Atoms and the Periodic Table Lecture Notes

Elements and the Periodic Table

Elements are pure substances that cannot be broken down into simpler substances via a chemical reaction. Every element is uniquely identified by a one- or two-letter symbol and is organized within the periodic table. The specific position of an element in the periodic table provides significant information regarding its chemical properties.

Table 2.1: Common Elements and Their Symbols

Metals, Nonmetals, and Metalloids

The elements in the periodic table are categorized into three primary groups based on their properties and location.

Metals

Metals are situated on the left side of the periodic table. Their physical characteristics and properties include:

• Usually existing as shiny solids.
• Being excellent conductors of heat and electricity.
• Existing as solids at room temperature, with the singular exception of Mercury ($Hg$), which is a liquid.

Nonmetals

Nonmetals are located on the right side of the periodic table. Their physical characteristics and properties include:

• Usually lacking a shiny appearance.
• Generally being poor conductors of heat and electricity.
• Being able to exist in solid, liquid, or gaseous states at room temperature.
• Examples include solids (sulfur, carbon), liquids (bromine), and gases (nitrogen, oxygen).

Metalloids

Metalloids are located along the solid zigzag line that begins at Boron ($B$) and angles down toward Astatine ($At$). They possess chemical properties intermediate between those of metals and nonmetals. There are only seven elements classified as metalloids:

• Boron ($B$)
• Silicon ($Si$)
• Germanium ($Ge$)
• Arsenic ($As$)
• Antimony ($Sb$)
• Tellurium ($Te$)
• Astatine ($At$)

Focus on the Human Body: The Elements of Life

Four specific nonmetals compose $96\,\%$ of the total mass of the human body. These are referred to as the building-block elements:

• Oxygen ($O$)
• Carbon ($C$)
• Hydrogen ($H$)
• Nitrogen ($N$)

Chemical Compounds and Formulas

A compound is a pure substance formed by chemically combining two or more elements together. To represent these compounds, chemical formulas are used, consisting of:

• Element symbols to denote the identity of the constituent elements.
• Subscripts to indicate the ratio of atoms of each element in the compound.

Examples of chemical formulas:

• Water ($H_2O$): Contains $2$ Hydrogen ($H$) atoms and $1$ Oxygen ($O$) atom.
• Propane ($C_3H_8$): Contains $3$ Carbon ($C$) atoms and $8$ Hydrogen ($H$) atoms.

In chemical documentation, compounds can be visualized in multiple ways, using different colors to represent different elements.

Structure of the Atom

All matter is constructed from basic building blocks called atoms. Atoms are composed of three subatomic particles:

The Nucleus

• The dense core of the atom.
• Location of protons (positively charged) and neutrons (neutral charge).
• Most of the atom's mass resides here.

The Electron Cloud

• The region surrounding the nucleus where electrons (negatively charged) are located.
• Comprises the vast majority of the atom's volume.

Atomic Number, Mass Number, and Isotopes

Atomic Number ($Z$)

• The atomic number ($Z$) is defined as the number of protons and is found in the periodic table above the element symbol.
• Every atom of a specific element has the same number of protons in its nucleus.
• Different elements have different atomic numbers.
• In a neutral atom, there is no net charge, meaning:   $Z = \text{number of protons} = \text{number of electrons}$

Mass Number ($A$)

• The mass number is the total sum of protons and neutrons within the nucleus:   $A = \text{number of protons (Z)} + \text{number of neutrons}$

Isotopes

Isotopes are atoms of the same element that contain the same number of protons but have a different number of neutrons. Example: Chlorine ($Cl$) has two common isotopes:

• $\binom{35}{17}Cl$: $17$ protons, $17$ electrons, and $18$ neutrons ($35 - 17 = 18$).
• $\binom{37}{17}Cl$: $17$ protons, $17$ electrons, and $20$ neutrons ($37 - 17 = 20$).

Atomic Weight

The atomic weight is the weighted average of the masses of the naturally occurring isotopes of a specific element. This value is reported in atomic mass units ($\text{amu}$). In the periodic table entry for Lead ($Pb$):

• Atomic number ($Z$): $82$
• Atomic weight: $207.2\,\text{amu}$

The Periodic Table Structure

• Period: A horizontal row in the periodic table.
• Group: A vertical column in the periodic table.

Categories of Elements

• Main Group Elements: Consist of the two columns on the far left and the six columns on the far right. These are numbered Group $1A$ through $8A$.
• Transition Metal Elements: Located in the $10$ short columns in the middle of the table, numbered Group $1B$ through $8B$.
• Inner Transition Elements: Consist of the Lanthanides and Actinides located below the main body of the table. No group numbers are assigned to these.

Characteristics of Element Groups

Elements within a specific group possess similar chemical properties.

Groups 1A and 2A

• Group $1A$ (Alkali metals): Includes soft and shiny metals with low melting points that are good conductors and react with water to form basic solutions.
• Group $2A$ (Alkaline earth elements): Share similar properties with alkali metals, including reactivity with water.

Groups 7A and 8A

• Group $7A$ (Halogens): These exist as diatomic molecules (two atoms joined together). They are very reactive and combine with many other elements to form compounds.
• Group $8A$ (Noble gases): These are very stable and rarely combine with other elements.

The Unusual Nature of Carbon

Carbon is unique because it exists in three distinct elemental forms (allotropes):

• Diamond: A three-dimensional network of Carbon atoms.
• Graphite: Parallel sheets of Carbon atoms.
• Buckminsterfullerene: A sphere composed of $60$ Carbon atoms.

Electronic Structure: Shells, Subshells, and Orbitals

The chemistry of an element is determined by the number of electrons. Electrons occupy specific energy levels called principal energy levels or shells ($n$).

• Shells are numbered $n = 1, 2, 3, \dots$
• Lower numbered shells (lower $n$) are closer to the nucleus and lower in energy.
• Higher numbered shells (higher $n$) are further from the nucleus, higher in energy, have larger volumes, and can hold more electrons.

Electron Distribution by Shell

• Shell $1$ ($n=1$): Max $2$ electrons.
• Shell $2$ ($n=2$): Max $8$ electrons.
• Shell $3$ ($n=3$): Max $18$ electrons.
• Shell $4$ ($n=4$): Max $32$ electrons.

Subshells and Orbitals

Shells are divided into subshells labelled $s, p, d,$ and $f$. Subshells contain orbitals, which are regions where the probability of finding an electron is high. Each orbital can hold a maximum of $2$ electrons.

• $s$ subshell: $1$ orbital ($2$ electrons).
• $p$ subshell: $3$ orbitals ($6$ electrons).
• $d$ subshell: $5$ orbitals ($10$ electrons).
• $f$ subshell: $7$ orbitals ($14$ electrons).

Orbital Shapes

• $s$ orbital: Spherical shape.
• $p$ orbital: Dumbbell shape.

Electron Configuration

Electron configuration describes the arrangement of electrons in an atom’s orbitals. The lowest energy arrangement is the ground state.

Rules for Ground State Configuration

• Rule 1: Electrons fill the lowest energy orbitals first, starting with $1s$. Energy increases in the order: $1s, 2s, 2p, 3s, 3p$. Within a shell, orbital energy increases from $s < p < d < f$.
• Rule 2: Each orbital can hold a maximum of $2$ electrons.

Examples of Configurations

• Hydrogen ($Z=1$): $1s^1$
• Helium ($Z=2$): $1s^2$
• Lithium ($Z=3$): $1s^2 2s^1$
• Carbon ($Z=6$): $1s^2 2s^2 2p^2$
• Neon ($Z=10$): $1s^2 2s^2 2p^6$

Valence Electrons and Electron-Dot Symbols

• Valence Shell: The outermost shell ($n$ with the highest value).
• Valence Electrons: Electrons located in the valence shell. These determine the chemical properties of an element.

Examples:

• Beryllium ($Be$): $1s^2 2s^2$ (Valence shell $n=2$, valence electrons = $2$).
• Chlorine ($Cl$): $1s^2 2s^2 2p^6 3s^2 3p^5$ (Valence shell $n=3$, valence electrons = $7$).

Group Relationships

• Elements in the same group have the same number of valence electrons.
• For Main Group elements ($1A–8A$), the group number equals the number of valence electrons (Exception: Helium has $2$).

Electron-Dot Symbols

Dots representing valence electrons are placed around the element symbol:

• One dot per side for the first four electrons.
• Dots are paired for elements with more than four valence electrons.
• $H$: $1$ dot.
• $C$: $4$ dots.
• $O$: $6$ dots (two pairs, two singles).
• $Cl$: $7$ dots (three pairs, one single).

Periodic Trends

Atomic Size

• Increases down a column: Valence electrons occupy shells that are further from the nucleus.
• Decreases across a row: The number of protons in the nucleus increases, pulling the electron cloud closer to the center.

Ionization Energy

Ionization energy is the energy required to remove an electron from a neutral atom: $Na + \text{energy} \rightarrow Na^+ + e^-$

• Decreases down a column: Valence electrons are farther from the positive attraction of the nucleus.
• Increases across a row: The increasing number of protons creates a stronger pull on the electrons.