atomic number and mass number
Atoms and Elements
Atoms are the foundation of our sensations.
Typical seaside rocks are composed of silicates, which are compounds made of silicon and oxygen atoms.
Seaside air consists of nitrogen and oxygen molecules.
Seaside air may contain amines, which are organic compounds.
Example: Triethylamine is an amine emitted by decaying fish, contributing to the fishy smell of the seaside.
How Many Atoms Are in a Pebble?
Atoms are incredibly small.
A single pebble from the shoreline contains an inconceivable number of atoms, far exceeding the count of pebbles on the bottom of San Francisco Bay.
Small Size and Large Number of Atoms in a Pebble
To illustrate the smallness of atoms:
If every atom within a small pebble were the size of the pebble itself, the resulting pebble would be larger than Mount Everest.
Atoms and Elements in Depth
Atoms compose all matter, and the properties of atoms dictate the properties of the matter they form.
Definition of an Atom: The smallest identifiable unit of an element.
Definition of an Element: A substance that cannot be broken down into simpler substances.
Approximately 91 different elements occur naturally, and thus about 91 kinds of atoms exist naturally.
Scientists have synthesized about 20 elements that do not exist in nature.
The quantity of naturally occurring elements is subject to debate; some elements previously deemed synthetic may exist naturally in trace amounts.
Historical Atomic Theory
Atomic Theory of Democritus
Democritus (460–370 B.C.E.) and his mentor Leucippus (fifth century B.C.E.) contributed early ideas on atomic theory.
Democritus theorized that continually dividing matter would ultimately yield tiny, indivisible particles termed "atomos" or atoms.
He is recognized as the first to postulate that matter is comprised of atoms.
Atomic Theory of Dalton (1808)
John Dalton formalized an atomic theory gaining widespread acceptance over 2000 years later:
Each element is composed of tiny, indestructible particles called atoms.
Atoms of a specific element have identical mass and properties that differentiate them from the atoms of other elements.
Atoms combine in simple, whole-number ratios to form compounds.
Modern Evidence Supporting Atomic Theory
Writing with Atoms: Scientists at IBM utilized a scanning tunneling microscope (STM) to manipulate xenon atoms, forming the letters I, B, and M. The cone shape of the atoms results from instrument characteristics, while atoms themselves are typically spherical.
Thomson’s Discovery of Electrons
Key Discoveries by J. J. Thomson
J. J. Thomson (1856–1940) was an English physicist who discovered the electron.
Key findings include:
Electrons possess a negative charge.
Electrons are significantly smaller and lighter than atoms.
Electrons are present across diverse substances.
Thomson proposed that atoms contain a balance of positive charge to counter the negative charge of electrons.
Plum Pudding Model: In this model, negatively charged electrons are embedded in a uniform positive sphere, likened to a plum pudding.
Rutherford's Experiment and Nuclear Theory
Gold Foil Experiment
Rutherford’s Gold Foil Experiment (1909):
Alpha particles were directed at thin gold foil.
Most alpha particles passed through, but some were deflected sharply.
Proposed Nuclear Model of the Atom
Atomic Nucleus Discovery:
If Thomson’s plum pudding model were accurate, the alpha particles would experience minimal deflection.
Actual outcomes indicated some alpha particles were deflected or bounced back, indicating the presence of a nucleus.
Rutherford’s Nuclear Theory:
Most of the atom’s mass and all positive charge reside in a central core called the nucleus.
The volume of the atom consists mainly of empty space where negatively charged electrons inhabit.
The number of electrons equals the number of protons in the nucleus, rendering the atom electrically neutral.
Mass Distribution in the Atom
The nucleus comprises more than 99.9% of the atom's mass but occupies only a small volume, while electrons lack significant mass despite occupying a larger region.
Matter is less uniform than perceived; if atomic nuclei were packed like marbles, densities would be astronomical.
A single grain of sand composed solely of solid atomic nuclei could weigh around 5 million kg.
Black holes and neutron stars probably consist of such dense matter.
Relative Size of Proton and Electron
If a proton had the mass of a baseball, an electron would weigh as little as a grain of rice.
A proton is approximately 2000 times as massive as an electron.
Electrical Charge Properties
Definition of Electrical Charge: A fundamental property of protons and electrons.
Interactions:
Positive and negative charges attract each other.
Like charges repel (positive-positive; negative-negative).
A pair of a proton and electron is charge-neutral.
Properties of Subatomic Particles
Protons and neutrons have approximately equal masses.
Electrons have negligible mass in comparison.
Data tables for these particles are essential for problem-solving in chemistry.
Evidence of Charge in Matter
Matter is typically electrically neutral, with equal positive and negative charges that cancel out.
During electrical storms, charge imbalances occur, leading to phenomena like lightning.
Variations Among Atoms of Elements
Elements are defined by the count of protons they contain.
Atomic Number (Z): The number of protons in an atom's nucleus, which categorizes the atom as a specific element.
Periodic Table and Atomic Symbols
The periodic table organizes all known elements by their atomic numbers.
Formation of Ions
Atoms may gain or lose electrons during chemical reactions, forming ions:
Cations: Positively charged ions resulting from loss of electrons.
Anions: Negatively charged ions resulting from the gain of electrons.
Ion charge notation appears in the upper right corner of their symbol (e.g., Mg²⁺, O²⁻).
Examples of Ion Formation
Ion Formation by Losing Electrons
Lithium loses one electron (e⁻) to create Li⁺ ions.
Example Equation for Charge Calculation: Charge of ion = $p^+$ (protons) - $e^-$ (electrons).
Ion Formation by Gaining Electrons
Fluorine gains one electron (e⁻) to form F⁻ ions.
Example Equation for Charge Calculation: Charge of ion = $p^+$ - $e^-$.
Isotopes and Neutron Variations
All atoms of an element contain the same number of protons but can vary in neutron count.
Isotopes: Atoms with identical proton counts but different neutron counts.
Natural isotopes have a unique percent abundance by element.
Isotopes of Neon
Naturally occurring neon has three isotopes:
Ne-20: 10 protons, 10 neutrons
Ne-21: 10 protons, 11 neutrons
Ne-22: 10 protons, 12 neutrons
Isotope Notation
Isotopes are often denoted as follows:
For neon: Ne-20, Ne-21, Ne-22.
Mass Number Definition
Mass Number (A): The total count of protons and neutrons in an atom.
Neutron count can be found using: (A - Z), where A is mass number and Z is atomic number.
Calculating Atomic Mass
Atomic Mass: The average weight of an element’s atoms.
For chlorine: composed of 75.77% Cl-35 and 24.23% Cl-37.
To calculate atomic mass, apply the formula:
{ ext{Atomic mass} = ext{(Fraction of isotope 1} imes ext{Mass of isotope 1)} + ext{(Fraction of isotope 2} imes ext{Mass of isotope 2)} + …}
Example: Calculating Atomic Mass of Gallium
Gallium has two isotopes—Ga-69 and Ga-71:
Ga-69: 68.9256 amu, 60.11% natural abundance.
Ga-71: 70.9247 amu, 39.89% natural abundance.
Atomic Mass Calculation Method
To compute the atomic mass:
Convert natural abundances to decimal:
e.g., 60.11% = 0.6011
Use fractions with atomic masses:
e.g., atomic mass = $(0.6011 imes 68.9256) + (0.3989 imes 70.9247)$.
Chapter 4 Review
The Atomic Theory Overview
Ancient Greeks proposed matter comprised of small, undestroyable particles.
Dalton established matter as atoms with unique properties for each element and their combinations.
The Nuclear Model of the Atom
The atom consists of protons and neutrons in a nucleus, with electrons surrounding it, contributing to overall volume.
Protons and neutrons share similar masses, with electrons being significantly lighter.
Charge Insights
Protons (1+) and electrons (1-) exhibit opposite charges, while neutrons possess no charge.
Charge cancellation occurs in atoms with protons and electrons.
Atomic Characteristics
Atomic number (Z) signifies an element’s unique identity based on proton count (the defining characteristic).
Ions and Isotopes
Ions are formed during electron gain or loss.
Isotopes share proton counts but differ in neutron counts, represented through mass numbers (A). Atomic mass denotes a weighted average of isotopes' masses.