Atoms, Ions, and the Periodic Table: Study Notes

Atoms, Ions, and the Periodic Table

Atomic Structure: Protons, Neutrons, and Electrons

Atoms are composed of three fundamental subatomic particles: protons, neutrons, and electrons. The number of each determines the identity and properties of an atom or ion.

• Protons: Positively charged particles found in the nucleus. The number of protons defines the atomic number (Z) and the element.

• Neutrons: Neutral particles found in the nucleus. The sum of protons and neutrons gives the mass number (A).

• Electrons: Negatively charged particles found in orbitals around the nucleus. In a neutral atom, the number of electrons equals the number of protons.

Example: For Cl (Chlorine), atomic number = 17, typical isotope mass number = 35 or 37.

• Protons: 17

• Neutrons: 18 (for Cl-35), 20 (for Cl-37)

• Electrons: 17 (neutral atom)

Ions: Cations and Anions

An ion is an atom or molecule with a net electric charge due to the loss or gain of electrons.

• Cation: Positively charged ion (fewer electrons than protons).

• Anion: Negatively charged ion (more electrons than protons).

• Neutral atom: Number of electrons equals number of protons.

Example: For W7+ (Tungsten ion with +7 charge):

• Protons: 74 (atomic number of W)

• Electrons: 67 (74 - 7)

• Charge: +7 (cation)

Periodic Table of the Elements

The Periodic Table organizes elements by increasing atomic number and groups elements with similar chemical properties into columns.

• Groups: Vertical columns, elements share similar valence electron configurations.

• Periods: Horizontal rows, elements have the same number of electron shells.

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

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

Example: Sulfur (S) is in group 16, period 3, atomic number 16.

Table: Atomic Properties and Classification

The following table summarizes atomic and ionic properties for selected examples:

Protons	Neutrons	Electrons	Atomic Number (Z)	Mass Number (A)	Charge	Atomic Symbol	Name of Element
7	8	10	7	15	-3	N3-	Nitrogen
33	60	35	33	93	+2	As2+	Arsenic
53	73	53	53	126	0	I	Iodine
16	17	17	16	33	0	S	Sulfur

Additional info: Some entries inferred based on standard atomic numbers and periodic table data.

Writing Chemical Symbols for Ions

Chemical symbols for ions indicate the element and its charge. The number of electrons determines the charge:

• Atoms or cations with 6 electrons: Examples include C4+ (Carbon ion), N5+ (Nitrogen ion), O6+ (Oxygen ion).

• Atoms or anions with 36 electrons: Examples include Br- (Bromide ion), Kr (Krypton, neutral atom), Se2- (Selenide ion).

Example: For Br-, atomic number = 35, electrons = 36.

Key Equations and Relationships

• Atomic Number: $Z = \text{Number of Protons}$

• Mass Number: $A = \text{Number of Protons} + \text{Number of Neutrons}$

• Charge: $\text{Charge} = \text{Number of Protons} - \text{Number of Electrons}$

Summary Table: Cations vs. Anions

Type	Charge	Electron Count	Example
Cation	Positive	Fewer than protons	Na+, Ca2+
Anion	Negative	More than protons	Cl-, O2-
Neutral Atom	Zero	Equal to protons	H, He, Ne

Applications

• Identifying elements and ions using the periodic table.

• Calculating atomic and mass numbers for isotopes.

• Writing chemical symbols for ions based on electron count.