Elements

Definition and Significance

An element is a pure substance consisting of only one type of atom. Elements are considered the fundamental building blocks of matter because all substances are composed of combinations of elements.

• Element Symbols: Each element is represented by a unique symbol, with the first letter capitalized and the second letter (if present) lowercase (e.g., C for carbon, Ca for calcium).

• Common Element Symbols: Examples include C (carbon), N (nitrogen), O (oxygen), Ca (calcium), Mg (magnesium).

Periodic Table Organization

Structure and Arrangement

The periodic table organizes elements by increasing atomic number and groups them based on similar properties.

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

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

• Arrangement: Elements are ordered by increasing atomic number (number of protons).

Metals, Metalloids, and Nonmetals

Classification and Properties

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

• Metals: Located on the left and center of the periodic table; conductive, shiny, ductile, malleable.

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

• Metalloids: Found between metals and nonmetals; have properties of both groups.

Structure of the Atom

Atomic Components

An atom consists of a central nucleus and an electron cloud surrounding it.

• Nucleus: Contains protons and neutrons.

• Electron Cloud: Region where electrons are found, surrounding the nucleus.

Subatomic Particles

Properties and Location

Atoms are made up of three main subatomic particles: protons, neutrons, and electrons.

• Proton: Positive charge (+1), mass ≈ 1 amu, located in the nucleus.

• Neutron: No charge (0), mass ≈ 1 amu, located in the nucleus.

• Electron: Negative charge (–1), mass ≈ 1/1836 amu, located in the electron cloud.

• Most of an atom’s mass is concentrated in the nucleus.

Atomic Number

Definition and Importance

The atomic number is the number of protons in an atom and determines the element’s identity.

• Atomic number = number of protons.

• In a neutral atom, number of protons = number of electrons.

Mass Number

Calculation and Use

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

• Formula:

• To find neutrons:

Isotopes

Definition and Examples

Isotopes are atoms of the same element with the same number of protons but different numbers of neutrons.

• Examples: carbon-12, carbon-13, carbon-14.

• Isotopes have different mass numbers but identical chemical properties.

Electron Energy Levels

Shells and Electron Movement

Electrons occupy specific energy levels (shells) around the nucleus.

• Lower energy levels are closer to the nucleus.

• Electrons can move between energy levels by absorbing or releasing energy.

Bohr Model

Quantized Energy and Electron Transitions

The Bohr model describes electrons in quantized energy levels.

• When electrons absorb energy, they move to a higher energy level (excited state).

• When electrons release energy, they return to a lower energy level (relaxation).

Electron Shell Capacity

Maximum Electrons per Shell

The maximum number of electrons in a shell is determined by the formula:

• Formula: (where n = shell number)

• First shell (n=1): 2 electrons; Second shell (n=2): 8 electrons.

Valence Electrons

Definition and Chemical Properties

Valence electrons are electrons in the outermost shell and determine an atom’s chemical properties.

• The Group A number on the periodic table corresponds to the number of valence electrons.

Lewis Dot Structures

Representation of Valence Electrons

Lewis dot structures use dots to represent valence electrons around an element’s symbol.

• Each dot represents one valence electron.

• Helps visualize bonding and chemical reactivity.

Electron Configuration

Writing and Principles

Electron configuration shows the arrangement of electrons in an atom’s orbitals.

• Electrons fill the lowest energy orbitals first (Aufbau principle).

• No two electrons in the same atom can have identical quantum numbers (Pauli Exclusion Principle).

• Electrons fill orbitals singly before pairing (Hund’s Rule).

Condensed Electron Configuration

Noble Gas Notation and Electron Types

Condensed electron configuration uses noble gas notation to simplify electron arrangements.

• Core electrons: Electrons in inner shells.

• Valence electrons: Electrons in the outermost shell.

• Example: Sodium (Na): [Ne] 3s1

Subatomic Particle Comparison Table

Properties of Protons, Neutrons, and Electrons