Campbell Biology in Focus - Chapter 2

Campbell Biology in Focus Third Edition Chapter 2 The Chemical Context of Life Copyright © 2022, 2020, 2016 Pearson Education, Inc. All Rights Reserved The Importance of Chemistry to Life How Does Life on Ear...

Campbell Biology in Focus Third Edition Chapter 2 The Chemical Context of Life Copyright © 2022, 2020, 2016 Pearson Education, Inc. All Rights Reserved The Importance of Chemistry to Life How Does Life on Earth Depend on the Chemistry of Water? Copyright © 2022, 2020, 2016 Pearson Education, Inc. All Rights Reserved  Matter Consists of Chemical Elements in Pure Form and in Combinations Called Compounds Organisms are composed of matter Matter is anything that takes up space and has mass Matter is made up of elements An element is a substance that cannot be broken down to other substances by chemical reactions Copyright © 2022, 2020, 2016 Pearson Education, Inc. All Rights Reserved Figure 2.2:The Emergent Properties of a Compound A compound is a substance consisting of two or more elements in a fixed ratio A compound has emergent properties, characteristics different from those of its elements Copyright © 2022, 2020, 2016 Pearson Education, Inc. All Rights Reserved Table 2.1: Elements in the Human Body Of the 90+ natural elements, about 20-25% are essential elements needed by an organism to live a healthy life and reproduce Trace elements are required in only minute quantities For example, in vertebrates, iodine (I) is required for normal activity of the thyroid gland In humans, an iodine deficiency can cause goiter Copyright © 2022, 2020, 2016 Pearson Education, Inc. All Rights Reserved An Element’s Properties Depend on the Structure of Its Atoms Each element consists of a certain type of atom, different from the atoms of any other element An atom is the smallest unit of matter that still retains the properties of an element Simplified Models of a Helium (H e) Atom Copyright © 2022, 2020, 2016 Pearson Education, Inc. All Rights Reserved  Subatomic Particles Atoms are composed of smaller parts called subatomic particles Relevant subatomic particles include – neutrons (no electrical charge) – protons (positive charge) – electrons (negative charge) Neutrons and protons form the atomic nucleus Electrons form a “cloud” of negative charge around the nucleus Neutron mass and proton mass are almost identical and are measured in daltons Copyright © 2022, 2020, 2016 Pearson Education, Inc. All Rights Reserved  Atomic Number and Atomic Mass Atoms of the various elements differ in number of subatomic particles An element’s atomic number is the number of protons in its nucleus An element’s mass number is the sum of protons plus neutrons in the nucleus Atomic mass, the atom’s total mass, can be approximated by Image from: the mass number https://nuclearsafety.gc.ca/eng/resources/radiation/introduction-to- radiation/atoms-nuclides-radioisotopes.cfm?pedisable=true Copyright © 2022, 2020, 2016 Pearson Education, Inc. All Rights Reserved Atomic Number and Atomic Mass Because neutrons and protons each have a mass of approximately 1 dalton, we can estimate the atomic mass (total mass of one atom) of sodium as 23 daltons Copyright © 2022, 2020, 2016 Pearson Education, Inc. All Rights Reserved Isotopes All atoms of an element have the same number of protons but may differ in number of neutrons Isotopes are two atomic forms of an element that differ in number of neutrons These isotopes of Hydrogen and Carbon are STABLE. Copyright © 2022, 2020, 2016 Pearson Education, Inc. All Rights Reserved One of the best explanations I’ve seen of this… “When there is an imbalance between protons and neutrons, usually when the ratio of neutrons to protons is too low, the isotope will want to transform itself into a more stable form – a different atom. When this happens, the atom decreases its mass by emitting alpha particles, beta particles, positrons and/or gamma rays, but some may also gain stability through spontaneous fission or electron capture. It is a spontaneous process that is known as radioactive decay.” Copyright © 2022, 2020, 2016 Pearson Education, Inc. All Rights Reserved The take-home message right here is to be aware of the energy of subatomic particles and their placement in atoms. The prior two slides contain images and information from: https://nuclearsafety.gc.ca/eng/resources/radiation/introduction-to-r adiation/atoms-nuclides-radioisotopes.cfm?pedisable=true  Isotopes Radioactive isotopes decay spontaneously, giving off particles and energy Some applications of radioactive isotopes in biological research are – dating fossils – tracing atoms through metabolic processes A PE T Scan, a Medical – diagnosing medical disorders Use for Radioactive Radiation from decaying isotopes also Isotopes pose a hazard to life by damaging cellular molecules Copyright © 2022, 2020, 2016 Pearson Education, Inc. All Rights Reserved The Energy Levels of Electrons Energy is the capacity to cause change Potential energy is the energy that matter has because of its location or structure The electrons of an atom have potential energy due to their distance from the nucleus Changes in potential energy occur in steps of fixed amounts An electron’s energy level is correlated with its average distance from the nucleus Copyright © 2022, 2020, 2016 Pearson Education, Inc. All Rights Reserved  The Energy Levels of Electrons (2 of 2) Electrons are found in different electron shells, each with a characteristic average distance from the nucleus The energy level of each shell increases with distance from the nucleus Electrons can move to higher or lower shells by absorbing or releasing energy, respectively Copyright © 2022, 2020, 2016 Pearson Education, Inc. All Rights Reserved Electron Distribution and Chemical Properties The chemical behavior of an atom is determined by the distribution of electrons in electron shells The periodic table of the elements shows the electron distribution for each element (Let’s look at an electron- centric representation!) Also, note to all of us: please take a look at notes under some of the slides - the publisher provides text under some with information you might find helpful from the book! Copyright © 2022, 2020, 2016 Pearson Education, Inc. All Rights Reserved Figure 2.6 Electron Distribution Diagrams for the First 18 Elements in the Periodic Table Copyright © 2022, 2020, 2016 Pearson Education, Inc. All Rights Reserved Electron Distribution and Chemical Properties of Atoms Chemical behavior of an atom depends mostly on the number of electrons in its outermost shell, or valence shell Valence electrons are those that occupy the valence shell The reactivity of an atom arises from the presence of one or more unpaired electrons in the valence shell Atoms with completed valence shells are unreactive, or inert This is one of our most important slides to date!! Copyright © 2022, 2020, 2016 Pearson Education, Inc. All Rights Reserved  The Formation and Function of Molecules Depend on Chemical Bonding Between Atoms Atoms with incomplete valence shells can share or transfer valence electrons with certain other atoms This usually results in atoms staying close together, held by attractions called chemical bonds Completed outermost C has 6 electrons, 2 in the O has how many valence shell innermost shell, then 4 on electrons? What does this the outermost mean? Where are they? Copyright © 2022, 2020, 2016 Pearson Education, Inc. All Rights Reserved Figure 2.7: Formation of a Covalent Bond A covalent bond is the sharing of a pair of valence electrons by two atoms In a covalent bond, the shared electrons count as part of each atom’s valence shell Two or more atoms held together by covalent bonds constitute a molecule Copyright © 2022, 2020, 2016 Pearson Education, Inc. All Rights Reserved Covalent Bonds The molecular formula, for example, H2, indicates that the molecular consists of two hydrogen atoms. Electron sharing can be shown by an electron distribution diagram or a structural formula For example, in the structural formula, H – H, the line represents a pair of shared electrons. Copyright © 2022, 2020, 2016 Pearson Education, Inc. All Rights Reserved Covalent Bonds Pure elements are composed of molecules of one type of atom, such as H2 and O2 Molecules composed of a combination of two or more types of atoms, such as H2O or CH4 , are called compounds In a nonpolar covalent bond, the atoms share the electrons equally In a polar covalent bond, one atom is more electronegative, and the atoms do not share the electron equally Unequal sharing of electrons causes a partial positive or negative charge for each atom or molecule Copyright © 2022, 2020, 2016 Pearson Education, Inc. All Rights Reserved Electronegativity and its impact on covalent bonding Atoms in a molecule attract electrons to varying degrees Electronegativity is an atom’s attraction for the electrons of a covalent bond The more electronegative an atom, the more strongly it pulls shared electrons toward itself Copyright © 2022, 2020, 2016 Pearson Education, Inc. All Rights Reserved Figure 2.9 Polar Covalent Bonds in a Water Molecule Copyright © 2022, 2020, 2016 Pearson Education, Inc. All Rights Reserved