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Atomic Structure and Dalton's Atomic Theory

Learn about the atomic structure and Dalton's Atomic Theory, including the concept of atoms, subatomic particles, isotopes, and the laws of conservation of mass and constant composition.

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Atomic Structure and Dalton's Atomic Theory

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  1. Unit 2: Atoms, Ions and Ionic Compounds Atoms Nuclear Symbols Ions Names and Formulas of Ionic Compounds Atomic Models

  2. Atoms • Atom:the smallest representative particle of an element • Democritus (460-370 BC) first suggested the concept of atoms • Rejected by Aristotle and the Greek thinkers in favor of fire, earth, air, water & heavenly ether as the basic components of matter • Dalton’s Atomic Theory (1803 – 1807) led to the re-emergence of the idea that atoms are the building blocks of all matter.

  3. Dalton’s Atomic Theory • Dalton:Each element is composed of extremely small, indivisible, indestructible particles called atoms. • We now know that atoms are not indivisible. Each atom contains subatomic particles including: • Protons • Neutrons • Electrons

  4. Dalton’s Atomic Theory • Dalton: All atoms of a given element are identical to each other in mass and other properties but different from the atoms of all other elements. • Dalton wasn’t quite correct. Some atoms of the same element have different masses. They are, however, chemically identical to each other.

  5. Dalton’s Atomic Theory • Dalton: Atoms of one element cannot be changed into atoms of another element. • Dalton was correct when talking about ordinary chemical reactions. • Nuclear reactions, however, can convert one element into another.

  6. Dalton’s Atomic Theory • Dalton: Compounds are formed when atoms of more than one element combine. • A given compound contains atoms combined in definite whole number ratios. • Fe2O3(rust) • H2O (water) • H2O2 (hydrogen peroxide) • These ratios are the same for any sample of a particular pure substance.

  7. Dalton’s Atomic Theory • Dalton’s Atomic Theory explained several laws that were already known: • Law of Conservation of Mass (Matter) • Matter cannot be created or destroyed. • The total mass of substances present before and after a chemical reaction is constant. 5.0 g of reactants  5.0 g of products

  8. Dalton’s Atomic Theory • Dalton’s Atomic Theory explained several laws that were already known (cont.): • Law of Constant Composition • A given compound always contains the same relative numbers and kinds of atoms. Ethyl alcohol C2H6O Methane always contains 25% H and 75% C

  9. Dalton’s Atomic Theory • Dalton’s atomic theory was based on evidence gained in the macroscopic world. • Today, scientists can actually see individual atoms using special techniques. • See figure 2.2 of your text for a scanning tunneling micrograph of gallium arsenide • http://www.almaden.ibm.com/vis/stm/gallery.html

  10. Modern View of Atomic Structure • Atoms are tiny spherical particles with a: • nucleus: • The small positively charged center of an atom • contains most of the mass of the atom • contains two important subatomic particles: protons and neutrons • diffuse, negatively charged outer layer that contains a third type of subatomic particle, the electron

  11. Modern View of Atomic Structure • Most of the volume of an atom is empty space. Diffuse layer containing electrons in constant motion

  12. Modern View of Atomic Structure • Important subatomic particles • Protons • positively charged subatomic particles found in the nucleus • Neutrons • neutral (uncharged) subatomic particles found in the nucleus • Electrons • very small, negatively charged subatomic particles found in a diffuse layer surrounding the nucleus

  13. Modern View of Atomic Structure Relative Particle Symbol Charge Mass Proton p +1 1.0073 amu Neutron n 0 1.0087 amu Electron e- -1 5.468 x 10-4 amu amu (atomic mass unit): • a unit used to express very small masses • 1.66054 x 10-24 g

  14. Modern View of Atomic Structure • If all atoms are composed of protons, neutrons, and electrons, then why is an atom of gold different from an atom of carbon? • The atoms of each element have a unique number of protons in the nucleus. • Atomic number: the number of protons in the nucleus • Carbon: 6 p (atomic # = 6) • Gold: 79 p (atomic # = 79)

  15. Modern View of Atomic Structure • Atoms are electrically neutral (i.e. the total charge on an atom is zero). • Each atom contains the same number of protons and electrons. • Helium: 2 p, 2 e- • Magnesium: 12 p, 12 e- • Iron: 26 p, 26 e- • The atomic number tells you both the number of protons and electrons in an atom.

  16. Modern View of Atomic Structure • The number of neutrons found in atoms of the same element can vary. • Hydrogen atoms: • Always 1 p and 1 e- • May have 0, 1, or 2 n • Atoms that have the same number of protons but different numbers of neutrons are referred to as isotopes of an element.

  17. Modern View of Atomic Structure • Isotopes: • Atoms with the same number of protons but different numbers of electrons • Atoms with the same atomic number but different mass numbers • Mass number = # protons + # neutrons • a calculated value • A hydrogen atom with 2 neutrons: • Mass number = 1 p + 2 n = 3

  18. Modern View of Atomic Structure • Atoms are small pieces of matter and therefore have mass. • Atomic weight (atomic mass) • The average atomic mass of an element expressed in amu • Calculated using the weighted average of each of the naturally occurring isotopes of an element

  19. Atomic number Atomic weight (atomic mass) 5 10.81 B Elementsymbol electronegativity en = 2.04 boron Element name Modern View of Atomic Structure • Finding information on the periodic table: • So where do you find mass number???

  20. Nuclear Symbols • Information about a particular isotope of an element is often depicted using its nuclear symbol: • For example, the nuclear symbol for a hydrogen atom with 2 neutrons would be: • Since atoms are electrically neutral, the charge is zero and is not shown.

  21. Nuclear Symbols Example: Use the following nuclear symbol to determine the information listed below: Mass number = Atomic number = # protons = # neutrons = # electrons = Charge =

  22. Nuclear Symbols Example: An atom has 21 neutrons and 19 protons. Identify the missing information. Mass number = Atomic number = # protons = # neutrons = # electrons = Charge = Nuclear symbol:

  23. Ions • An atom can gain or lose electrons to become a charged particle called an ion. • A positively or negatively charged chemical particle that is formed when an atom (or group of atoms) gains or loses one or more electrons • Cation:a positively charged ion • Formed when an atom loses one or more electrons • Anion:a negatively charged ion • Formed when an atom gains one or more electrons

  24. 3p 4n 3p 4n e- e- e- e- e- Ions Formation of a cation + e- Lithium atom 3 p, 4 n, 3 e- Lithium ion (cation) 3 p, 4 n, 2 e- Net charge = 0 Net charge = +1

  25. 8p 8n 8e- 8p 8n + 2e- 10e- Ions Formation of an anion Oxygen atom 8p, 8 n, 8e- Oxide ion (anion) 8p, 8n, 10e- Net charge = -2 Net charge = 0

  26. 137 56 2+ Ba Nuclear Symbols of Ions • Since protons  electrons for an ion, the charge of the ion must be shown in the nuclear symbol. • Charge = # p - # e- • Show the magnitude of the charge first and then show the sign (+ or -)

  27. 137 56 2+ Ba Nuclear Symbols of Ions Example: Use the following nuclear symbol to determine the missing information. Mass number = Atomic number = # protons = # neutrons = # electrons = Charge =

  28. Nuclear Symbols of Ions Example: Write the nuclear symbol. 1) 53 p, 74 n, 54 e- • 23 e-, 30 n, charge = +3 • 10 e-, 10 n, charge = -1

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