Atoms and Elements Study Notes

Learning Unit 2: Atoms and Elements

Introduction

• Learning Goal:
  • Name an element and write its correct symbol.
  • From the symbol, derive the correct name.

Outline of Topics

1. Element and Symbols
2. The Periodic Table
3. The Atom
4. Atomic Number and Mass Number
5. Isotopes and Atomic Mass
6. Electron Energy Levels
7. Trends in the Periodic Properties

Element and Symbols

• Definition of Elements:
  • All matter is composed of elements.
  • Elements are pure substances from which all things are built.
  • Elements cannot be broken down into simpler substances.
• Chemical Symbols:
  • Learn the chemical symbols (one or two-letter abbreviations) for all elements.
  • Be able to write the chemical symbols for all elements in the periodic table.
• Practice:
  • Complete the questions and problems at the end of each section in the textbook.

The Atom

• Definition of Atom:
  • Atoms are the building blocks of matter, the basic units from which nature builds.
  • An atom is the smallest identifiable unit of an element and cannot be broken down into simpler substances.
• Significance of Atoms:
  • The properties of atoms determine the properties of matter.
  • Atoms are fundamental building blocks of everything we see, feel, hear, and experience.

The Periodic Table

• Definition:
  • The periodic table is a systematic arrangement of all known chemical elements (118 elements).
  • Based on their atomic number, electron configuration, and recurring chemical properties.
• Utility:
  • Essential tool in chemistry to predict the properties of elements and their compounds.
• Organization:
  • Periods:
  • Horizontal rows (7 periods total), elements have the same number of electron shells.
  • Groups:
  • Vertical columns (18 groups total), elements in the same group have similar chemical properties due to the same number of valence electrons.

Properties of Elements

Categories of Elements

• Metals:
  • Hard, shiny materials with good thermal and electrical conductivity.
• Nonmetals:
  • Lack metallic properties; often brittle and poor conductors of heat and electricity.
• Metalloids:
  • Exhibit qualities of both metals and nonmetals, more conductive than nonmetals but not as much as metals.

Atomic Structure

Composition of the Atom

• Subatomic Particles:
  • Electrons (e−): Negative charge, found in orbitals, involved in bonding and reactions.
  • Protons (p+): Positive charge, found in the nucleus, defines the element (atomic number, Z).
  • Neutrons (n): Neutral charge, in nucleus, stabilize the atom.
• Nucleus:
  • Dense core composed of protons and neutrons, contains most of the mass of the atom.

Atomic Number and Mass Number

• Atomic Number (Z):
  • The number of protons in an atom, distinguishes one element from another.
  • In a neutral atom, the number of protons equals the number of electrons.
• Mass Number (A):
  • Total number of protons and neutrons in the nucleus.
  • $A = ext{Number of Protons} + ext{Number of Neutrons}$
  • Example: An oxygen atom with 8 protons and 8 neutrons has a mass number of 16.

Isotopes

• Definition of Isotopes:
  • Atoms of the same element with the same number of protons but different numbers of neutrons, leading to different mass numbers.
• Chemical Behavior:
  • Isotopes do not change the chemical behavior of an atom.
• Examples:
  • Isotopes of Carbon:
  • Carbon-12: 6 protons + 6 neutrons
  • Carbon-13: 6 protons + 7 neutrons
  • Carbon-14: 6 protons + 8 neutrons

Atomic Mass

• Definition:
  • The weighted average of the masses of all naturally occurring isotopes of an element.
• Calculation of Atomic Mass:
  • $ext{Atomic Mass (amu)} = \frac{(A<em>1 imes ext{fractional abundance}</em>1) + (A<em>2 imes ext{fractional abundance}</em>2) + ext{…}}{100}$
• Example:
  • For chlorine:
  • $ext{Cl}^{35}$ (75.76%) and $ext{Cl}^{37}$ (24.24%)
  • $ext{Average Atomic Mass} = 34.969(0.7576) + 36.699(0.2424) = 35.453 ext{ amu}$

Electron Energy Levels

• Energy Levels:
  • Electrons occupy specific energy levels or orbitals around the nucleus.
  • Each level can hold a maximum determined by the formula: $2(n^2)$, where n is the level number.
  • Valence Electrons:
  • Electrons in the outermost energy level, involved in bonding.
• Organization of Electrons:
  • The first level holds 2 electrons, the second level holds 8, third level typically holds 8 or 18, and the fourth level can hold more.

Trends in Periodic Properties

• Atomic Size:
  • Increases from top to bottom in a group due to additional energy levels.
  • Decreases from left to right due to increased nuclear charge pulling electrons closer.
• Electronegativity:
  • Increases left to right and decreases top to bottom.
• Ionization Energy:
  • Amount of energy required to remove an outer electron; increases left to right and decreases top to bottom.

Summary of Periodic Trends

• Trends in Valence Electrons:
  • Remain constant down a group, increase across a period.
• Trends in Atomic Radius:
  • Increase down a group and decrease across a period due to increased proton count attracting electrons.
• Ionization Energy and Metallic Character:
  • Ionization energy decreases down a group and increases across a period, while metallic character increases down a group and decreases across a period.