Atomic Structure and the Periodic Table

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Every heartbeat, muscle contraction, and nerve impulse in the human body is fundamentally a transaction of invisible subatomic particles. When a nurse administers a saline intravenous drip, evaluates a patient’s blood potassium levels, or prepares a patient for a radioactive PET scan, they are directly manipulating and relying upon atomic structure. The physical properties of elements and the ways they interact to form life-sustaining molecules are not arbitrary; they are strictly governed by the architecture of atoms and their arrangement on the periodic table. To understand human physiology at a chemical level, one must first understand the blueprint of matter itself.

A whole-body PET scan utilizes radioactive isotopes to visualize metabolic processes, demonstrating how healthcare and human physiology directly rely on the manipulation of atomic structure.

At the core of all matter is the atom, a structure defined by three distinct subatomic particles. The atom is divided into two main regions: a dense, heavy center and a vast, mostly empty periphery.

Protons and neutrons are located within the nucleus of an atom. They provide the atom with its physical mass.

Protons have a positive electrical charge of plus one.
Neutrons have a neutral electrical charge of zero.

Because they form the dense core, the mass of a single proton is approximately one atomic mass unit (amu). The mass of a single neutron is also approximately one atomic mass unit.

Surrounding this dense nucleus is a frantic, orbiting swarm. Electrons orbit the atomic nucleus within a surrounding electron cloud.

Electrons have a negative electrical charge of minus one.
The mass of an electron is negligible compared to the masses of protons and neutrons. It takes nearly 2,000 electrons to equal the mass of a single proton.

The Rule of Neutrality: In a neutral atom, the total number of electrons is exactly equal to the total number of protons. The positive and negative charges cancel each other out perfectly.

The structure of a helium atom, featuring a dense nucleus composed of protons and neutrons, surrounded by a probabilistic cloud of orbiting, negatively charged electrons.

Identity and Mass

The number of protons in a nucleus is not just a structural detail; it is the fundamental defining characteristic of an element. The atomic number equals the total number of protons in the nucleus of an atom. Consequently, the atomic number defines the chemical identity of an element. If you have an atom with 6 protons, you have carbon. If you add a proton, it becomes nitrogen (7 protons).

While protons define identity, protons and neutrons together define weight. The mass number of an atom is the sum of its protons and its neutrons. Because electrons have negligible mass, we safely ignore them in this calculation.

In healthcare, you will frequently encounter elements like Iodine-131, used to treat thyroid disorders, or Carbon-14, used in radioactive tracing. These are isotopes.

Isotopes are atoms of the exact same element that contain different numbers of neutrons. Because they are the same element, isotopes of a specific element share the exact same atomic number (number of protons). However, because the neutron count varies, isotopes of a specific element have different mass numbers.

The three naturally occurring isotopes of hydrogen. While each contains exactly one proton (maintaining their chemical identity as hydrogen), they differ in their number of neutrons, which alters their mass number.

If you look at a periodic table, you will notice that the mass listed for an element is rarely a perfect whole number. Carbon, for instance, is listed as 12.011, not exactly 12. This is because the atomic mass listed on the periodic table is a calculated weighted average of all naturally occurring isotopes of that element. Most carbon is Carbon-12, but a tiny fraction is Carbon-13 and Carbon-14. The decimal reflects the natural abundance of these heavier isotopes mixed in.

If the nucleus is the heavy vault of the atom, the electrons are the active negotiators. Electrons are arranged in designated energy levels around the atomic nucleus. They do not orbit randomly; they occupy specific shells, expanding outward like the layers of an onion.

The first electron energy level of an atom can hold a maximum of two electrons.
The second electron energy level of an atom can hold a maximum of eight electrons.

Within these principal energy levels, electrons exist in specialized zones. Electron subshells within principal energy levels are designated by the letters s, p, d, and f.

The s electron subshell can hold a maximum of two electrons.
The p electron subshell can hold a maximum of six electrons.

Notice the math: an s subshell (2) plus a p subshell (6) equals 8 electrons. This number is paramount in chemistry. The octet rule states that atoms are most stable when their outermost electron shell contains eight electrons.

Electrons are arranged in specific principal energy levels. As seen in neon (far right), achieving a full outer shell of eight valence electrons satisfies the octet rule, creating a highly stable atom.

Valence Electrons and Ions

The electrons situated furthest from the nucleus are called valence electrons. Because they are on the outside, valence electrons determine the chemical reactivity and bonding behavior of an element.

Atoms will enthusiastically steal, donate, or share electrons to satisfy the octet rule. When this happens, the balance of protons to electrons is broken, and the atom acquires an electrical charge. An ion is formed when a neutral atom gains or loses electrons.

A cation is a positively charged ion formed by the loss of electrons. (If you lose negativity, you become more positive).
An anion is a negatively charged ion formed by the gain of electrons. (If you gain negativity, you become more negative).

In a clinical setting, we refer to these ions as electrolytes. Sodium ( $Na^+$ ), Potassium ( $K^+$ ), and Chloride ( $Cl^-$ ) are literally cations and anions conducting electrical currents across cell membranes to keep a patient alive.

Ion formation involves the transfer of electrons to achieve stability. When a neutral atom loses an electron, it becomes a positively charged cation; when it gains an electron, it becomes a negatively charged anion.

The modern periodic table arranges elements in order of increasing atomic number. However, it is not just a sequential list; it is a grid carefully organized by atomic behavior.

The horizontal rows on the periodic table are called periods.
The vertical columns on the periodic table are called groups or families.

This column structure is the key to predicting chemical behavior: elements within the same group on the periodic table possess the same number of valence electrons. Because they share the same outer electron configuration, elements within the same group on the periodic table exhibit similar chemical properties.

Broad Categories: Metals, Nonmetals, and Metalloids

The periodic table is broadly divided into three territories based on physical and chemical traits.

Category	Location on Periodic Table	Physical Properties	Chemical Behavior
Metals	Located on the left side and center of the periodic table.	Typically shiny, malleable, and highly conductive of heat and electricity.	Tend to lose electrons during chemical reactions to form positively charged cations.
Nonmetals	Located on the far right side of the periodic table. (Exception: Hydrogen)	Typically dull in appearance, brittle, and poor conductors of heat and electricity.	Tend to gain electrons during chemical reactions to form negatively charged anions.
Metalloids	Located along a zigzag diagonal line separating metals and nonmetals.	Possess a mixture of physical and chemical properties characteristic of both metals and nonmetals.	Can behave as electron donors or acceptors depending on the reaction.

Critical Exception: Hydrogen is classified as a nonmetal despite being located on the top left side of the periodic table. It is placed in Group 1 because it has one valence electron, but it behaves as a gaseous nonmetal.

Important Families to Know

Certain vertical columns (groups) have specific names and predictable, extreme behaviors based on how close they are to achieving the octet rule.

Group 1 elements on the periodic table are known as alkali metals. Alkali metals are highly reactive elements that possess exactly one valence electron. They violently want to lose this single electron to reveal a stable full shell underneath.
Group 2 elements on the periodic table are known as alkaline earth metals. They possess two valence electrons.
Group 17 elements on the periodic table are known as halogens. Halogens are highly reactive nonmetals that possess exactly seven valence electrons. They violently want to steal one electron to complete their octet (forming anions like Chloride, $Cl^-$ ).
Group 18 elements on the periodic table are known as noble gases. Noble gases are highly unreactive elements due to possessing a completely full outermost electron shell. They do not need to steal, lose, or share electrons with anyone.

The periodic table groups elements by shared chemical properties and electron configurations. This layout physically categorizes reactive metals on the left, nonmetals on the right, and highly stable noble gases in the final column.

Because of how the table is organized, we can observe distinct patterns—or "trends"—in how elements behave depending on whether you move horizontally across a period or vertically down a group.

1. Atomic Radius

Atomic radius represents the total distance from the center of an atom's nucleus to its outermost electron shell. Think of it as the physical size of the atom.

Atomic radius generally decreases as you move from left to right across a period on the periodic table. Why? As you move right, you add more protons to the nucleus and more electrons to the same energy level. The increasingly positive nucleus exerts a stronger magnetic pull, drawing the electron cloud tighter and shrinking the atom.
Atomic radius generally increases as you move from top to bottom down a group on the periodic table. Moving down a group means adding entirely new principal energy levels (new layers to the onion), naturally making the atom larger.

2. Electronegativity

Electronegativity is the measure of an atom's ability to attract shared electrons within a chemical bond. Imagine a tug-of-war between two atoms sharing an electron; the more electronegative atom will pull the electron closer to itself.

Electronegativity generally increases as you move from left to right across a period. Elements on the right (like halogens) are just one electron away from a perfect octet; they pull aggressively.
Electronegativity generally decreases as you move from top to bottom down a group. As atoms get larger (greater atomic radius), the outer electrons are further from the attractive pull of the nucleus, making the atom worse at attracting new electrons.
Note: Fluorine is the most electronegative element present on the periodic table. It is small, incredibly close to an octet, and pulls on electrons with unmatched ferocity.

3. Ionization Energy

Ionization energy is the amount of energy required to completely remove an electron from an isolated neutral atom. If an atom wants to keep its electrons, this energy will be very high.

Ionization energy generally increases as you move from left to right across a period. Atoms on the right side are trying to gain electrons to complete their octet; forcing them to give one up requires massive energy.
Ionization energy generally decreases as you move from top to bottom down a group. Because the outermost electrons are physically further away from the nucleus in larger atoms, the "magnetic hold" the nucleus has on them is weaker, making them easier to pluck away.

First ionization energy trends across the periodic table. Energy peaks at the noble gases (which resist losing electrons due to their full octet) and drops at the alkali metals (which readily lose their single valence electron).