Atoms and Elements

Introduction

This chapter introduces the foundational concepts of atoms and elements, their organization in the periodic table, and the structure of atoms. Understanding these concepts is essential for studying chemical reactions, properties of matter, and the role of elements in biological systems.

Elements and Symbols

Definition and Properties of Elements

• Elements are pure substances from which all other substances are built.

• They cannot be broken down into simpler substances by chemical means.

• Each element is represented by a unique chemical symbol, usually one or two letters (first letter capitalized).

Examples: C for carbon, N for nitrogen, F for fluorine, O for oxygen, Co for cobalt, Ca for calcium.

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

Practice: Names and Symbols

• Iodine: I

• Iron: Fe

• Magnesium: Mg

• Zinc: Zn

• Nitrogen: N

The Periodic Table

Organization of the Periodic Table

• The periodic table arranges elements by increasing atomic number and groups them by similar properties.

• Groups (vertical columns) contain elements with similar chemical properties.

• Periods (horizontal rows) are numbered from 1 to 7.

• Group numbers can be written as 1A–8A (representative elements) and 3B–12B (transition elements), or as 1–18.

Group Names and Properties

• Alkali Metals (Group 1A/1): Highly reactive metals (e.g., Li, Na, K).

• Alkaline Earth Metals (Group 2A/2): Shiny, less reactive than alkali metals (e.g., Be, Mg, Ca).

• Halogens (Group 7A/17): Highly reactive nonmetals (e.g., F, Cl, Br, I).

• Noble Gases (Group 8A/18): Inert gases (e.g., He, Ne, Ar).

Metals, Nonmetals, and Metalloids

• Metals: Shiny, ductile, good conductors, mostly solids (except Hg).

• Nonmetals: Dull, brittle, poor conductors, low densities/melting points.

• Metalloids: Properties intermediate between metals and nonmetals; used as semiconductors (e.g., Si, Ge).

The Atom

Structure of the Atom

• An atom is the smallest particle of an element that retains its properties.

• Composed of a dense nucleus (protons and neutrons) and electrons in a large surrounding space.

Subatomic Particles

Atomic Number and Mass Number

• Atomic number (Z): Number of protons in the nucleus; unique for each element.

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

• For a neutral atom: number of protons = number of electrons.

Example: Lead (Pb) with atomic number 82 and mass number 207 has 82 protons, 125 neutrons, and 82 electrons.

Isotopes and Atomic Mass

Isotopes

• Atoms of the same element with different numbers of neutrons (different mass numbers).

• Isotopes are represented as , where X is the element symbol, A is the mass number, and Z is the atomic number.

Example: Magnesium has three isotopes: , , .

Calculating Atomic Mass

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

• Formula:

Example: Gallium has two isotopes: 60.10% Ga (68.926 amu) and 39.90% Ga (70.925 amu). amu

Electron Energy Levels

Energy Levels, Sublevels, and Orbitals

• Electrons occupy energy levels (n = 1, 2, 3, ...), with higher n farther from the nucleus and higher in energy.

• Each energy level contains one or more sublevels (s, p, d, f).

• Each sublevel contains a specific number of orbitals:

  • s: 1 orbital (2 electrons max)

  • p: 3 orbitals (6 electrons max)

  • d: 5 orbitals (10 electrons max)

  • f: 7 orbitals (14 electrons max)

Electron Configurations

• Describes the arrangement of electrons in an atom.

• Electrons fill orbitals in order of increasing energy (Aufbau principle).

• Notation: 1s2 2s2 2p2 (for carbon).

• Abbreviated notation uses the previous noble gas in brackets (e.g., [Ne] 3s2 3p4 for sulfur).

Trends in Periodic Properties

Valence Electrons and Lewis Symbols

• Valence electrons are the electrons in the outermost energy level; they determine chemical properties.

• The group number for representative elements indicates the number of valence electrons.

• Lewis symbols use dots to represent valence electrons around the element symbol.

Periodic Trends

• Atomic size increases down a group and decreases across a period.

• Ionization energy (energy to remove an electron) decreases down a group and increases across a period.

• Metallic character increases down a group and decreases across a period.

Elements Essential to Health

Major and Trace Elements in the Human Body

• 20 elements are essential for human health.

• Oxygen, carbon, hydrogen, and nitrogen make up 96% of body mass.

• Macrominerals (e.g., Ca, P, K, Cl, S, Na, Mg) are important for bones, heart, muscles, and metabolism.

• Microminerals (trace elements) are needed in small amounts (e.g., Fe, Zn, Cu, I).

Summary

• Elements are the building blocks of matter, each with a unique symbol and atomic number.

• The periodic table organizes elements by properties and atomic structure.

• Atoms consist of protons, neutrons, and electrons; isotopes differ in neutron number.

• Electron configurations and periodic trends explain chemical behavior and reactivity.

• Essential elements are critical for biological functions and health.