Atoms and Elements

Atomic Symbols

Atomic symbols are one- or two-letter abbreviations used to represent an atom of a particular element. These symbols are standardized and recognized internationally.

• First letter is always capitalized; if there is a second letter, it is lowercase.

• Some symbols are derived from Latin names (e.g., Na for sodium from Natrium).

• Examples: Nitrogen: N, Sodium: Na

Elements to Know

Students should be familiar with the names, atomic symbols, and possible charges of common elements. The following table summarizes some important elements and their properties:

Important People in Early Chemistry

• Antoine Lavoisier (1743–1794): Led the experimental foundation of modern chemistry; known as the "Father of Modern Chemistry."

• John Dalton (1766–1844): Developed Dalton's Atomic Theory, which described atoms as indivisible and indestructible particles.

• J.J. Thomson (1856–1940): Discovered the electron, proving that atoms are divisible.

• Ernest Rutherford (1871–1937): Discovered the nucleus and characterized alpha, beta, and gamma radiation.

• Dmitri Mendeleev (1834–1907): Created the periodic table, arranging elements by increasing atomic mass and predicting the properties of undiscovered elements.

Dalton's Atomic Theory

Postulates of Dalton's Atomic Theory

1. All matter is composed of indivisible atoms.

2. An element is a type of matter composed of only one kind of atom.

3. Atoms of different elements have different chemical and physical properties.

4. A compound is a type of matter composed of atoms of two or more elements in simple, whole-number ratios.

5. Chemical reactions involve the rearrangement of atoms; atoms are not destroyed during reactions.

Modern Modifications: Atoms are divisible (composed of protons, neutrons, and electrons), and atoms of the same element can have different masses (isotopes).

Structure of the Atom

Discovery of Subatomic Particles

• Electrons (e-): Negatively charged particles discovered by J.J. Thomson in 1897.

• Protons (p+): Positively charged particles found in the nucleus.

• Neutrons (n0): Neutral particles found in the nucleus.

Atoms are mostly empty space, with a dense nucleus containing protons and neutrons, surrounded by a cloud of electrons.

The Nuclear Atom

• Rutherford's gold foil experiment demonstrated the existence of a small, dense, positively charged nucleus.

• The nucleus contains nearly all the mass of the atom, but occupies a tiny fraction of its volume.

Example: If the nucleus were the size of a baseball, the atom would be about 5 miles in diameter.

Atomic Number, Mass Number, and Isotopes

• Atomic Number (Z): Number of protons in the nucleus; defines the element.

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

• Isotopes: Atoms of the same element (same Z) with different mass numbers (A) due to different numbers of neutrons.

Example: 35Cl and 37Cl are isotopes of chlorine.

Formula:

• Number of neutrons = Mass number (A) – Atomic number (Z)

Atomic Masses

• Atomic masses listed on the periodic table are weighted averages based on the natural abundance of isotopes.

• Formula:

Example: Iron has several naturally occurring isotopes; its atomic mass is calculated using their masses and abundances.

The Periodic Table

Periods, Groups, and Families

• Period: Horizontal rows; elements arranged by increasing atomic number.

• Group/Family: Vertical columns; elements in the same group have similar chemical and physical properties due to similar valence electron configurations.

Classification of Elements

Physical State of the Elements

• Most elements are solids at room temperature.

• Some are gases (e.g., H, N, O, F, Cl, noble gases).

• Few are liquids (e.g., Hg, Br).

Arranging the Periodic Table: Electron Shells and Subshells

• Electrons are arranged in shells (energy levels) around the nucleus.

• Each shell can be divided into subshells: s, p, d, f.

• Subshell capacities:

  • s: 2 electrons

  • p: 6 electrons

  • d: 10 electrons

  • f: 14 electrons

• The chemical behavior of atoms is largely determined by the electrons in the outermost shell (valence electrons).

Valence Electrons and the Periodic Table

• Valence electrons are the electrons in the outermost shell of an atom.

• Elements in the same group have the same number of valence electrons, leading to similar chemical properties.

Example: Group 1 elements (alkali metals) all have 1 valence electron.

Lewis Dot Structures for Atoms

• Lewis dot structures represent the nucleus and all electrons except those in the valence shell.

• Dots are used to represent valence electrons around the element symbol.

• Example:

  • Li: •

  • Cl: •••••••

Chemical Behavior and Valence Electrons

Similarities in the chemical behavior of elements are related to the number of valence electrons. Elements with the same number of valence electrons tend to form similar compounds.

Summary

• Atoms are the fundamental building blocks of matter, composed of protons, neutrons, and electrons.

• Elements are defined by their atomic number (number of protons).

• Isotopes are atoms of the same element with different numbers of neutrons.

• The periodic table organizes elements by increasing atomic number and groups elements with similar properties together.

• Valence electrons determine chemical reactivity and bonding behavior.

Additional info: The study of atomic structure and the periodic table forms the foundation for understanding chemical reactions, bonding, and the properties of matter in general chemistry.