Freeman Quillin Allison

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BIOLOGICAL SCIENCEFIFTH EDITION

2Water and Carbon: The Chemical Basis of Life

Lecture Presentation by Cindy S. Malone, PhD, California State University Northridge

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Basic Atomic Structure

§ Atoms are composed of– Protons—positively charged particles– Neutrons—neutral particles– Electrons—negatively charged particles

§ Protons and neutrons are located in the nucleus§ Electrons are found in orbitals surrounding the

nucleus

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Figure 2.1 (a) Diagrams of atoms

(b) Most of an atom’s volume is empty space.

HydrogenCarbon

Electron

ProtonNeutron

Nucleus

If an atom occupied the samevolume as this stadium, the nucleuswould be about the size of a pea

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Elements—The Building Blocks of Chemical Evolution

§ The atomic number– Every different atom has a characteristic number of

protons in the nucleus

§ Atoms with the same atomic number– Have the same chemical properties – Belong to the same element

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Elements—The Building Blocks of Chemical Evolution

§ Isotopes are– Forms of an element with different numbers of

neutrons

§ The mass number is– The number of protons + neutrons of the most

common isotope

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Figure 2.2

Mass number(number of protons+ neutrons)

Atomic number(number of protons)

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Electron Arrangement around the Nucleus

§ Electrons move around atomic nuclei in specific regions called orbitals– Each orbital can hold up to two electrons

– Orbitals are grouped into levels called electron shells

§ Electron shells are numbered 1, 2, 3; or lettered K, L, M, and so on – Numbers indicate their relative distance from the

nucleus – Smaller numbers are closer to the nucleus

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Electron Arrangement around the Nucleus

§ Each electron shell contains a specific number of orbitals

– An electron shell comprising a single orbital can hold up to two electrons

– A shell with four orbitals can contain up to eight electrons

§ The electrons of an atom fill the innermost shells first, then fill the outer shells

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Figure 2.3

Hydrogen

Lithium

Sodium

Beryllium

Magnesium

Boron

Aluminum

Carbon

Silicon

Nitrogen

Phosphorus

Oxygen

Sulfur

Fluorine

Chlorine

Helium

Neon

Argon

Valence = 1 2 3 4 3 2 1 0

Electron shell

Nucleus

The highlightedelements are the mostabundant elementsfound in organisms

Number of unpairedelectrons = valence

Unfilled electron orbitals allow formation of chemical bonds

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Chemical Bonding

– Atoms– Are most stable when each electron orbital is filled

– Molecules– Are substances held together by covalent bonds

– Covalent bond– Each atom’s unpaired valence electrons are shared by

both nuclei to fill their orbitals

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Covalent Bonds

– Nonpolar covalent bond– Electrons are evenly shared between two atoms – The bond is symmetrical

Hydrogen atoms eachhave one unpairedelectron

Covalent bond

H2 molecule hastwo sharedelectrons

Figure 2.4

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Polar Covalent Bonds

§ Electrons are not always shared equally – Polar covalent bond

– Electrons are asymmetrically shared– Differences in electronegativity dictate how electrons are

distributed in covalent bonds

§ An atom in a molecule with a high electronegativity– Holds electrons more tightly

—has a partial negative charge (d–)– The other atom

—will have a partial positive charge (d+)

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Figure 2.5

(a) Nonpolar covalent bond in hydrogen molecule

(b) Polar covalent bonds in water molecule

Electrons are halfway betweenthe two atoms, shared equally

Electrons are not shared equally(O is more electronegative thanH), so partial charges exist onthe O and H atoms