Chapter 4 - Atoms and Elements: Structure, Properties, and the Periodic Table

Chapter 4 – Atoms and Elements

Section 4.1: Elements and Symbols

Elements are the fundamental substances from which all matter is constructed. Each element is represented by a unique chemical symbol.

• Elements are pure substances that cannot be broken down into simpler substances by chemical means.

• There are 118 known elements, each with a unique symbol.

• Chemical symbols consist of one or two letters, starting with a capital letter (e.g., C for carbon, S for sulfur, Au for gold, Al for aluminum).

Examples:

• One-letter symbols: Carbon (C), Sulfur (S)

• Two-letter symbols: Gold (Au), Aluminum (Al)

Section 4.2: The Periodic Table

The periodic table organizes all known elements based on their properties and atomic structure.

• There are 118 elements arranged in order of increasing atomic mass (now atomic number).

• Elements are grouped by similar chemical properties into groups (vertical columns) and periods (horizontal rows).

Groups and Periods

• Groups: Vertical columns containing elements with similar properties.

• Periods: Horizontal rows numbered from 1 to 7.

Group Numbers and Names

• Groups are labeled at the top of each column.

• Representative elements: Groups 1A–8A (or 1, 2, 13–18 in the IUPAC system).

• Transition elements: Groups 3B–8B, 1B, 2B (or 3–12 in IUPAC).

Alkali and Alkaline Earth Metals

• Group 1A (1): Alkali metals – lithium (Li), sodium (Na), potassium (K), rubidium (Rb), cesium (Cs)

• Group 2A (2): Alkaline earth metals – beryllium (Be), magnesium (Mg), calcium (Ca), strontium (Sr), barium (Ba), radium (Ra)

Halogens and Noble Gases

• Group 7A (17): Halogens – fluorine (F), chlorine (Cl), bromine (Br), iodine (I), astatine (At), tennessine (Ts)

• Group 8A (18): Noble gases – helium (He), neon (Ne), argon (Ar), krypton (Kr), xenon (Xe), radon (Rn), oganesson (Og)

Metals, Nonmetals, and Metalloids

• The zigzag line on the periodic table separates metals (left) from nonmetals (right).

• Metalloids are located along the zigzag line and have properties intermediate between metals and nonmetals.

Characteristics of Metals, Nonmetals, and Metalloids

Type	Properties
Metals	Shiny, ductile, conduct heat and electricity, mostly solids (except Hg, which is liquid)
Nonmetals	Dull, brittle, poor conductors, good insulators, low densities and melting points
Metalloids	Exhibit properties of both metals and nonmetals, used as semiconductors and insulators

Section 4.3: The Atom

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

• Atoms are the building blocks of matter.

• Atoms cannot be broken down further by chemical means.

Dalton’s Atomic Theory (1808)

• All matter is made of atoms.

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

• Atoms combine in fixed ratios to form compounds.

• Atoms are rearranged in chemical reactions but are not created or destroyed.

Additional info: Dalton's theory laid the foundation for modern atomic theory, though later discoveries (e.g., isotopes, subatomic particles) refined it.

Key Experiments

• J.J. Thomson (1897): Discovered electrons (negatively charged particles) using cathode ray experiments; proposed the "plum pudding" model.

• Ernest Rutherford (1911): Gold foil experiment showed atoms are mostly empty space with a small, dense, positively charged nucleus.

Electromagnetic Radiation

• Consists of energy "particles" (photons) that move as waves.

• Wavelength (λ): Distance between wave peaks; measured in nm, μm, mm, m, etc.

• High-energy radiation has short wavelengths; low-energy radiation has long wavelengths.

Electromagnetic Spectrum: Includes radio waves, microwaves, infrared, visible light, ultraviolet, X-rays, and gamma rays.

Charged Particles in an Atom

• Protons: Positive charge (+1)

• Electrons: Negative charge (–1)

• Neutrons: No charge (neutral)

• Like charges repel; unlike charges attract.

Structure of the Atom

• Nucleus: Center of the atom; contains protons and neutrons; most of the atom's mass.

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

Mass of the Atom

• Measured in atomic mass units (amu).

• 1 amu = 1/12 the mass of a carbon-12 atom ($1.66 \times 10^{-27}$ kg).

• 1 amu = 1 Dalton (Da) in biology.

• Electrons have negligible mass compared to protons and neutrons.

Particle	Symbol	Charge	Mass (amu)	Location
Proton	p	+1	1.007	Nucleus
Neutron	n	0	1.008	Nucleus
Electron	e–	–1	0.00055	Outside nucleus

Section 4.4: Atomic Number and Mass Number

Each element is defined by its 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.

• Atomic number is written below the element symbol; mass number is written above.

For neutral atoms:

• Number of protons = number of electrons

• If protons ≠ electrons, the particle is an ion (cation: more protons; anion: more electrons).

Example: Magnesium ($^{24}_{12}\text{Mg}$): 12 protons, 12 electrons, 12 neutrons (24 – 12 = 12).

Counting Protons, Electrons, and Neutrons

Isotope	Atomic Number	Mass Number	Number of Protons	Number of Neutrons
$^{16}_8$O	8	16	8	8
$^{31}_{15}$P	15	31	15	16

Practice Example: Lead (Pb) with mass number 207 and atomic number 82 has 125 neutrons (207 – 82 = 125).

Section 4.5: Isotopes and Atomic Mass

Isotopes are atoms of the same element with the same number of protons but different numbers of neutrons, resulting in different mass numbers.

• Isotopes are represented by their atomic symbols (e.g., $^{24}$Mg, $^{25}$Mg, $^{26}$Mg).

Average Atomic Mass is the weighted average of the masses of all naturally occurring isotopes of an element.

• Calculated using the formula:

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

Example: If gallium has 60.10% $^{69}$Ga (68.926 amu) and 39.90% $^{71}$Ga (70.925 amu):

$\text{Average atomic mass} = (0.6010 \times 68.926) + (0.3990 \times 70.925) = 69.72$ amu

Practice and Application

• Use the periodic table to identify elements, their symbols, and properties.

• Calculate the number of protons, neutrons, and electrons for any isotope.

• Distinguish between metals, nonmetals, and metalloids based on their location and properties.

Additional info: These foundational concepts are essential for understanding chemical bonding, reactions, and the behavior of matter in GOB Chemistry.