Atoms, Elements & the Periodic Table

Chemistry begins with one extraordinary idea: everything in the universe — your blood, a glass of water, the air you breathe, the tablet you are reading this on — is made of atoms. An atom is the smallest unit of an element that still has the chemical properties of that element. Atoms are almost incomprehensibly small. A single strand of human hair is about a million atoms wide. Yet atoms are mostly empty space — if you scaled up an atom to the size of a football stadium, the nucleus would be a marble at the centre, and the electrons would be tiny specks near the outer walls. Despite their size, atoms determine everything: the colour of a flame, why water is wet, how medicines work in your body, why your bones are hard. Understanding atoms is the foundation of understanding all chemistry — and all biology.

Every atom has two regions: a dense central nucleus and an outer electron cloud. The nucleus contains: - Protons — positively charged particles. The number of protons defines which element an atom is. Carbon always has 6 protons; oxygen always has 8. This number is called the atomic number. - Neutrons — neutral particles (no charge). They add mass and help hold the nucleus together. Atoms of the same element can have different numbers of neutrons — these are called isotopes. Outside the nucleus: - Electrons — negatively charged, extremely light particles that orbit the nucleus in shells (energy levels). A neutral atom has the same number of electrons as protons, so the charges balance. Medical connection: Carbon-14, a radioactive isotope of carbon, is used in radiocarbon dating and some medical diagnostic scans. Technetium-99m — an isotope of technetium — is the most widely used radioactive tracer in nuclear medicine, used in bone scans, heart scans, and more.

An element is a pure substance made of only one type of atom. There are 118 known elements, arranged in the periodic table — one of the greatest achievements in science. The periodic table organises elements by atomic number (left to right) and by chemical behaviour (top to bottom in groups). Elements in the same column (group) behave similarly because they have the same number of outer-shell electrons. The elements most important to medicine and the human body: | Element | Symbol | Role in the body | |---------|--------|-----------------| | Oxygen | O | Cellular respiration, carried by haemoglobin | | Carbon | C | The backbone of ALL organic molecules | | Hydrogen | H | Part of water, acids, all organic molecules | | Nitrogen | N | Found in amino acids (proteins) and DNA | | Calcium | Ca | Bones, teeth, muscle contraction, nerve signals | | Phosphorus | P | DNA backbone, ATP (energy), bone mineral | | Iron | Fe | Haemoglobin carries oxygen in red blood cells | | Sodium | Na | Nerve impulses, fluid balance | | Potassium | K | Heart rhythm, nerve impulses | | Iodine | I | Thyroid hormone production | About 96% of your body's mass is just four elements: oxygen, carbon, hydrogen, and nitrogen. The rest is trace elements — small amounts that are nevertheless essential.

Electrons occupy shells around the nucleus. The first shell holds up to 2 electrons; the second holds up to 8; the third up to 18. What matters most chemically is the outermost shell — called the valence shell. The key rule: Atoms are most stable when their outermost shell is full. This drive toward stability is what causes atoms to form chemical bonds — they share or transfer electrons to achieve a full outer shell. - Atoms with a nearly full outer shell tend to gain electrons (like oxygen, which needs 2 more) - Atoms with only a few outer electrons tend to give them away (like sodium, which has 1 to lose) - Carbon is special — it has 4 outer electrons and can form 4 bonds, making it incredibly versatile. Every biological molecule is built around carbon chains. Why carbon is the basis of life: Carbon can bond to itself in long chains, rings, and branching structures. It can bond to hydrogen, oxygen, nitrogen, sulfur, and phosphorus. This flexibility allows carbon to form millions of different molecules — from glucose to DNA to cholesterol to morphine. No other element comes close.

Isotopes are atoms of the same element with different numbers of neutrons. For example, carbon has three natural isotopes: C-12 (most common), C-13 (stable), and C-14 (radioactive). Radioactive isotopes (radioisotopes) are unstable — their nuclei decay, releasing radiation. This can be harnessed medically: Diagnosis — nuclear medicine: - PET scans use fluorine-18 labelled glucose (FDG). Cancer cells take up more glucose than normal cells → they light up on the scan. - Bone scans use technetium-99m. It accumulates in areas of high bone activity → detects fractures, cancer spread to bone, infections. - Thyroid scans use iodine-123 or iodine-131, because the thyroid naturally absorbs iodine. Treatment — radiotherapy: - Iodine-131 treats thyroid cancer — concentrated in thyroid tissue, its radiation destroys cancer cells locally. - External beam radiotherapy uses high-energy X-rays or gamma rays to damage DNA in cancer cells, preventing them from dividing. The same property that makes radiation dangerous (DNA damage) is precisely what makes it useful as a cancer treatment when aimed carefully.