Atoms and Elements - Exam Study Guide

Atoms and Elements

Small Size and Large Number of Atoms

• Atoms are incredibly small.
  • If each atom in a pebble were the size of the pebble, the pebble would be larger than Mount Everest.

Atoms and Elements

• Atoms compose matter, and their properties determine the properties of matter.
• An atom is the smallest identifiable unit of an element.
• An element cannot be broken down into simpler substances.
• There are about 91 naturally occurring elements.
• Scientists have created about 20 synthetic elements.

Democritus and Leucippus

• Democritus and Leucippus theorized that matter is made of tiny, indestructible particles called "atomos" meaning indivisible.

John Dalton's Atomic Theory (1808)

• Each element consists of tiny, indestructible particles called atoms.
• Atoms of a given element have the same mass and properties.
• Atoms combine in simple, whole-number ratios to form compounds.

Modern Evidence for the Atomic Theory

• Scanning tunneling microscopes (STM) can precisely move atoms to form words and images.

Discovery of Electrons: J. J. Thomson

• J.J. Thomson discovered electrons, which are:
  • Negatively charged.
  • Much smaller and lighter than atoms.
  • Present in many different substances.
• Thomson proposed that atoms also contain positive charge to balance the negative charge of electrons.

Thomson’s Plum-Pudding Model

• Negatively charged electrons are held in a sphere of positive charge.

Rutherford’s Gold Foil Experiment

• Alpha-particles were directed at a thin gold foil.
• Most particles passed through, but some were deflected at sharp angles.

Results

• Expected (Plum-Pudding Model): Alpha-particles would pass through with minimal deflection.
• Actual: Most alpha particles passed through, but some were deflected or bounced back.

Rutherford's Nuclear Theory of the Atom

• Most of the atom’s mass and positive charge are in a small core called the nucleus.
• Most of the atom's volume is empty space where tiny, negatively charged electrons are dispersed.
• The number of electrons equals the number of protons in the nucleus, making the atom electrically neutral.
• The nucleus contains more than 99.9% of the atom's mass but occupies a small fraction of its volume.
• Matter is less uniform than it appears; atomic nuclei piled would be incredibly dense.

Protons, Neutrons, and Electrons

• Protons and neutrons have similar masses.
• Electrons have almost negligible mass.
• The proton is nearly 2000 times as massive as an electron.

Electrical Charge

• Electrical charge is a fundamental property of protons and electrons.
• Positive and negative charges attract; like charges repel.
• Positive and negative charges cancel each other when paired.

Subatomic Particles Summary

• Proton:
  • Mass: $1.67262 \times 10^{-27}$ kg, 1.0073 amu
  • Charge: 1+
• Neutron:
  • Mass: $1.67493 \times 10^{-27}$ kg, 1.0087 amu
  • Charge: 0
• Electron:
  • Mass: $0.00091 \times 10^{-27}$ kg, 0.00055 amu
  • Charge: 1−

Lightning and Charge

• Matter is normally charge-neutral.
• Electrical storms disturb the charge balance.
• Negative charge builds up on clouds, and positive charge on the ground.
• Rebalancing of charge can cause lightning.

Elements and Protons

• Elements are defined by their number of protons (atomic number, Z).
• Z = #p^+
• Changing the number of protons changes the element.

Periodic Table

• Lists all known elements by their atomic numbers.

Element Names and Symbols

• Most symbols are based on the English name of the element.
• Some symbols are based on Latin names (e.g., Na for sodium from natrium).

Examples of Symbols based on Latin names

• Lead: Pb (plumbum)
• Mercury: Hg (hydrargyrum)
• Iron: Fe (ferrum)
• Silver: Ag (argentum)
• Tin: Sn (stannum)
• Copper: Cu (cuprum)

Origins of Element Names

• Early scientists named elements after their properties (e.g., Argon from Greek "argos" meaning inactive).
• Some elements are named after countries (e.g., Polonium after Poland).
• Some elements are named after scientists (e.g., Curium after Marie Curie).

Dmitri Mendeleev and the Periodic Law

• Mendeleev arranged elements by increasing relative mass, and similar properties recurred in a regular pattern (Periodic Law).

Metals, Nonmetals, and Metalloids

• Elements are classified as metals, nonmetals, and metalloids.

Metals

• Good conductors of heat and electricity.
• Malleable (can be pounded into sheets).
• Ductile (can be drawn into wires).
• Often shiny (lustrous).
• Tend to lose electrons.

Nonmetals

• Poor conductors of heat and electricity.
• Can be solids or gases at room temperature (Bromine is a liquid).
• Tend to gain electrons.

Metalloids

• Also called semimetals.
• Have mixed properties.
• Are semiconductors (intermediate electrical conductivity).

Main Group and Transition Elements

• The periodic table is grouped into:
  • Main group elements (predictable properties based on position).
  • Transition elements (less predictable properties).

Groups (Families) in the Periodic Table

• Columns in the periodic table are called groups or families.
• Elements in a family have similar properties.

Alkali Metals (Group 1A)

• Very reactive metals (not including Hydrogen).

Alkaline Earth Metals (Group 2A)

• Fairly reactive metals.

Halogens (Group 7A)

• Very reactive nonmetals.

Noble Gases (Group 8A)

• Chemically inert (unreactive).

Protons, Neutrons, and Electrons in an Atom

• Atomic number (Z) = #protons ($p^+$)
• Neutral atoms: Z = #$p^+$ = #electrons ($e^-$)
• Ions (charged atoms): #$p^+ \neq e^-$
• Isotopes (different number of neutrons): #$p^+ \neq$ #neutrons

Ions: Gaining and Losing Electrons

• Atoms gain or lose electrons to form ions.
• Positive ions are cations.
• Negative ions are anions.
• Ion charge = #protons - #electrons

Ions and the Periodic Table

• The "A" group number (e.g., 1A, 7A) indicates the number of valence electrons.
• Main-group elements tend to form ions with the same number of valence electrons as the nearest noble gas.

Isotopes

• Atoms of an element have the same number of protons but can have different numbers of neutrons.
• Atoms with same number of protons but different number of neutrons are called isotopes.
• All elements have their own unique percent natural abundance of isotopes.

Isotopes: Mass Number

• Mass number (A) = #protons + #neutrons
• A = Z + #n
• #neutrons = A – Z

Isotopes: Symbol Notation

• Isotopes are symbolized as:
  • $^{A}_{Z}X$
• A second notation is: element-mass number (e.g., Ne-20).

Calculating Atomic Mass (Weighted Average)

• Atomic mass = (Fraction of isotope 1 x Mass of isotope 1) + (Fraction of isotope 2 x Mass of isotope 2) + …
• Fraction abundance = (Percentage natural abundance) / 100

Radioactive Isotopes

• Nuclei of some isotopes are unstable.
• These atoms emit energetic subatomic particles (nuclear radiation) and are radioactive.
• Technetium-99 (Tc-99) is used in medicine for diagnosis.