The Periodic Table of the Elements

Table of Contents

The periodic table of the elements is a way of arranging all known chemical elements so that their properties become easier to understand and predict. For biology, it is especially helpful for seeing how life’s most important elements are related to one another—above all, carbon.

What an Element Is (Chemically)

In chemistry, an element is a substance made of only one kind of atom.
Atoms of a given element all have the same number of protons in their nucleus; this is the atomic number.

Hydrogen atoms all have 1 proton → hydrogen is element 1
Carbon atoms all have 6 protons → carbon is element 6
Oxygen atoms all have 8 protons → oxygen is element 8

The periodic table sorts and organizes these elements using this atomic number and their recurring (“periodic”) chemical behavior.

Basic Structure of the Periodic Table

Periods (Rows)

The horizontal rows are called periods.

Each period corresponds to atoms with electrons filling the same main energy level (shell).
As you move from left to right in a period, the atomic number increases by 1 each step, so each element has one more proton (and, in a neutral atom, one more electron) than the previous element.
Across a period, properties change gradually: from metals on the left, through semimetals, to nonmetals on the right.

Biologically important examples along periods:

Period 1: hydrogen (H), helium (He)
Period 2: lithium (Li), beryllium (Be), boron (B), carbon (C), nitrogen (N), oxygen (O), fluorine (F), neon (Ne)
Period 3: sodium (Na), magnesium (Mg), aluminum (Al), silicon (Si), phosphorus (P), sulfur (S), chlorine (Cl), argon (Ar)

Groups (Columns)

The vertical columns are called groups or families.
Elements in the same group have similar outer electron configurations and therefore similar chemical properties.

Typical group names (from left to right):

Group 1: alkali metals (e.g., Li, Na, K)
Group 2: alkaline earth metals (e.g., Mg, Ca)
Groups 3–12: transition metals (e.g., Fe, Cu, Zn)
Group 17: halogens (e.g., F, Cl, Br, I)
Group 18: noble gases (e.g., He, Ne, Ar)

Biologically important groups:

Group 1 (Na, K): key for nerve impulses and fluid balance.
Group 2 (Mg, Ca): Mg in chlorophyll; Ca in bones and cell signaling.
Group 15 (N, P): N in amino acids and nucleic acids; P in ATP and DNA backbone.
Group 16 (O, S): O in water and respiration; S in some amino acids.
Group 17 (Cl): Cl in body fluids, gastric acid.

Carbon (C) is in Group 14, together with silicon (Si), which is also relevant in some organisms (e.g., diatoms use silica for their shells).

Atomic Number, Mass Number, and Isotopes

Each element in the periodic table is usually represented with:

Its symbol (e.g., C)
Its atomic number, $ Z $ (number of protons)
Its average atomic mass (in atomic mass units, u), which reflects the typical mix of isotopes

Atomic Number $ Z $

Defines the element.
For carbon, $ Z = 6 $. Any atom with 6 protons is carbon.

Mass Number $ A $

Total number of protons + neutrons in the nucleus.
Often written as a superscript in front of the symbol, e.g. $ {}^{12}\mathrm{C} $, $ {}^{14}\mathrm{C} $.

The relationship:
$$
A = Z + N
$$
where $ N $ is the number of neutrons.

Isotopes

Isotopes are atoms of the same element (same $ Z $) with different numbers of neutrons (different $ A $).

Example: Carbon

$ {}^{12}\mathrm{C} $: 6 protons, 6 neutrons → stable
$ {}^{13}\mathrm{C} $: 6 protons, 7 neutrons → stable
$ {}^{14}\mathrm{C} $: 6 protons, 8 neutrons → radioactive

Isotopes matter for biology because:

Some stable isotopes are used as tracers to follow atoms through metabolic pathways.
Radioactive isotopes such as $ {}^{14}\mathrm{C} $ allow dating of formerly living material (radiocarbon dating).

In the periodic table, the mass listed for an element is the average atomic mass, weighted by how common each isotope is in nature.

Electron Shells and Valence Electrons (In Brief)

The arrangement of electrons around the nucleus explains why the periodic table groups elements as it does.

Electrons occupy shells (energy levels) and subshells (orbitals). The electrons in the outermost shell are called valence electrons. These determine how an element forms chemical bonds.

General pattern:

Elements in the same group have the same number of valence electrons (except some transition metals), giving them similar chemical behavior.
As you go across a period, the number of valence electrons increases, and properties gradually change.

Examples:

Group 1 (Na, K): 1 valence electron → tend to lose 1 electron, forming $ \mathrm{Na^+} $, $ \mathrm{K^+} $.
Group 17 (Cl): 7 valence electrons → tends to gain 1 electron, forming $ \mathrm{Cl^-} $.
Group 18 (Ne, Ar): 8 valence electrons (except He with 2) → very stable, hardly reactive.

Carbon, in Group 14, has 4 valence electrons. This half-filled valence shell is crucial for its ability to form up to four covalent bonds and an enormous variety of stable compounds—this is developed in detail in the following chapter on carbon as an element.

Metals, Nonmetals, and Metalloids

The periodic table also reflects broad categories of elements:

Metals (left and center): usually shiny, conductive, malleable; tend to lose electrons.
Nonmetals (upper right): often gases or brittle solids; poor conductors; tend to gain or share electrons.
Metalloids/semimetals (border between metals and nonmetals, e.g., B, Si): properties in between.

Life largely relies on nonmetals (C, H, N, O, P, S) for the main frameworks of molecules, and on selected metals (e.g., Na, K, Ca, Mg, Fe, Cu, Zn) in smaller amounts for more specialized roles (enzymes, signaling, structures).

Periodic Trends Relevant to Biology

Several properties change systematically across the periodic table. Two especially matter for biological chemistry: electronegativity and ionization energy.

Electronegativity

Electronegativity is a measure of how strongly an atom attracts shared electrons in a bond.

Trends in the table:

Increases from left to right across a period.
Decreases from top to bottom in a group.

Biologically important electronegativities (relative values):

Hydrogen (H): moderate
Carbon (C): moderate
Nitrogen (N), oxygen (O): high
Halogens (e.g. F, Cl): very high

Consequences in biological molecules:

Bonds between atoms of similar electronegativity (C–C, C–H) are mostly nonpolar.
Bonds where one atom is much more electronegative (O–H, N–H) become polar, with partial charges.
Many key interactions in cells (hydrogen bonds, solubility in water, protein folding) depend on these polar and nonpolar patterns, which in turn are rooted in the periodic trends of the elements involved.

Ionization Energy and Chemical Reactivity (Overview)

Ionization energy is the energy required to remove an electron from an atom.

Low ionization energy → electrons are removed easily → atoms are more likely to form positive ions (cations), as in alkali metals (Na, K).
High ionization energy → electrons are held tightly → atoms tend to attract electrons instead (as in O, N, F).

These tendencies explain why:

Sodium (Na) and potassium (K) readily form $ \mathrm{Na^+} $ and $ \mathrm{K^+} $, crucial in nerve impulses.
Chlorine (Cl) forms $ \mathrm{Cl^-} $, combining with Na to make table salt (NaCl).
Oxygen (O) is a powerful oxidizing agent in respiration, accepting electrons from fuel molecules.

Blocks of the Periodic Table (s, p, d, f)

The periodic table can also be divided into blocks based on which type of orbital (s, p, d, f) is being filled by electrons:

s-block: Groups 1–2 and helium → alkali and alkaline earth metals, plus hydrogen and helium.
p-block: Groups 13–18 → includes C, N, O, P, S, halogens, and noble gases.
d-block: Transition metals (Groups 3–12) → Fe, Cu, Zn, etc.
f-block: Lanthanoids and actinoids (separate rows at the bottom).

Most lighter elements crucial for life come from the s- and p-blocks, while certain transition metals serve as essential trace elements.

Biologically Important Regions of the Periodic Table

If you highlight elements most used in living organisms, several zones stand out:

Main biogenic elements (bulk of organic matter):

C, H, O, N, P, S (all in the main groups, mostly periods 2 and 3)

Macro- and microelements (needed in smaller but essential amounts):

Na, K, Ca, Mg (electrolytes, structural roles)
Fe, Cu, Zn, Mn, Co, Mo, etc. (enzyme cofactors, oxygen transport)

Light, high-electronegativity elements (N, O, F, Cl):

Central in water chemistry, acids/bases, and energy metabolism.

These elements cluster in particular parts of the periodic table and share related properties because of their positions. Understanding this layout makes it easier to see why certain elements, especially carbon, were “chosen” by evolution as the main building blocks of life.

How the Periodic Table Helps in Biology

For biological chemistry, the periodic table is not just a list; it is a map of chemical behavior that allows you to:

Predict which elements will form ions (e.g., Na⁺, K⁺, Ca²⁺, Cl⁻).
Anticipate which atoms will form polar versus nonpolar bonds.
Recognize which elements can form many diverse compounds (like C, N, O, S).
Understand why certain metals appear again and again in enzymes and proteins.

In the following chapters, this foundation will be used to examine carbon in more detail and to understand the types of chemical bonds and molecules that make up living organisms.