Atomic Structure
Defining the Atom
Defining the AtomEssential Question
How did the concept of the atom move from philosophy to science?
Defining the Atom
• An atom is the smallest particle of an element that retains its identity in a chemical reaction.
Philosophy or Science?
• Democritus believed that matter consisted of tiny, indivisible, unchangeable particles called atoms.
• Until recent centuries the existence of the atom was a philosophical question rather than a scientific one.
Philosophy to Science
• Greater accuracy in measurement allowed for experiments which indicated the existence of atoms.
• Law of Conservation of Mass, Law of Definite Proportions, and Law of Multiple Proportions were clues to the existence of atoms.
Dalton’s Atomic Theory
• 1) All elements are composed of tiny indivisible particles called atoms.
• 2) Atoms of one element are identical. The atoms of one element are different from those of any other element.
Dalton’s Theory (cont.)
• 3) Atoms of different elements can physically mix together, or can chemically combine in simple whole-number ratios to form compounds.
• 4) Chemical reactions occur when atoms are separated, joined, or rearranged. Atoms of one element, however, are never changed into atoms of another element as a result of a chemical reaction.
Sizing Up the Atom
• A copper penny contains about 2.4 x 1022 atoms.
• Earth’s population measures about 6 x 109 people.
• There are about 4 x 1012 times as many atoms in a penny as people on the earth.
Sizing Up the Atom
• A line of 100,000,000 copper atoms side by side would be only 1 cm long.
Sizing Up the Atom
• Sizes of atoms are expressed in picometers (trillionths of a meter).
• 1 cm is the same fractional part of 600 miles as 100 picometers is of 1 cm!
Sizing Up the Atom
• Dry air contains 0.002% Neon.
• Yet there are about 5 x 1017 atoms of Neon in every breath you inhale!
• That’s 500,000,000,000,000,000 atoms!
Sizing Up the Atom
• In Summary:
• Atoms are itty-bitty.
Atomic Structure
Structure of the Nuclear Atom
Structure of the Nuclear Atom
Essential Question
What are the three principle sub-atomic particles and what part do they play in the structure of the atom?
The Structure of the Nuclear Atom
• Atoms are known to be divisible.
• Sub-atomic particles include neutrinos, quarks, baryons, hadrons, fermions, bosons, mesons, leptons, photons, gravitons, gluons, electrons, protons and neutrons.
• Not to mention futons, croutons, Nissans and morons.
The Structure of the Nuclear Atom
We will be most concerned with protons, neutrons and electrons.
Subatomic ParticlesThe Electron
• Discovered by Thomson
• Negatively charged particles
• Relative charge = 1–
• Symbol = e–
• 1 / 1840 the mass of a proton
• Reside outside of the nucleus
Subatomic ParticlesThe Proton
• Discovered by Goldstein
• Positively charged particles
• Relative charge = 1+
• Symbol = p+
• Mass of 1 atomic mass unit (amu)
Subatomic ParticlesThe Neutron
• Discovered by Chadwick
• Neutrally charged particles
• Relative charge = 0
• Symbol = n0
• Mass of 1 atomic mass unit (amu)
The Atomic Nucleus
• Discovered by Rutherford (Gold Foil Experiment)
• The tiny central core of an atom
• Composed of protons and neutrons
The Atomic Nucleus
• The nucleus is highly compacted and extremely dense.
• Most of the mass of atoms is in the nucleus.
• Nuclei have a density of about 2 x 108 metric tons / cm3 !
Atomic Structure
Distinguishing Among Atoms
Distinguishing Among AtomsEssential Question
What part do protons, neutrons and electrons play in the identity and characteristics of atoms?
Atomic Number
• The number of protons determines the identity of the element.
• The number of protons is indicated by the atomic number.
Mass Number
• The mass number indicates the number of protons AND neutrons in the nucleus.
• How do you find the number of neutrons from the mass number of a given element?
Isotopes
• Isotopes are atoms of that have the same number of protons (the same element) but have different numbers of neutrons.
• They, therefore, have different mass numbers and different masses.
Atomic Mass
• Masses of atoms are incredibly small and impractical to express in grams
• It is more practical to compare the masses of atoms to a different standard.
Atomic Mass Unit
• An AMU (atomic mass unit) is defined as 1/12 the mass of a Carbon-12 atom.
• Since the Carbon-12 atom is made up of 6 protons and 6 neutrons, an AMU is approximately equal to the mass of one proton or one neutron.
Atomic Mass
• An element’s atomic mass is a weighted average mass of all the atoms in a naturally occurring sample of the element.
• To calculate atomic mass, multiply the mass of each isotope by its percent abundance (expressed as a decimal) and add the products.