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 currently 118 known elements, each with distinct properties.

Example: Magnesium (Mg) is an element that cannot be separated into simpler substances by chemical reactions.

Chemical Symbols

Chemical symbols are shorthand notations for elements, usually consisting of one or two letters. The first letter is always capitalized, and the second letter (if present) is lowercase.

• One-letter symbols: H (hydrogen), N (nitrogen), O (oxygen), F (fluorine), P (phosphorus), S (sulfur), K (potassium), U (uranium), V (vanadium), Y (yttrium), I (iodine), W (tungsten).

• Two-letter symbols: Co (cobalt), Ca (calcium), Al (aluminum), Si (silicon), etc.

• Some symbols are derived from Latin names, e.g., Ag (silver, argentum), Au (gold, aurum).

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

4.2 The Periodic Table

Structure and Organization

The periodic table organizes elements by increasing atomic number and groups elements with similar properties into columns (groups) and rows (periods).

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

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

• Groups are often labeled 1–18 or with Roman numerals and letters (e.g., IA, IIA).

Example: Group 1 elements (alkali metals) are highly reactive metals.

Group Names

• Alkali metals: Group 1 (except hydrogen)

• Alkaline earth metals: Group 2

• Halogens: Group 17

• Noble gases: Group 18

• Transition metals: Groups 3–12

Metals, Nonmetals, and Metalloids

The periodic table can be divided into metals, nonmetals, and metalloids based on their properties and location.

• Metals: Located to the left and center; good conductors, malleable, ductile, shiny.

• Nonmetals: Located to the right; poor conductors, brittle, dull.

• Metalloids: Located along the zigzag line; have properties intermediate between metals and nonmetals.

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

Table: Comparison of Metals, Nonmetals, and Metalloids

Type	Location	Properties
Metals	Left/center	Shiny, malleable, ductile, good conductors
Nonmetals	Right	Dull, brittle, poor conductors
Metalloids	Zigzag line	Intermediate properties; semiconductors

4.3 The Atom

Dalton's Atomic Theory

Dalton's atomic theory laid the foundation for modern chemistry by describing atoms as indivisible particles that combine in fixed ratios to form compounds.

• All matter is made of atoms.

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

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

• Atoms are not created or destroyed in chemical reactions.

Subatomic Particles and Electrical Charges

Atoms are composed of three main subatomic particles:

• Protons: Positive charge (+1), located in the nucleus.

• Neutrons: No charge (neutral), located in the nucleus.

• Electrons: Negative charge (–1), located outside the nucleus.

Like charges repel; unlike charges attract.

Table: Subatomic Particles in the Atom

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

Atomic Models

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

• Rutherford's Gold Foil Experiment: Showed that atoms have a small, dense, positively charged nucleus with electrons in the surrounding space.

Structure of the Atom

• The nucleus contains protons and neutrons.

• Electrons occupy a large, mostly empty space around the nucleus.

Mass of the Atom

The mass of an atom is concentrated in the nucleus. The atomic mass unit (amu) is used to express atomic and subatomic masses.

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

• Protons and neutrons each have a mass of approximately 1 amu.

• Electrons have negligible mass compared to protons and neutrons.

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 a given element have the same atomic number.

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

$\text{Atomic number} = \text{Number of protons}$

Mass Number ($A$)

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

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

$\text{Mass number} = \text{Number of protons} + \text{Number of neutrons}$

Atoms Are Neutral

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

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

Table: Composition of Some Atoms of Different Elements

Element	Symbol	Atomic Number	Mass Number	Protons	Neutrons	Electrons
Hydrogen	$^1_1$H	1	1	1	0	1
Carbon	$^{12}_6$C	6	12	6	6	6
Oxygen	$^{16}_8$O	8	16	8	8	8
Magnesium	$^{24}_{12}$Mg	12	24	12	12	12
Aluminum	$^{27}_{13}$Al	13	27	13	14	13

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)

4.5 Isotopes and Atomic Mass

Isotopes

Isotopes are atoms of the same element (same atomic number) 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 $^{A}_{Z}X$, where $A$ is the mass number, $Z$ is the atomic number, and $X$ is the element symbol.

Example: $^{12}_{6}$C and $^{14}_{6}$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.

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

Table: Atomic Symbols and Subatomic Particles

Symbol	Atomic Number	Mass Number	Protons	Neutrons	Electrons
$^{16}_8$O	8	16	8	8	8
$^{31}_{15}$P	15	31	15	16	15
$^{65}_{30}$Zn	30	65	30	35	30

Learning Check: Isotopes

• Atoms with the same number of protons but different numbers of neutrons are isotopes.

• Atoms with the same mass number but different atomic numbers are not isotopes.

Additional info: These notes are based on standard GOB Chemistry curriculum and expand on the provided lecture slides for clarity and completeness.