Elements, Atoms, and the Periodic Table: General Chemistry Study Guide

Atoms and Subatomic Particles

Structure of the Atom

The atom represents the smallest unit of matter that retains the chemical properties of an element. It consists of a dense nucleus surrounded by electrons.

• Protons: Positively charged subatomic particles found in the nucleus.

• Neutrons: Neutral subatomic particles also located in the nucleus.

• Electrons: Negatively charged subatomic particles that orbit the nucleus in electron shells.

Example: The number of protons in an atom determines its identity as an element.

Forces in the Atom

Two major forces act within the atom:

• Nuclear Force: Holds protons and neutrons together in the nucleus.

• Electrostatic Force: Attraction between positively charged protons and negatively charged electrons.

Subatomic Particles: Mass and Charge

Properties of Subatomic Particles

Subatomic particles differ in mass and charge. Their properties are summarized below:

Subatomic Particle	Actual Mass (kg)	Relative Mass (amu)	Relative Charge
Neutron	1.67493 × 10-27	1	0
Proton	1.67262 × 10-27	1	+1
Electron	0.00091 × 10-27	~0	-1

1 atomic mass unit (amu) is defined as 1/12 the mass of a carbon-12 atom.

Isotopes and Atomic Notation

Isotopes

Isotopes are atoms of the same element with different numbers of neutrons, resulting in different mass numbers.

• Mass Number (A): Total number of protons and neutrons.

• Atomic Number (Z): Number of protons; determines the element's identity.

• Number of Electrons: Equal to the number of protons in a neutral atom.

Isotope Notation: AZX, where X is the element symbol.

Example: 4320Ca represents a calcium isotope with 20 protons and 23 neutrons.

Ion Formation

Cations and Anions

Atoms can gain or lose electrons to form ions:

• Cation: Formed when an atom loses electrons; positively charged.

• Anion: Formed when an atom gains electrons; negatively charged.

Example: 27Al3+ has 13 protons, 14 neutrons, and 10 electrons.

Atomic Mass and Isotopic Abundance

Calculating Atomic Mass

The atomic mass of an element is the weighted average of the masses of its isotopes, based on their natural abundances.

Atomic Mass Formula:

$\text{Atomic Mass} = \sum (\text{Isotope Mass} \times \text{Fractional Abundance})$

Example: To calculate the atomic mass of gallium, use the masses and percent abundances of its isotopes.

The Periodic Table

History and Organization

The periodic table arranges elements by increasing atomic number and groups elements with similar chemical properties.

• Element Symbol: One- or two-letter abbreviation for each element.

• Periodic Law: Properties of elements repeat periodically when arranged by atomic number.

Classifications

• Metals: Largest classification; typically shiny, malleable, and good conductors.

• Nonmetals: Opposite properties to metals; often gases or brittle solids.

• Metalloids: Elements with properties intermediate between metals and nonmetals.

Groups and Periods

• Groups: Vertical columns; elements in the same group have similar properties.

• Periods: Horizontal rows; elements in the same period have the same number of electron shells.

Representative and Transition Elements

• Representative Elements: Groups 1A-8A (main group elements).

• Transition Metals: Groups 3-12; often have variable charges and form colored compounds.

Phases of Elements

• Gases: Assume both the shape and volume of their container (e.g., N2, O2).

• Liquids: Assume the shape but not the volume of their container (e.g., Br2, Hg).

• Solids: Maintain their own shape and volume (e.g., Na, Fe).

Main Group Element Charges

Predicting Ion Charges

• Metals: Lose electrons to form cations with positive charges.

• Nonmetals: Gain electrons to form anions with negative charges.

• Group Number: Often indicates the charge of ions formed by main group elements.

Example: Group 1A elements form +1 cations; Group 7A elements form -1 anions.

Atomic Theory

Development of Atomic Theory

• Democritus: Proposed the existence of atoms as indivisible particles.

• Dalton: Formulated the Atomic Theory, stating that atoms are the fundamental building blocks of matter.

Dalton's Atomic Theory:

• All matter is composed of atoms.

• Atoms of the same element are identical.

• Atoms combine in simple whole-number ratios to form compounds.

• Atoms are rearranged in chemical reactions but are not created or destroyed.

Electronic Structure

Shells, Subshells, and Orbitals

Electrons are arranged in shells around the nucleus, which are further divided into subshells and orbitals.

• Shell: Principal energy level, designated by n (n = 1, 2, 3, ...).

• Subshell: Subdivision of a shell, labeled s, p, d, f.

• Orbital: Region of space where an electron is likely to be found.

Example: The third shell (n = 3) contains s, p, and d subshells.

Electron Configuration

Electron configuration describes the arrangement of electrons in an atom.

• Aufbau Principle: Electrons fill the lowest energy orbitals first.

• Hund's Rule: Electrons occupy degenerate orbitals singly before pairing.

• Pauli Exclusion Principle: No two electrons in an atom can have the same set of quantum numbers.

Example: Fluorine (Z = 9): $1s^2 2s^2 2p^5$

Condensed Electron Configuration

Condensed electron configuration uses the previous noble gas in brackets to simplify notation.

Example: Aluminum (Z = 13): [Ne] $3s^2 3p^1$

Valence Electrons

Main Group Elements

Valence electrons are the outermost electrons involved in chemical bonding. Inner core electrons are those not involved in bonding.

Example: Chlorine (Z = 17) has 7 valence electrons and 10 inner core electrons.

Periodic Trends

Metallic Character

Metallic character increases down a group and decreases across a period from left to right.

• Metals: Tend to lose electrons easily.

• Nonmetals: Tend to gain electrons.

Example: Cesium (Cs) has greater metallic character than sodium (Na).

Tables and Data

Subatomic Particle Table

Isotope	Mass Number	Atomic Number	Neutrons	Protons	Electrons
Zirconium-54	54	41	13	41	41
Aluminum-27	27	13	14	13	13

Ion Table

Symbol	Protons	Neutrons	Electrons	Mass Number	Net Charge
Cu2+	29	36	27	65	2+
O2-	8	10	10	18	2-

Electron Configuration Table

Shell Number (n)	Subshell
1	s
2	s, p
3	s, p, d
4	s, p, d, f

Additional info: These notes cover the foundational concepts of atomic structure, periodic table organization, isotopes, ions, electron configuration, and periodic trends, which are essential for General Chemistry students preparing for exams or coursework.