Atoms and Elements - Comprehensive Notes

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 itself, 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 is a substance that cannot be broken down into simpler substances.
There are about 91 naturally occurring elements, each with a different kind of atom.
Scientists have created about 20 synthetic elements.
The exact number of naturally occurring elements is debated due to the presence of trace amounts of some synthetic elements in nature.

Democritus and Leucippus: Matter is Made of Particles

Democritus (460–370 B.C.E.) and Leucippus (fifth century B.C.E.) theorized that matter consists of tiny, indestructible particles.
They named these particles atomos, meaning "indivisible".

Atomic Theory: John Dalton

In 1808, John Dalton formalized the atomic theory.
1. Each element is composed of tiny, indestructible particles called atoms.
2. All atoms of a given element have the same mass and properties, distinguishing them from atoms of other elements.
3. Atoms combine in simple, whole-number ratios to form compounds.

Modern Evidence for the Atomic Theory: Writing with Atoms

IBM scientists used a scanning tunneling microscope (STM) to move atoms precisely to form words and images.
They created the world’s smallest movie, A Boy and His Atom, using 250 frames.

Discovery of Electrons: J. J. Thomson

J. J. Thomson (1856–1940) discovered the electron, a smaller, more fundamental particle within the atom.
Electrons are negatively charged and much smaller and lighter than atoms.
Electrons are uniformly present in many different kinds of substances.
Thomson proposed that atoms must contain positive charge to balance the negative charge of electrons.

Thomson’s Plum-Pudding Model

The plum-pudding model suggests that negatively charged electrons are held within a sphere of positive charge.

Rutherford’s Gold Foil Experiment

In 1909, Rutherford conducted the gold foil experiment, where alpha-particles were directed at a thin sheet of gold foil.
Most particles passed through the foil, but some were deflected at sharp angles.
Expected result (plum-pudding model): alpha-particles would pass through with minimal deflection.
Actual result: most alpha-particles passed through, but a small number were deflected or bounced back.

Rutherford: Nuclear Theory of the Atom

Most of the atom’s mass and all its positive charge are concentrated 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 negatively charged electrons outside the nucleus equals the number of positively charged particles (protons) inside the nucleus, making the atom electrically neutral.

The atom’s mass is concentrated in the nucleus.
Electrons are portrayed as a cloud around the nucleus.
The nucleus makes up more than 99.9% of the atom’s mass but occupies a small fraction of its volume.
Matter is less uniform than it appears.
If matter were composed of atomic nuclei piled on each other, it would be incredibly dense.

Protons, Neutrons, and Electrons: Relative Mass

If a proton had the mass of a baseball, an electron would have the mass of a rice grain.
The proton is nearly 2000 times as massive as an electron.
Protons and neutrons have similar masses.
Electrons have almost negligible mass.

Protons, Neutrons, and Electrons: Electrical Charge

Electrical charge is a fundamental property of protons and electrons.
Positive and negative electrical charges attract each other.
Positive–positive and negative–negative charges repel each other.
Positive and negative charges cancel each other, so a proton and an electron are charge-neutral when paired.

Protons, Neutrons, and Electrons: Summary

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

What Causes Lightning: Evidence of Charge in Matter

Matter is normally charge-neutral, with equal numbers of positive and negative charges that cancel exactly.
In an electrical storm, the charge balance of matter is disturbed.
Negative charge builds up on clouds, and positive charge builds up on the ground.
The quick rebalancing of charge often occurs in dramatic ways, such as lightning.

Elements: Defined By Their Number of Protons

Elements are defined by their number of protons.
The number of protons in the nucleus of an atom identifies the atom as a particular element.
If an atom had a different number of protons, it would be a different element.
The number of protons in the nucleus of an atom is its atomic number and is given the symbol Z.
Z = #p^+

Periodic Table of the Elements: Organized by Atomic Number

The periodic table of the elements lists all known elements according to their atomic numbers.

Periodic Table: Names and Symbols of the Elements

Most chemical symbols are based on the English name of the element.
- For example, the symbol for carbon is C and bromine is Br.
Some symbols are based on Latin names.
- The symbol for potassium is K, from the Latin kalium, and the symbol for sodium is Na, from the Latin natrium.
Additional elements with symbols based on their Greek or Latin names:
- Lead: Pb (plumbum)
- Mercury: Hg (hydrargyrum)
- Iron: Fe (ferrum)
- Silver: Ag (argentum)
- Tin: Sn (stannum)
- Copper: Cu (cuprum)

Periodic Table: Origins of the Names of the Elements

Early scientists named newly-discovered elements based on their properties.
- Argon, from the Greek argos, means “inactive.”
- Bromine originates from the Greek word bromos, meaning “stench.”
Scientists named some elements after countries:
- Polonium after Poland
- Francium after France
- Americium after the United States of America.
Other elements were named after scientists.
- Curium is named after Marie Curie, who helped discover radioactivity and two new elements, and won two Nobel Prizes.

Looking for Patterns: Dmitri Mendeleev

Dmitri Mendeleev (1834-1907) noticed that listing elements in order of increasing relative mass resulted in similar properties recurring in a regular pattern.
He summarized these patterns in the periodic law.

Looking for Patterns: Recurring Properties

Elements with similar properties are arranged in vertical columns in the periodic table.

Looking for Patterns: Periodic Law

Mendeleev’s periodic law was based on observation.
Like scientific laws, the periodic law summarized observations without explaining the underlying reason.
The periodic law is accepted as is, but Chapter 9 will examine a theory that explains the law.

Locating Metals, Nonmetals, Metalloids on the Periodic Table

Elements in the periodic table are broadly classified as metals, nonmetals, and metalloids.

Properties of Metals

Metals are on the left side of the periodic table and have similar properties:
- Good conductors of heat and electricity
- Malleable (can be pounded into flat sheets)
- Ductile (can be drawn into wires)
- Often shiny (lustrous)
- Tend to lose electrons during chemical changes
- Examples: iron (Fe), magnesium (Mg), chromium (Cr), and sodium (Na).

Properties of Nonmetals

Nonmetals are on the upper right side of the periodic table.
The dividing line between metals and nonmetals is the zigzag diagonal line from boron to astatine.
Their properties are varied.
- States: some are solids, others are gases at room temperature; one is a liquid.
- Poor conductors of heat and electricity.
In chemical reactions, nonmetals tend to gain electrons.
Examples: oxygen (O), nitrogen (N), chlorine (Cl), bromine (Br), and iodine (I).

Properties of Metalloids

Metalloids lie along the zigzag diagonal line that divides metals and nonmetals.
Also called semimetals, they display mixed properties.
Semiconductors because of their intermediate electrical conductivity, which can be changed and controlled.
Useful in the manufacture of electronic devices for computers, cell phones, and other modern gadgets.
Examples: silicon (Si), arsenic (As), and germanium (Ge).

Main Group Elements and the Transition Elements

The periodic table can be broadly grouped into:
- Main group elements: properties are predictable based on their position in the periodic table.
- Transition elements: properties are less predictable based on their position.

Groups in the Periodic Table

Each column in the periodic table is a group (or family).
Elements within a family of main group elements usually display similar properties and may have a group name.

Groups in the Periodic Table: Alkali Metals (Group 1A)

Very reactive metals.
Include lithium, sodium, potassium, rubidium, and cesium.
Note that hydrogen is NOT an alkali metal (although it is in Group 1A).

Groups in the Periodic Table: Alkaline Earth Metals (Group 2A)

Fairly reactive, but not as reactive as the alkali metals.
Include beryllium, magnesium, calcium, strontium, and barium.

Groups in the Periodic Table: Halogens (Group 7A)

Very reactive nonmetals.
Include fluorine, chlorine, bromine, iodine, and astatine.

Groups in the Periodic Table: Noble Gases (Group 8A)

Chemically inert (unreactive).
Chemically stable and won’t combine with other elements to form compounds.
Include helium, neon, argon, krypton, and xenon.

Number of Protons, Neutrons, and Electrons in an Atom

Atomic number is always equal to the number of protons: Z = #p^+
In neutral atoms (no charge): Z = #p^+ = #e^-
- Example: sodium (Na): Z = 11, #p^+ = 11, #e^- = 11
In ions (charged atoms): #p^+ \neq #e^-
In isotopes (different no. of neutrons): #p^+ \neq #n

Ions: Gaining and Losing Electrons

In chemical reactions, atoms often lose or gain electrons to form charged particles called ions.
Positive ions are called cations.
Negative ions are called anions.
The charge of an ion is shown in the upper right corner of the symbol.
Ion charges are usually written with the magnitude of the charge first, followed by the sign of the charge.
Examples: $Mg^{2+}, O^{2-}$
$Li \rightarrow Li^+ + e^-$
$F + e^- \rightarrow F^-$
The charge of an ion depends on how many electrons were gained or lost and is given by the formula:
- ion \ charge = #p^+ - #e^-
  - For the lithium ion: $ion \ charge = 3 - 2 = +1$
  - For the fluoride ion: $ion \ charge = 9 - 10 = -1$
- Cations are named as the element name plus "cation."
  - Example: $Li^+$ is Lithium cation
- Anions are named with the stem of the element name plus the suffix "-ide".
  - Example: $F^-$ is Fluoride

Ions and the Periodic Table

The number associated with the letter “A” above each main group column (e.g. 1A, 7A) in the periodic table—1A through 8A—gives the number of valence electrons for the elements in that column.
- e.g. Group 2A elements have 2 valence e-
- Group 7A elements have 7 valence e-
Valence electrons are the outermost electrons of an atom which are readily involved in chemical reactions.
Main-group elements tend to form ions that have the same number of valence electrons as the nearest noble gas.

Isotopes

All atoms of a given element have the same number of protons, but they do not necessarily have the same number of neutrons.
Atoms with the same number of protons but different numbers of neutrons are called isotopes.
All elements have their own unique percent natural abundance of isotopes.

Isotopes: Natural Abundance of Isotopes in Neon

Naturally occurring neon contains three different isotopes: Ne-20 (with 10 neutrons), Ne-21 (with 11 neutrons), and Ne-22 (with 12 neutrons). All three isotopes contain 10 protons.

Isotopes: Mass Number

The mass number (A) is the sum of the number of protons (#p+) and the number of neutrons (#n).
- A = #p^+ + #n
- A = Z + #n
The number of neutrons in an isotope is the difference between the mass number and the atomic number.
- #n = A – Z

Isotopes: Symbol Notation

Isotopes are often symbolized in the following way:
$\ _{Z}^{A}X$
- where X is the element symbol, A is the mass number, and Z is the atomic number
The symbols for the isotopes of neon are as follows:
- $\ _{10}^{20}Ne, \ _{10}^{21}Ne, \ _{10}^{22}Ne$
A second notation for isotopes is the chemical symbol (or chemical name) followed by a hyphen and the mass number of the isotope.
- Ne-20 (neon-20)
- Ne-21 (neon-21)
- Ne-22 (neon-22)

Calculating Atomic Mass as the Weighted Average

The atomic mass of each element listed in the periodic table represents the average mass of the atoms that compose that element.
For elements that contain isotopes, atomic mass is calculated according to the following equation:
- $Atomic \ mass = (Fraction \ of \ isotope \ 1 \times Mass \ of \ isotope \ 1) + (Fraction \ of \ isotope \ 2 \times Mass \ of \ isotope \ 2) + (Fraction \ of \ isotope \ 3 \times Mass \ of \ isotope \ 3) + …$
The fraction abundance of each isotope is the percentage natural abundance divided by 100.
- Example: Naturally occurring chlorine consists of 75.77% chlorine-35 (mass 34.97 amu) and 24.23% chlorine-37 (mass 36.97 amu). Calculate the average atomic mass of chlorine.
  - $atomic \ mass = (0.7577 \times 34.97amu) + (0.2423 \times 36.97amu) = 35.45amu$

Radioactive Isotopes

The nuclei of some isotopes of a given element are unstable.
These atoms emit energetic subatomic particles from their nuclei and are converted into different isotopes of different elements.
The emitted subatomic particles are called nuclear radiation.
The unstable isotopes that emit them are termed radioactive.

Radioactive Isotopes in Medicine

Radioactive isotopes are not always harmful; they can also have beneficial uses.
For example, technetium-99 (Tc-99) is often given to patients to diagnose disease.
The radiation emitted by Tc-99 helps doctors to image internal organs or detect infection.