Atoms and Elements: Structure, Periodic Table, and Atomic Mass

Atoms and Elements

Introduction

Atoms are the fundamental building blocks of matter, and elements are pure substances consisting of only one type of atom. Understanding the structure of atoms and how elements are organized in the periodic table is essential for studying chemistry.

Atomic Number and Chemical Symbol

Definition and Importance

• Atomic number (Z): The number of protons in the nucleus of an atom. It uniquely identifies an element.

• Chemical symbol: A one- or two-letter abbreviation for an element, listed below its atomic number on the periodic table (e.g., He for helium, C for carbon, N for nitrogen).

• The number of protons determines the identity of the element.

Example: Helium has an atomic number of 2 (2 protons), and its chemical symbol is He. Carbon has an atomic number of 6 (6 protons), and its symbol is C.

The Periodic Table

Organization and Structure

• Elements are arranged in order of increasing atomic number.

• Each element is represented by its atomic number, chemical symbol, and name.

• The periodic table is divided into groups (vertical columns) and periods (horizontal rows).

• Groups are often labeled with a number and the letter A or B (e.g., 1A, 2A, 7A).

Classification of Elements

• Metals: Located on the lower-left side and middle of the table. Properties include:

  • Good conductors of heat and electricity

  • Malleable (can be pounded into sheets)

  • Ductile (can be drawn into wires)

  • Often shiny

  • Tend to lose electrons in chemical reactions

• Nonmetals: Located on the upper-right side. Properties include:

  • Poor conductors of heat and electricity

  • Not ductile or malleable

  • Gain electrons in chemical reactions

  • Exist as solids, liquids, or gases at room temperature

• Metalloids (Semimetals): Found along the zigzag line dividing metals and nonmetals. Exhibit mixed properties and are often semiconductors.

Special Groups

• Noble gases (Group 8A): Chemically stable, do not readily form compounds (e.g., helium, neon, argon).

• Alkali metals (Group 1A): Highly reactive metals (e.g., lithium, sodium, potassium).

• Alkaline earth metals (Group 2A): Reactive metals, less so than alkali metals (e.g., magnesium, calcium).

• Halogens (Group 7A): Very reactive nonmetals (e.g., fluorine, chlorine, bromine, iodine).

Isotopes and Atomic Mass

Isotopes

• Atoms of the same element can have different numbers of neutrons, resulting in different isotopes.

• Isotopes have the same number of protons but different numbers of neutrons and thus different mass numbers.

• Example: Neon has three isotopes: all have 10 protons, but may have 10, 11, or 12 neutrons.

Notation for Isotopes

• The mass number (A) is the sum of protons and neutrons: $A = p + n$

• Isotopes are represented as $^A_ZX$, where X is the chemical symbol, A is the mass number, and Z is the atomic number.

Natural Abundance

• The relative amount of each isotope in a natural sample is called its natural abundance.

• Example: In natural neon: 90.48% is Ne-20, 0.27% is Ne-21, and 9.25% is Ne-22.

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 is listed below the chemical symbol on the periodic table.

• Formula:

$\text{Atomic mass} = \sum (\text{fraction of isotope}_n \times \text{mass of isotope}_n)$

• Example: Chlorine consists of 75.77% Cl-35 (34.97 amu) and 24.23% Cl-37 (36.97 amu):

  • Cl-35: $0.7577 \times 34.97 = 26.4968$ amu

  • Cl-37: $0.2423 \times 36.97 = 8.9578$ amu

  • Total atomic mass: $26.4968 + 8.9578 = 35.45$ amu

Ions: Losing and Gaining Electrons

Formation of Ions

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

• Atoms can lose or gain electrons during chemical changes, forming ions (charged particles).

• Cations: Positively charged ions formed by losing electrons (e.g., Na+).

• Anions: Negatively charged ions formed by gaining electrons (e.g., S2−).

Predicting Ion Charges from the Periodic Table

• Group 1A metals form 1+ ions.

• Group 2A metals form 2+ ions.

• Group 7A nonmetals form 1− ions.

• Group 6A nonmetals form 2− ions.

• Transition metals may form ions with various charges.

Counting Atoms: The Mole and Avogadro's Number

The Mole Concept

• A mole (mol) is a counting unit for atoms, molecules, or other particles.

• 1 mole = $6.022 \times 10^{23}$ particles (Avogadro's number).

• The mole is defined as the number of atoms in exactly 12 grams of pure carbon-12.

Conversions Involving Moles

• To convert between number of moles and number of atoms:

  • $1 \text{ mol atoms} = 6.022 \times 10^{23} \text{ atoms}$

• To convert between mass and moles, use the molar mass (grams per mole):

  • Molar mass (g/mol) = atomic mass (amu) numerically

  • Example: $1 \text{ mol C} = 12.01 \text{ g C}$

General Conversion Plan

• To find the number of atoms in a sample:

  1. Measure the mass of the sample.

  2. Convert mass to moles using molar mass.

  3. Convert moles to number of atoms using Avogadro's number.

Example: How many atoms are in a 3.10 g copper penny? (Assume pure copper.)

$\text{Moles of Cu} = \frac{3.10 \text{ g}}{63.55 \text{ g/mol}} = 0.0488 \text{ mol}$ $\text{Number of atoms} = 0.0488 \text{ mol} \times 6.022 \times 10^{23} \text{ atoms/mol} = 2.94 \times 10^{22} \text{ atoms}$

Summary Table: Element Classification and Properties

Type	Location on Periodic Table	Key Properties	Typical Ion Formation
Metals	Left and center	Conductive, malleable, ductile, shiny	Lose electrons (form cations)
Nonmetals	Upper right	Poor conductors, brittle, varied states	Gain electrons (form anions)
Metalloids	Along zigzag line	Mixed properties, semiconductors	Varied

Additional info: Some details, such as specific atomic masses and example calculations, were inferred and expanded for clarity and completeness.