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Chapter 3: Atoms and moles
Honors Chemistry AB
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Atomic theory • All materials are made up of small particles called atoms that cannot be subdivided, created or destroyed by ordinary means
• Atoms of a given element are identical in physical and chemical properties
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Atomic theory • Atoms of different elements combine in whole number ratios to form compounds
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Conservation of mass • Matter can neither be created or destroyed
• In a chemical reaction, all of the atoms that are present at the start of the reaction are present at the end
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Conservation of mass • Atoms do not change from one element to another during the course of a chemical reaction
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Subatomic particles • Atoms are made up of protons, neutrons, and electrons
• Electrons have very little mass and have a negative charge
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Subatomic particles • Protons and neutrons have almost the same mass; protons have a positive charge, and neutrons have no charge.
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The nucleus • Protons and neutrons are found in the center of the atom, in the nucleus.
• The nucleus is held together by a strong force called the nuclear force.
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The nucleus • The nuclei of large atoms tend to be more unstable than the nuclei of small atoms
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Atomic number and mass number • The number of protons in the nucleus is called the atomic number
• All atoms of the same element have the same atomic number
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Atomic number and mass number • The total number of particles in the nucleus is called the mass number, which varies from atom to atom of the same element
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Energy levels • Electrons are located on the outside of the atoms in regions called energy levels
• Electrons close to the center of the atom tend to have lower energy than those far away
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Energy levels • Electrons tend to go to the lowest possible energy level
Hydrogen Emission Possibilities
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Emission Lines of Certain Elements
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Electron configurations • No two electrons in an atom can share energy levels
• The location of the electrons, or their energy levels, is identified by their quantum numbers
Quantum Numbers of the First 30 Atomic Orbitals
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Electron configurations • The Pauli Exclusion Principle states that no two electrons in an atom can have the same quantum state
Electron Configurations
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Counting atoms • Because the mass of an atom is so small, chemist use the mole as a unit for measuring chemicals
• The mole is based on the number of atoms in 12 grams of carbon-12
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Counting atoms • One mole of any pure element has a mass in grams equal to the atomic weight of that element
1. Which of these is always equal to the number of protons in an atom?
A. the mass number B. the number of isotopes C. the number of neutrons D. the number of electrons
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2. Which of these events occurs when an electron in an excited state returns to its
ground state?
A. Light energy is emitted. B. Energy is absorbed by the atom. C. The atom undergoes spontaneous
decay. D. The charge increases because an
electron is added.
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3. Which of the following represents an electron configuration of a calcium atom,
whose atomic number is 20? 3.
A. 1s22s22p63s23p64s2 B. 1s22s22p63s23p64s3 C. 1s22s22p63s13p64s23d1 D. 1s22s22p63s23d8
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4. The model of the atom has changed over time because
A. earlier models were proven to be wrong B. electrons do not revolve around the nucleusC. as new properties of atoms were
discovered, models had to be revised to account for those properties
D. new particles were discovered, so the model had to be changed to explain how they could exist
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5. Why do scientists need models as opposed to directly observing electrons?
A. Models can be changed. B. There is no technology that allows direct
observation of electrons. C. The charges on the electrons and protons
interfere with direct observation of the atom. D. Scientists cannot measure the speed of
electrons with sufficient accuracy to determine which model is correct.
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