5.1 Atomic Structure & the Periodic Table

Atomic structure and the periodic table provide standard vocabulary for introductory chemistry. This local editorial section covers atoms, isotopes, elements, oxidation states, and electron configuration without claiming that the Navy has published them as NAPT content.

Protons, Neutrons & Electrons

Every atom is built from three subatomic particles:

Protons and neutrons together make up the dense, positively charged nucleus at an atom's center; electrons occupy the much larger surrounding space in shells (energy levels). Because protons and electrons carry equal and opposite charge, a neutral atom always has the same number of protons as electrons.

Atomic Number vs. Mass Number

Two numbers describe every atom:

• Atomic number (Z) — the number of protons in the nucleus. This number defines which element the atom is; every atom of a given element has the same atomic number, and no two elements share one.
• Mass number (A) — the integer total of protons plus neutrons in one specific nuclide's nucleus. It is not the periodic table's average atomic mass rounded to a whole number.

For example, a neutral atom of potassium always has atomic number 19 (19 protons). If that same atom has 20 neutrons, its mass number is 19 + 20 = 39, written as potassium-39.

Isotopes

Isotopes are atoms of the same element (same atomic number, same proton count) that have different numbers of neutrons — and therefore different mass numbers. Carbon, for instance, has three naturally occurring isotopes:

All three are chemically carbon — same 6 protons, same chemical behavior — but they have different masses and, in the case of carbon-14, different nuclear stability. (Radioactive decay of unstable isotopes is covered in depth in the Nuclear Science chapter.)

When an atom gains or loses electrons rather than neutrons, it becomes an ion — a charged particle — rather than an isotope. Sodium (Na) losing its outer electron becomes Na⁺ (11 protons, 10 electrons); chlorine (Cl) gaining an electron becomes Cl⁻ (17 protons, 18 electrons). Ions are the building blocks of ionic bonding, covered in the next section.

Periodic Table Organization

The periodic table arranges all known elements by increasing atomic number into 7 periods (horizontal rows) and 18 groups (vertical columns).

• Periods correspond to the number of electron shells an atom has. Elements in Period 1 (H, He) use only one shell; elements in Period 3 (Na through Ar) use three.
• Groups collect elements with related chemical patterns. For many main-group elements, group position corresponds to a similar valence-electron pattern, but transition metals are more complex. Hydrogen is positioned in Group 1 because of its electron configuration yet is a nonmetal and is not an alkali metal; alkali metals below hydrogen generally react with water, with reactivity varying down the group.

Key groups to know by name:

The table is also divided by broad category: metals (most of the left and center of the table) tend to lose electrons and conduct electricity; nonmetals (upper right) tend to gain electrons; and metalloids — boron, silicon, germanium, arsenic, antimony, and tellurium — sit along the "staircase" between them and share properties of both.

The First 20 Elements

This local review uses the first 20 elements as a manageable introductory reference set:

Notice the pattern: elements 3, 11, and 19 (Li, Na, K) are all alkali metals; elements 9 and 17 (F, Cl) are both halogens; elements 2, 10, and 18 (He, Ne, Ar) are all noble gases. Recognizing these families by atomic number—not just by memorized symbol—supports the local exercises.

Common Oxidation States

An oxidation state (or oxidation number) describes the charge an atom effectively has in a compound, based on how many electrons it has gained, lost, or unequally shared. For selected main-group elements among the first 20, the following common oxidation-state patterns are useful simplifications; real compounds include exceptions:

Hydrogen is a special case: it usually shows +1 (as in H2O or HCl) but takes -1 in metal hydrides such as NaH, where it bonds to an even less electronegative metal.

Electron Configuration Basics

Electrons fill an atom's shells from the lowest energy level outward, a pattern called the Aufbau principle ("building-up" principle). Two equivalent descriptions are useful in this local review:

1. Simplified shell (Bohr) model — count electrons per shell, capped at 2 in the first shell and 8 in the second and third shells for the elements covered here:

2. Standard subshell notation — write out sublevels (s, p) in order of filling:

• Hydrogen (Z=1): 1s¹
• Oxygen (Z=8): 1s² 2s² 2p⁴
• Neon (Z=10): 1s² 2s² 2p⁶
• Sodium (Z=11): 1s² 2s² 2p⁶ 3s¹ (shorthand: [Ne] 3s¹)
• Chlorine (Z=17): 1s² 2s² 2p⁶ 3s² 3p⁵ (shorthand: [Ne] 3s² 3p⁵)

The outermost shell's electrons — the valence electrons — are what actually participate in bonding. Many introductory main-group bonding examples are described by a full outer shell of 8 valence electrons (2 for hydrogen and helium), a pattern called the octet rule; it is a model with important exceptions. Sodium's single valence electron is loosely held and easily lost; chlorine's 7 valence electrons need just one more to complete an octet, so it's easily gained. That imbalance — one atom wanting to give an electron away, another wanting to take one — is exactly what drives the ionic and covalent bonding covered in the next section.

Key Takeaways

• Atomic number (Z) = protons; mass number (A) = protons + neutrons; isotopes share Z but differ in neutron count and A.
• The table has 18 groups and 7 periods; similar valence patterns are mainly a main-group simplification, transition metals are more complex, and hydrogen is a Group 1 exception.
• Know the first 20 elements (H through Ca) by atomic number, and recognize the alkali metals, halogens, and noble gases among them.
• Selected main-group elements show common oxidation-state patterns, but group number alone does not determine every compound's oxidation state.
• Valence electrons drive bonding; the octet rule is a useful introductory main-group model with exceptions.