Chapter 4 Notes: Atoms and Elements

Chemistry in Agriculture

• Farmers utilize chemical tests and apply fertilizers, pesticides, and herbicides to enhance crop production.

Chapter 4 Objectives

• Classify matter as either a pure substance or a mixture.
• Distinguish elements as metals or nonmetals.
• Describe the subatomic particles (protons, neutrons, and electrons) within an atom, including their location, charges, and relative masses.
• Determine the number of protons, neutrons, and electrons in an atom.
• Understand the relationship between an element's atomic mass and its position on the periodic table.
• Understand electron shells, subshells, and orbitals.
• Write electron configurations and create orbital diagrams for atoms.
• Explain the correlation between electron arrangement, group number, and the periodic law.
• Determine the number of valence electrons in an atom.

Chapter 4 Readiness

• Key Math Skills:
  • Using Positive and Negative Numbers in Calculations (1.4B)
  • Calculating a Percentage (1.4C)
• Core Chemistry Skills:
  • Counting Significant Figures (2.2)
  • Rounding Off (2.3)
  • Using Significant Figures in Calculations (2.3)

4.1 Elements and Symbols

• Learning Goal: Given an element's name, write its correct symbol, and vice versa.
• Elements:
  • Pure substances that cannot be broken down into simpler substances.
  • Listed on the inside front cover of the textbook.

Element Names

• Element names are derived from various sources:
  • Planets
  • Mythological figures
  • Minerals
  • Colors
  • Geographic locations
  • Famous people

Chemical Symbols

• Represent names of elements.
• Consist of one or two letters, with the first letter capitalized.
  • One-Letter Symbols:
    • C - Carbon
    • N - Nitrogen
    • F - Fluorine
    • O - Oxygen
  • Two-Letter Symbols:
    • Co - Cobalt
    • Ca - Calcium
    • Al - Aluminum
    • Mg - Magnesium

Chemical Symbols from Latin Names

• Ag (argentum) - silver, from argentum
• Au (aurum) - gold, from aurum

Chemistry Link to the Environment: Carbon

• Carbon (C) atoms can arrange in multiple ways, forming different substances.

Chemistry Link to Health: Mercury

• Mercury (Hg) is a silvery, shiny element that is a liquid at room temperature.
• Mercury can enter the body via:
  • Inhaled mercury vapor
  • Skin contact
  • Ingestion of contaminated food and water
• Mercury can damage the brain and kidneys, cause mental retardation, and decrease physical development by destroying proteins and disrupting cell function.
• Scientists use blood, urine, and hair samples to test for mercury levels in the body.

4.2 The Periodic Table

• Learning Goal: Use the periodic table to identify the group and period of an element and classify it as a metal, nonmetal, or metalloid.
• The periodic table organizes 118 elements into groups with similar properties, ordered by increasing atomic mass.

Groups and Periods

• Elements are arranged according to similar properties.
• Groups: Vertical columns containing elements with similar properties.
• Periods: Horizontal rows of elements, numbered 1-7 from top to bottom.

Group Numbers

• Two systems for numbering groups:
  • A system: Uses the letter A for representative elements (Groups 1A-8A) and the letter B for transition elements (Groups 3B-12B).
  • Alternative system: Assigns numbers 1-18 to all groups from left to right.

Names of Groups

• Alkali Metals: Group 1A (Li, Na, K, Rb, Cs)
• Halogens: Group 7A (Cl, Br, I)

Chemistry Link to Health

• 20 elements are essential for human well-being and survival.
• Oxygen, carbon, hydrogen, and nitrogen make up 96% of body mass.
• Macrominerals (Ca, P, K, Cl, S, Na, Mg) are representative elements involved in:
  • Bone and teeth formation
  • Maintenance of heart and blood vessels
  • Muscle contraction
  • Nerve impulses
  • Acid-base balance of body fluids
  • Regulation of cellular metabolism

4.3 The Atom

• Learning Goal: Describe the electrical charge and location of protons, neutrons, and electrons within an atom.
• Atom: The smallest particle of an element that retains the characteristics of that element.

Dalton’s Atomic Theory

• Atoms are tiny particles of matter.
• Atoms of an element are similar to each other and different from those of other elements.
• Atoms of two or more different elements combine to form compounds.
• Atoms are rearranged to form new combinations in chemical reactions but are never created or destroyed.

Electrical Charges in an Atom

• Protons: Positive (+) charge.
• Electrons: Negative (–) charge.
• Neutrons: No charge (neutral).
• Like charges repel, and unlike charges attract.

J. J. Thomson’s Cathode Ray Experiment

• Thomson discovered that cathode rays contain negatively charged particles, now called electrons, which had a much smaller mass than the atom.
• He proposed the Plum-Pudding model, where protons and electrons were randomly distributed in a positively charged cloud.

Rutherford’s Gold Foil Experiment

• Positively charged particles were aimed at atoms of gold.
• Most particles passed straight through, but some were occasionally deflected.
• Conclusion: Atoms have a small, dense, positively charged nucleus that deflects positive particles that come close.

Structure of the Atom

• The atom consists of:
  • A nucleus containing protons and neutrons.
  • Electrons in a large, empty space around the nucleus.

Mass of the Atom

• Atomic Mass Unit (amu):
  • One amu is equal to one-twelfth of the mass of a carbon-12 atom.
  • Proton mass ≈ 1.007 amu.
  • Neutron mass ≈ 1.008 amu.
  • Electron mass ≈ 0.000549 amu (very small).

Subatomic Particles in the Atom

• Proton:
  • Location: Nucleus
  • Charge: +1
  • Mass (amu): 1.007
• Neutron:
  • Location: Nucleus
  • Charge: 0
  • Mass (amu): 1.008
• Electron:
  • Location: Outside nucleus
  • Charge: -1
  • Mass (amu): 0.000549

4.4 Atomic Number and Mass Number

• Learning Goal: Given the atomic number and mass number of an atom, state the number of protons, neutrons, and electrons.
• All atoms of an element have the same number of protons and the same atomic number.

Atomic Number

• A whole number specific to each element.
• Same for all atoms of an element.
• Equal to the number of protons in an atom.
• Located above the element symbol on the periodic table.
• The atomic number represents the number of protons.
  • H: atomic number = 1; every H atom has 1 proton.
  • C: atomic number = 6; every C atom has 6 protons.
  • Cu: atomic number = 29; every Cu atom has 29 protons.

Atoms Are Neutral

• For neutral atoms, the net charge is zero.
• Number of protons = Number of electrons.
• Example: Aluminum (Al) has 13 protons (+13) and 13 electrons (-13), resulting in a net charge of 0.

Mass Number

• Represents the number of particles in the nucleus.
• Equal to the number of protons + the number of neutrons.
• Always a whole number.
• Does not appear on the periodic table.
  \text{Number of protons = atomic number}
  \text{Number of protons + number of neutrons = mass number}
  \text{Number of neutrons = mass number – atomic number}
• Note: Mass numbers are given for specific isotopes only.

4.5 Isotopes and Atomic Mass

• Learning Goal: Determine the number of protons, electrons, and neutrons in one or more isotopes of an element; calculate the atomic mass of an element using the percent abundance and mass of its naturally occurring isotopes.
• Isotopes:
  • Atoms of the same element.
  • Have different mass numbers.
  • Have the same number of protons but different numbers of neutrons.
  • Can be distinguished by their atomic symbols.

Calculating Atomic Mass

• Atomic mass is the weighted average of all naturally occurring isotopes of an element.
• Found on the periodic table below the element symbol, with two decimal places.
• To calculate atomic mass:
  • Use the experimental percent abundance of each isotope.
  • Multiply the percent abundance by the atomic mass of that isotope.
  • Sum the total mass of each isotope.

4.6 Electron Energy Levels

• Learning Goal: Given the name or symbol of one of the first 20 elements, write the electron arrangement.
• Energy levels (electron shells) are assigned numbers n = 1, 2, 3, 4, \ldots
• Energy increases as the value of n increases.
• Energy levels have a maximum number of electrons equal to 2n^2
  • n = 1: 2(1)^2 = 2
  • n = 2: 2(2)^2 = 8
  • n = 3: 2(3)^2 = 18

Electron Shells

• An electron shell is a region of space about a nucleus containing electrons that have approximately the same energy and spend most of their time approximately the same distance from the nucleus.
• Electrons in the first shell are closer to the nucleus and have lower energy than electrons in the second shell.

Electron Subshells

• An electron subshell is a region of space within an electron shell that contains electrons that have the same energy.
  • s: 2 electrons
  • p: 6 electrons
  • d: 10 electrons
  • f: 14 electrons

Electron Orbitals

• An electron orbital is a region of space within an electron subshell where an electron with a specific energy is most likely to be found.
• An electron orbital can accommodate a maximum of 2 electrons.
• Number of Orbitals
  • s: 1
  • p: 3
  • d: 5

Electron Spin

• As an electron moves within an orbital, it spins on its own axis in either a clockwise or counterclockwise direction.
• When two electrons are present in an orbital, they always have opposite spins.

Rules for Assigning Electrons

• Electron subshells are filled in order of increasing energy.
• Electrons occupy the orbitals of a subshell such that each orbital acquires one electron before any orbital acquires a second electron. All electrons in such singly occupied orbitals must have the same spin.
• No more than two electrons may exist in a given orbital – and then only if they have opposite spins.

Electron Configurations

• A statement showing the number of electrons an atom has in each of its electron subshells.
  • Example: Oxygen: 1s^22s^22p^4

Orbital Diagrams

• A notation that shows how many electrons an atom has in each of its occupied electron orbitals.
  • Example: Oxygen: 1s 2s 2p

4.7 Trends in Periodic Properties

• Learning Goal: Use the electron arrangement of elements to explain the trends in periodic properties such as atomic size, ionization energy, and metallic character.
• Electron arrangements are an important factor in the properties of elements.

Group Number and Valence Electrons

• For representative elements (Groups 1A-8A), chemical properties are due to the number of valence electrons.
• Valence electrons: the number of electrons in the outermost energy level.
• The group number gives the number of valence electrons for the representative elements.

Group Number and Valence Electrons Table

• 1A(1): 1
• 2A(2): 2
• 3A(13): 3
• 4A(14): 4
• 5A(15): 5
• 6A(16): 6
• 7A(17): 7
• 8A(18): 8

Electron-Dot Symbols

• Also known as Lewis structures.
• Represent valence electrons as dots placed on the sides of a symbol.
• Example: Aluminum (Al) has electron arrangement 2,8,3. Three valence electrons are represented as dots.

Atomic Size

• Determined by the atom’s atomic radius, the distance between the nucleus and outermost electrons.
• Increases for representative elements from top to bottom of the periodic table.
• Decreases within a period due to the increased number of protons in the nucleus.

Ionization Energy

• The energy required to remove one of the outermost electrons.
• As the distance from the nucleus to the valence electrons increases, the ionization energy decreases.
  \text{Na}(g) + \text{energy (ionization)} \rightarrow \text{Na}^+(g) + e^-
• Ionization energy decreases down a group and increases across a period from left to right.

Metallic Character

• An element with metallic character is one that loses electrons easily.
• More prevalent in metals on the left side of the periodic table.
• Less for nonmetals, which do not lose electrons easily.
• Metallic character increases going down a group and decreases going from left to right across a period.