Elements, Atomic Structure, and the Periodic Table: GOB Chemistry Study Notes (Week 2)

4.1 Elements and Symbols

Elements

Elements are pure substances that cannot be broken down into simpler substances by chemical means. They are the fundamental building blocks of matter.

• Element: A substance made of only one type of atom.

• Each element is represented by a unique chemical symbol.

• There are over 100 known elements, each with distinct properties.

Chemical Symbols

Chemical symbols are shorthand notations for elements, typically consisting of one or two letters derived from their English or Latin names.

• One-letter symbols: Always capitalized (e.g., H for hydrogen, N for nitrogen).

• Two-letter symbols: First letter capitalized, second letter lowercase (e.g., He for helium, Ca for calcium).

• Some symbols are based on Latin names (e.g., Ag for argentum (silver), Au for aurum (gold)).

Example: The symbol for carbon is C; for sodium, it is Na (from natrium).

4.2 The Periodic Table

Organization of the Periodic Table

The periodic table arranges elements in order of increasing atomic number, grouping elements with similar properties into columns called groups or families.

• Periods: Horizontal rows (numbered 1–7).

• Groups: Vertical columns (numbered 1–18 or labeled with A/B system).

• Elements in the same group have similar chemical properties.

Groups and Periods

• Group numbers: Indicate the number of valence electrons for main group elements.

• Period numbers: Indicate the number of electron shells.

• Special group names: Alkali metals (Group 1), Alkaline earth metals (Group 2), Halogens (Group 17), Noble gases (Group 18).

Example: Sodium (Na) is in Group 1, Period 3.

HTML Table: Main Group Names

Group Number	Group Name
1	Alkali Metals
2	Alkaline Earth Metals
17	Halogens
18	Noble Gases

Metals, Nonmetals, and Metalloids

Classification on the Periodic Table

Elements are broadly classified as metals, nonmetals, or metalloids based on their physical and chemical properties.

• Metals: Located to the left and center of the periodic table.

• Nonmetals: Located to the right.

• Metalloids: Found along the zigzag (stair-step) line between metals and nonmetals.

Characteristics

• Metals: Shiny, good conductors of heat and electricity, malleable, ductile, usually solid at room temperature (except mercury).

• Nonmetals: Dull, poor conductors, brittle, can be solid, liquid, or gas at room temperature.

• Metalloids: Have properties intermediate between metals and nonmetals; often semiconductors.

Example: Silicon (Si) is a metalloid used in electronics.

4.3 The Atom

Dalton's Atomic Theory

John Dalton proposed that matter is composed of small, indivisible particles called atoms. His theory laid the foundation for modern chemistry.

• All matter consists of atoms.

• Atoms of the same element are identical; atoms of different elements are different.

• Atoms combine in simple ratios to form compounds.

• Atoms are not created or destroyed in chemical reactions.

Structure of the Atom

An atom consists of a dense nucleus containing protons and neutrons, surrounded by electrons in a cloud.

• Protons: Positively charged particles in the nucleus.

• Neutrons: Neutral particles in the nucleus.

• Electrons: Negatively charged particles in orbitals around the nucleus.

Electrical Charges in an Atom

• Protons: +1 charge

• Neutrons: 0 charge

• Electrons: -1 charge

• Like charges repel; unlike charges attract.

Historical Models of the Atom

• J.J. Thomson's Plum Pudding Model: Atoms are spheres of positive charge with embedded electrons.

• Rutherford's Gold Foil Experiment: Demonstrated that atoms have a small, dense, positively charged nucleus.

Mass of the Atom

Atomic Mass

The mass of an atom is primarily due to its protons and neutrons, as electrons have negligible mass. Atomic mass is measured in atomic mass units (amu).

• 1 amu is defined as 1/12 the mass of a carbon-12 atom.

• Proton mass ≈ 1 amu; neutron mass ≈ 1 amu; electron mass ≈ 0.0005 amu.

HTML Table: Subatomic Particles

Particle	Symbol	Charge	Mass (amu)	Location
Proton	p+	+1	1	Nucleus
Neutron	n0	0	1	Nucleus
Electron	e-	-1	~0	Outside nucleus

4.4 Atomic Number and Mass Number

Atomic Number ($Z$)

The atomic number is the number of protons in the nucleus of an atom and uniquely identifies an element.

• All atoms of an element have the same atomic number.

• Atomic number appears above the element symbol in the periodic table.

$Z = \text{number of protons}$

Mass Number ($A$)

The mass number is the total number of protons and neutrons in an atom's nucleus.

• Mass number is not found on the periodic table; it is specific to each isotope.

$A = \text{number of protons} + \text{number of neutrons}$

Atoms Are Neutral

In a neutral atom, the number of protons equals the number of electrons.

$\text{Number of protons} = \text{Number of electrons}$

HTML Table: Composition of Some Atoms

Element	Protons	Neutrons	Electrons	Mass Number
Hydrogen	1	0	1	1
Carbon	6	6	6	12
Oxygen	8	8	8	16
Sodium	11	12	11	23

4.5 Isotopes and Atomic Mass

Isotopes

Isotopes are atoms of the same element (same number of protons) that have different numbers of neutrons, and therefore different mass numbers.

• Isotopes have nearly identical chemical properties but different physical properties (such as mass).

• Isotopes are represented as $^{A}_{Z}\text{X}$, where X is the element symbol, A is the mass number, and Z is the atomic number.

Example: $^{12}_{6}\text{C}$ and $^{14}_{6}\text{C}$ are isotopes of carbon.

Atomic Mass

The atomic mass of an element is the weighted average of the masses of its naturally occurring isotopes.

$\text{Atomic mass} = \sum (\text{fractional abundance} \times \text{isotope mass})$

HTML Table: Isotope Data Example

Isotope	Protons	Neutrons	Electrons	Mass Number
$^{16}$O	8	8	8	16
$^{19}$F	9	10	9	19
$^{27}$Al	13	14	13	27

Learning Tips: Protons and Neutrons

• Number of protons = atomic number

• Number of neutrons = mass number - atomic number

• Number of electrons = atomic number (for neutral atoms)

Summary Table: Key Atomic Concepts

Term	Definition
Element	Pure substance of one type of atom
Atomic Number (Z)	Number of protons in nucleus
Mass Number (A)	Number of protons + neutrons
Isotope	Atoms of same element with different numbers of neutrons
Atomic Mass	Weighted average mass of all isotopes

Additional info: These notes expand on the provided slides by including definitions, formulas, and tables for clarity and completeness, as would be expected in a mini-textbook for GOB Chemistry students.