a mass of 4 amu & charge of +2 c.u. • gold has a mass of 197 amu & is very malleable

  11. Figure 4.5: Rutherford’s experiment on -particle bombardment of metal foil.

  12. Rutherford’s Results • Over 98% of the  particles went straight through • About 2% of the  particles went through but were deflected by large angles • About 0.01% of the  particles bounced off the gold foil

  13. Rutherford’s Nuclear Model The atom contains a tiny dense center called the nucleus • the volume is about 1/10 trillionth the volume of the atom The nucleus is essentially the entire mass of the atom The nucleus is positively charged • the amount of positive charge of the nucleus balances the negative charge of the electrons The electrons move around in the empty space of the atom surrounding the nucleus

  14. Figure 4.9: A nuclear atom viewed in cross section.

  15. Structure of the Nucleus • The nucleus was found to be composed of two kinds of particles • Some of these particles are called protons • charge = +1 • mass is about the same as a hydrogen atom • Since protons and electrons have the same amount of charge, for the atom to be neutral there must be equal numbers of protons and electrons • The other particle is called a neutron • has no charge • has a mass slightly more than a proton

  16. The Modern Atom • We know atoms are composed of three main pieces - protons, neutrons and electrons • The nucleus contains protons and neutrons • The nucleus is only about 10-13 cm in diameter • The electrons move outside the nucleus with an average distance of about 10-8 cm • therefore the radius of the atom is about 105 times larger than the radius of the nucleus

  17. Isotopes • All atoms of an element have the same number of protons • The number of protons in an atom of a given element is the same as the atomic number • found on the Periodic Table • Atoms of an element with different numbers of neutrons are called isotopes • All isotopes of an element are chemically identical • undergo the exact same chemical reactions • Isotopes of an element have different masses • Isotopes are identified by their mass numbers • mass number = protons + neutrons

  18. Section 4-3 Section 4.3 How Atoms Differ 3. How Atoms Differ The number of protons and the mass number define the type of atom.

  19. Section 4-3 Isotopes and Mass Number Math used more in chemistry used more in physics Problem: How many neutrons does Carbon-14 have? Convert isotope name to symbol form. 2. Subtract lower number from higher number.

  20. Section 4-3 Mass of Atoms • The atomic mass of an element is the weighted average mass of the isotopes of that element. The mass number on the periodic table is the average mass. That’s why they have decimal places and are not whole numbers like mass numbers! • Always use 2 decimal places. • Round the third decimal place.

  21. Section 4-3 • A • B • C • D An unknown element has 19 protons, 19 electrons, and 3 isotopes with 20, 21 and 22 neutrons. What is the element’s atomic number? A.38 B.40 C.19 D.unable to determine

  22. Section 4-3 • A • B • C • D Elements with the same number of protons and differing numbers of neutrons are known as what? A.isotopes B.radioactive C.abundant D.ions

  23. Section 4-4 Unstable Nuclei and Radioactive Decay 4. Unstable Nuclei and Radioactive Decay Unstable atoms emit radiation to gain stability.

  24. Section 4-4 Radioactivity • Nuclear reactions can change one element into another element. • In the late 1890s, scientists noticed some substances spontaneously emitted radiation, a process they called radioactivity. • The rays and particles emitted are called radiation. • A reaction that involves a change in an atom's nucleus is called a nuclear reaction.

  25. Section 4-4 Radioactive Decay • Unstable nuclei lose energy by emitting radiation in a spontaneous process called radioactive decay. • Unstable radioactive elements undergo radioactive decay thus forming stable nonradioactive elements.

  26. Section 4-4 Radioactive Decay

  27. Section 4-4 Alpha Decay Simulation Radioactive Decay • Alpha radiation is made up of positively charged particles called alpha particles. • Each alpha particle contains two protons and two neutrons and has a 2+ charge. • This is a Helium (He) nuclei. Protons dictate the element!

  28. Section 4-4 Radioactive Decay • The figure shown below is a nuclear equation showing the radioactive decay of radium-226 to radon-222. • The mass is conserved in nuclear equations. • Lose 4 amu (top number) and 2 atomic numbers (bottom) To calculate the math, sometimes easier to just write He instead of α

  29. Section 4-4 Radioactive Decay Beta Decay Simulation • Beta radiation is radiation that has a negative charge and emits beta particles. • Each beta particleis an electron with a 1– charge. 0 +1 This one is the tricky one! A neutron splits into a proton and electron. • No loss of mass units, • Proton stays put in nucleus. • But the element increases by one number!

  30. Section 4-4 Radioactive Decay • Gamma rays(γ)are high-energy radiation with no mass and are neutral. • If it doesn’t have mass it isn’t matter so it must be energy! • Electromagnetic (EM) radiation of high frequency and therefore high energy. • Gamma rays are ionizing radiation and are thus biologically hazardous. • In 1903, Ernest Rutherford in 1903 coined "gamma rays"

  31. Section 4-4 Radioactive Decay • Gamma rays account for most of the energy lost during radioactive decay. • Gamma rays are produced alongside other forms of radiation such as alpha or beta, and are produced after the other types of decay occur. • The mechanism is that when a nucleus emits an α or β particle, the daughter nucleus is usually left in an excited state. • Gamma rays are classically produced by the decay from high energy states of atomic nuclei.

  32. Section 4-4 Radioactive Decay Electrons (β) are negative and are attracted to positive plate. Alpha (α) particles are positively charged and heavy so are attracted to negative, but not bent as much due to mass.

  33. Section 4-4 Radioactive Decay Characteristics of Radiation Relative masses are more important than absolute masses for this class.

  34. Section 4-4 Radioactive Decay • Atoms that contain too many or two few neutrons are unstable and lose energy through radioactive decay to form a stable nucleus. • Few exist in nature —most have already decayed to stable forms. • The high atomic number elements are gone! • There could actually be bigger, “new” elements, but they already decayed.

  35. Section 4-4 Radioactive Decay • Calculate the decay type for each atom from Lead-214 to Lead-210 Write nuclear reactions for each decay step. Annotate each type of decay and what it does to an atom. Alpha: down 2 Beta: up 1 Gamma: no effect Mass number Atomic number

  36. Radioactive Decay • In simple terms, when an isotope decays, it turns into a new isotope. • The amount of time it takes for 50% the original isotope to turn into the new isotope is called the half-life.

  37. Radioactive Decay There are two ways to calculate the amount of the original isotope that remains from the original isotope overtime as it decays from a known starting amount. Make a chart (useful for whole number half-life times) Use a formula (useful at any time that has transpired. The amount of an isotope that remains (N) after decaying from an initial amount (N0) of that isotope with a certain half-life (T) after a certain amount of time (t) has transpired.

  38. Chapter Assessment 3 D Two isotopes of an unknown element have the same number of: A.protons B.neutrons C.electrons D.both A and C • A • B • C • D

  39. Chapter Assessment 4 B Lithium has an atomic mass of 6.941 and two isotopes, one with 6 neutrons and one with 7 neutrons. Which isotope is more abundant? A.6Li B.7Li C.Both isotopes occur equally. D.unable to determine • A • B • C • D

  40. Chapter Assessment 5 D What happens when an element emits radioactive particles? A.It gains energy. B.It gains neutrons. C.It loses stability. D.It loses energy. • A • B • C • D

  41. STP 2 B How many neutrons, protons, and electrons does 12454Xe have? A.124 neutrons, 54 protons, 54 electrons B.70 neutrons, 54 protons, 54 electrons C.124 neutrons, 70 protons, 54 electrons D.70 neutrons, 70 protons, 54 electrons • A • B • C • D

  42. Elements (GET OUT YOUR PERIODIC TABLE) • Arranged in a pattern called the Periodic Table • Position on the table allows us to predict properties of the element • Metals • about 75% of all the elements • lustrous, malleable, ductile, conduct heat and electricity • Nonmetals • dull, brittle, insulators • Metalloids • also know as semi-metals • some properties of both metals & nonmetals

  43. The Modern Periodic Table • Elements with similar chemical and physical properties are in the same column • Columns are called Groups or Families • Rows are called Periods • Each period shows the pattern of properties repeated in the next period

  44. Figure 4.11: The periodic table.

  45. The Modern Periodic Table • Main Group = Representative Elements • “A” columns • Transition Elements • all metals • Bottom rows = Inner Transition Elements = Rare Earth Elements • metals • really belong in Period 6 & 7

  46. Group 8 = Noble Gases He, Ne, Ar, Kr, Xe, Rn all colorless gases at room temperature very non-reactive, practically inert found in nature as a collection of separate atoms uncombined with other atoms Noble Metals Ag, Au, Pt all solids at room temperature least reactive metals found in nature uncombined with other atoms Important Groups

  47. Group 7A = Halogens very reactive nonmetals react with metals to form ionic compounds HX all acids Fluorine = F2 pale yellow gas Chlorine = Cl2 pale green gas Bromine = Br2 brown liquid that has lots of brown vapor over it Only other liquid element at room conditions is the metal Hg Iodine = I2 lustrous, purple solid Important Groups - Halogens

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