Atoms and Periodic Table Basics

Atoms: Basic Structure and Key Concepts

Everything is made of atoms, including you.
An atom has a core (nucleus) and electrons orbiting around it.
The core (nucleus) contains protons and neutrons; electrons form surrounding shells.
The number of protons in the nucleus determines the element.
Water is made of hydrogen and oxygen; sodium is another element example; atoms come in many elements.
Quantum mechanics teaches that atoms don't look exactly like simple spheres with fixed shells; observable behavior is more nuanced, to be discussed later.
Think of atoms as having multiple electron shells; electrons in the outermost shell are called valence electrons.
Most chemistry is driven by the behavior of valence electrons.
The periodic table catalogs all elements with key information.
Elements in the same column (group) have the same number of valence electrons (general rule).
For the main groups, the number of valence electrons equals the group number from 1 to 8.
Helium is an exception: it has only 2 electrons in its shell but behaves like a noble gas, so it’s grouped with the noble gases.
Transition metals were claimed to follow a simple pattern, but that claim is not strictly true; the pattern is more complex (not essential for the moment).
Elements with the same number of valence electrons tend to show similar chemical behavior.
Example: the alkali metals are in Group 1 (the first group without hydrogen).
- Common properties: 1 valence electron; shiny metals; relatively soft; reactive with water (and may exhibit characteristic reactions).
All elements in the same row (period) have the same number of electron shells; the number of shells increases from top to bottom.
The atomic mass tends to increase along a period as protons, neutrons, and electrons are added, though the precise mass pattern is nuanced in real data.
Isotopes: the number of neutrons in the nucleus varies, creating isotopes of the same element; many isotopes are unstable and radioactive, emitting ionizing radiation which can be dangerous.
If an atom has the same number of electrons as protons, it has no net charge (neutral).
If there are more electrons than protons, the atom is negatively charged (anion).
If there are fewer electrons than protons, the atom is positively charged (cation).
The periodic table also serves as a quick reference: it lists the element name, symbol, number of protons (the atomic number, Z), total electrons (in a neutral atom), and the atomic mass (mass of protons and neutrons combined).
The periodic table is divided into categories:
- Metals are on the left side of a dividing line.
- Nonmetals are on the right side, and are often gases.
- The dividing line is called the semimetals (metalloids), with properties between metals and nonmetals.
A molecule is formed when two or more atoms bond together.
A compound is formed when two or more different elements are present in a bonded structure.
Compounds can behave very differently from the elements they’re made of (emergent properties).
A simple real-world illustration: combining a reactive metal with a toxic gas can yield a compound with novel properties; a classic example discussed is table salt (NaCl), formed from sodium (an alkali metal) and chlorine (a nonmetal).
Summary example: table salt NaCl is formed when sodium (Na) bonds with chlorine (Cl), illustrating how compounds differ from the constituent elements.
Note: the discussion includes a humorous aside about the complexity of transition metals’ patterns, signaling that not all blocks of the periodic table follow a simple rule.

Valence electrons, groups, and periodic trends (expanded)

Valence electrons are the outer-shell electrons that largely govern chemical reactivity.
In main-group elements, valence electron count equals the group number (1 through 8).
Helium, despite having only 2 electrons, behaves like a noble gas and is grouped with noble gases rather than with the alkali metals.
Elements with the same valence electron count tend to react similarly, leading to periodic trends in reactivity and bonding.
The alkali metals (Group 1, excluding hydrogen) share:
- 1 valence electron
- Shiny metallic luster
- Relatively soft texture
- High reactivity (with water, air, etc.)

The periodic table: structure and what’s listed

The periodic table is organized by increasing atomic number (number of protons Z).
Each cell provides:
- Element name
- Symbol
- Number of protons (Z) and typically the total number of electrons in a neutral atom
- Atomic mass (approximately the total mass of protons and neutrons in the nucleus)
The table is divided into:
- Metals on the left
- Nonmetals on the right (many are gases at room temperature)
- Semimetals (metalloids) along the dividing line with mixed properties
The arrangement mirrors how elements gain shells and electrons as you move across periods and groups.
The mass pattern described in the transcript notes that mass increases with added protons, neutrons, and electrons, but actual mass behavior includes isotopes and natural abundance variations.

Molecules, compounds, and chemical behavior (applications)

A molecule forms when two or more atoms bond together.
A compound forms when there are two or more different elements in the bonded structure.
Compounds can exhibit properties that are not present in their constituent elements (emergent properties).
Example concept highlighted: combining different elements can produce common substances with entirely different properties (e.g., table salt NaCl).
The statement about an explosive metal and a toxic gas combining to form table salt is likely a simplified illustrative error; the intended idea is that compounds form from elements, sometimes with hazardous consequences if reactive species are involved.

Quick reference formulas and key statements (LaTeX)

Mass number: A = Z + N
Net atomic charge (for an atom with E electrons and Z protons): q = Z - E
- If E = Z
  ightarrow q = 0 (neutral atom)
- If E > Z
  ightarrow q < 0 (anion)
- If E < Z ightarrow q > 0 (cation)
Valence electrons in main-group elements: v = g ext{ where } g ext{ is the group number } (1 ext{ to } 8)
Helium exception: v_{ ext{He}} = 2
Number of electron shells per atom corresponds to the period: S = ext{period number}
Isotopes: A = Z + N, ext{ with } N ext{ neutrons varying across isotopes; many isotopes are radioactive.}
Atomic mass approximation: m_{ ext{atom}} \,\approx\, A \, u where u is the atomic mass unit.

Connections to broader concepts and real-world relevance

Understanding valence electrons helps explain why certain elements react in similar ways and form similar compounds.
The division between metals, metalloids, and nonmetals correlates with properties like conductivity, reactivity, and physical state at room temperature.
Isotopes have practical implications in medicine, energy, and dating methods due to differences in stability and radioactive decay.
The idea that compounds behave differently from elements underpins why chemistry is about bonds, reaction conditions, and molecular structure rather than just elemental properties.
Ethical and practical implications: handling reactive metals, radioactive isotopes, and hazardous gases requires safety factors due to ionizing radiation and chemical reactivity.

Quick recall prompts (to test understanding)

What determines the element in an atom? (Number of protons, Z)
What is the role of valence electrons in chemistry? (Governs bonding and reactivity)
Why are isotopes of an element different? (Different numbers of neutrons N, yielding different mass numbers A = Z + N)
How is the periodic table organized? (By increasing Z; metals on left, nonmetals on right, metalloids on the dividing line)
What distinguishes a molecule from a compound? (A molecule is made of atoms bonded together; a compound contains two or more different elements)
What is table salt, and why is it a useful example in chemistry discussions? (NaCl, formed from sodium and chlorine; illustrates how compounds have properties different from constituent elements)