Fleming's discovery was serendipitous! He had left a Petri dish uncovered, and mold spores landed on it. When he returned from vacation, he observed the mold had killed the bacteria. Penicillin was first used to treat a human infection in 1941. By 1944, it was being mass-produced for Allied soldiers. Fleming warned about antibiotic resistance, which is now a major global health crisis. Fleming was a modest man; he was knighted in 1944. He died of a heart attack in 1955. The discovery of penicillin has been called the greatest medical breakthrough of the 20th century. Before penicillin, even a small cut could lead to fatal infection. The word "antibiotic" comes from Greek "anti" (against) and "bios" (life). Penicillin saved countless lives during World War II. Florey and Chain developed the mass production process. The discovery of penicillin opened the door to other antibiotics (streptomycin, tetracycline, etc.). Antibiotic resistance is a growing problem; we must use antibiotics responsibly.

Rosalind Franklin's Photo 51 was critical! Watson and Crick published a one-page paper in Nature in April 1953. Francis Crick famously said, "We have discovered the secret of life." The structure explained how DNA could replicate (unzip and copy itself). Franklin's contribution was largely overlooked during her lifetime. She is now recognized as a key contributor. Watson and Crick both lived long lives; Crick died in 2004, Watson is still alive (born 1928). The Human Genome Project (completed in 2003) sequenced all 3 billion base pairs of human DNA. The discovery of DNA's structure is one of the most important scientific breakthroughs of the 20th century. DNA is a double helix (like a twisted ladder). The sides are sugar-phosphate backbones, and the "rungs" are pairs of nitrogen bases (A-T, C-G). The genetic code is read in groups of three bases (codons). Each codon specifies an amino acid. The central dogma: DNA → RNA → protein. The discovery of DNA's structure opened the door to genetic engineering, gene therapy, and personalized medicine. The DNA molecule is about 2 nanometers wide. If stretched out, the DNA from one human cell would be about 6 feet (2 meters) long. The entire human genome (all DNA in one cell) contains about 3 billion base pairs. The Human Genome Project cost about $2.7 billion and took 13 years (1990-2003). Today, sequencing a human genome costs less than $1,000 and takes a few days. DNA fingerprinting is used in forensics, paternity testing, and genealogy. The discovery of DNA's structure was a turning point in biology.

Röntgen discovered X-rays! The discovery was accidental (he noticed a fluorescent screen glowing even when covered). X-rays are a form of electromagnetic radiation (like light, but with higher energy). They can pass through soft tissue but are absorbed by bone and metal. X-rays are used in medicine (radiography, CT scans), security (airport scanners), and industry (inspection of welds). Röntgen's wife's hand image was the first medical X-ray. She reportedly said, "I have seen my death." Early X-ray pioneers suffered radiation burns and cancers (they did not know the risks). X-ray tubes now have shielding. The unit of radiation exposure is called the röntgen. Röntgen was a conscientious scientist who declined financial gain from his discovery. He donated his Nobel Prize money to the University of Würzburg. The discovery of X-rays led to the discovery of radioactivity (Henri Becquerel). X-rays are ionizing radiation (can damage DNA). Excessive exposure can cause cancer. Modern X-ray machines use minimal doses. CT scans (computed tomography) create 3D images using X-rays. Fluoroscopy shows real-time moving images (used in surgery, barium swallows). Dental X-rays use very low doses. Mammography uses X-rays to screen for breast cancer. X-ray telescopes study celestial objects (black holes, neutron stars). The Chandra X-ray Observatory is named after Subrahmanyan Chandrasekhar. X-ray crystallography (using X-ray diffraction) was used to determine the structure of DNA (Rosalind Franklin) and many proteins. X-rays are an essential tool in modern medicine. The discovery of X-rays was one of the most important scientific breakthroughs of the late 19th century.

Rutherford discovered the nucleus! The gold foil experiment showed that atoms are mostly empty space. The nucleus is tiny (about 1/10,000 the diameter of the atom) but contains most of the atom's mass. Rutherford was a New Zealand-born physicist. He also discovered alpha and beta radiation and coined the terms. He was a mentor to many future Nobel laureates (Niels Bohr, James Chadwick). Rutherford was knighted and buried in Westminster Abbey (near Newton and Darwin). The element rutherfordium (Rf) is named after him. His student, Niels Bohr, improved the atomic model (electrons in fixed orbits). James Chadwick discovered the neutron in 1932. The current atomic model (quantum mechanical model) describes electrons as probability clouds (orbitals). The nucleus contains protons (positive charge) and neutrons (neutral). The number of protons determines the element (atomic number). Isotopes have the same number of protons but different numbers of neutrons. The discovery of the nucleus led to nuclear physics, nuclear energy, and nuclear weapons. The first nuclear reactor (Chicago Pile-1) was built by Enrico Fermi in 1942. The atomic bomb was developed during the Manhattan Project. Nuclear power plants generate electricity. Nuclear medicine uses radioactive isotopes for diagnosis and treatment. The discovery of the nucleus was a major breakthrough in understanding matter. Rutherford is considered one of the greatest experimental physicists of all time.

Natural selection is the mechanism of evolution! Darwin delayed publishing his theory for over 20 years because he feared controversy. When he finally published, he was shocked by the reaction. Thomas Huxley ("Darwin's Bulldog") defended him vigorously. The famous Oxford debate (1860) between Huxley and Bishop Wilberforce turned the tide of public opinion. Darwin was not the first to propose evolution (Lamarck did earlier), but he was the first to propose a plausible mechanism (natural selection). Darwin was a naturalist on the HMS Beagle (1831-1836). He collected thousands of specimens. The Galápagos finches are now called "Darwin's finches." Darwin also studied tortoises (different shell shapes on different islands). Darwin's theory was not fully accepted until the early 20th century, when genetics (Gregor Mendel) provided the mechanism. Modern evolutionary synthesis (1930s-1940s) combined Darwin's natural selection with Mendelian genetics. Evolution is both a fact (the pattern – life has changed over time) and a theory (the mechanism – how it happens). The theory of evolution is one of the most robust theories in science. Evidence for evolution includes the fossil record, comparative anatomy, comparative embryology, molecular biology (DNA), and direct observation (antibiotic resistance, beak size changes in finches). The theory of evolution is the unifying theory of biology. It explains the diversity of life on Earth. Darwin is buried in Westminster Abbey (near Isaac Newton). The Darwin Awards are named after him (though he would not approve).

Franklin proved lightning is electricity! The kite experiment was extremely dangerous (he could have been killed). Others who attempted it were electrocuted. Franklin was lucky. The experiment established the electrical nature of lightning. Lightning rods are still used today. Franklin was a prolific inventor and statesman. He was one of the Founding Fathers of the United States. He helped draft the Declaration of Independence and the U.S. Constitution. Franklin also invented the lightning rod, bifocal glasses, the Franklin stove (efficient heating), and the glass harmonica. He conducted experiments on electricity, coining terms like "battery," "conductor," and "electrician." Franklin's work on electricity was recognized internationally. He was elected to the Royal Society. Franklin's face appears on the $100 bill. He was a printer, author, scientist, inventor, diplomat, and postmaster. The Franklin Institute in Philadelphia is named after him. The kite experiment is a famous moment in the history of science. However, some historians doubt that Franklin actually flew the kite (the account may be apocryphal). Regardless, his contributions to electricity are significant. The unit of electric charge (franklin) is named after him (rarely used). Franklin's work on electricity laid the foundation for later discoveries by Volta, Ohm, Ampère, Faraday, and Maxwell. The study of electricity led to the development of motors, generators, batteries, and electronics. Without Franklin's experiments, modern electronics might not exist.

E = mc² is the most famous equation! Einstein won the Nobel Prize in Physics in 1921 for his discovery of the photoelectric effect (not for relativity, which was still controversial). Special relativity (1905) also introduced time dilation (time slows down at high speeds), length contraction (objects shrink in the direction of motion), and the equivalence of mass and energy (E=mc²). General relativity (1915) predicted that light bends around massive objects (gravitational lensing), that time runs slower in stronger gravity (time dilation), and that the universe is expanding. General relativity also predicted black holes (regions of spacetime where gravity is so strong nothing can escape). The first image of a black hole (M87*) was released in 2019. Einstein's theory of relativity has been confirmed by many experiments: the bending of starlight by the Sun (Eddington, 1919), gravitational redshift, the precession of Mercury's orbit, and the detection of gravitational waves (LIGO, 2015). Einstein was a German-born Jew who fled Nazi Germany in 1933. He settled in the United States and became a citizen. He was offered the presidency of Israel in 1952 (he declined). He signed a letter to President Franklin D. Roosevelt warning that Germany might develop an atomic bomb, which led to the Manhattan Project. Einstein later regretted his role. His brain was preserved after his death (without family consent). Einstein is often called the most influential physicist of the 20th century. His image is one of the most recognizable in the world.

Smallpox was eradicated! Jenner's vaccine was the first. The last naturally occurring case of smallpox was in Somalia in 1977. The WHO certified eradication in 1980. Smallpox vaccines are no longer routinely given because the disease no longer exists (samples remain in two labs: CDC in Atlanta and VECTOR in Russia). Jenner faced criticism and ridicule, but his vaccine saved countless lives. The word "vaccine" comes from "vacca" (cow). Jenner was a country doctor in Gloucestershire, England. He is buried in Berkeley, Gloucestershire. A statue of him stands in Trafalgar Square (London) and in Kensington Gardens. The Jenner Institute at Oxford University (developers of a COVID-19 vaccine) is named in his honor. Vaccination works by exposing the immune system to a harmless version of a pathogen (or part of it), so the body can recognize and fight the real pathogen. Vaccines have saved millions of lives. Other diseases targeted for eradication: polio (almost eradicated), measles, rubella, tetanus, diphtheria, pertussis (whooping cough). The Expanded Programme on Immunization (EPI) was launched by WHO in 1974. The Global Polio Eradication Initiative has reduced polio cases by 99.9% since 1988. Polio remains endemic in Afghanistan and Pakistan. COVID-19 vaccines were developed in record time (less than a year). Vaccination is one of the most effective public health interventions. Vaccines prevent about 2-3 million deaths per year. Herd immunity protects those who cannot be vaccinated (due to allergies, immune disorders). The anti-vaccination movement (which began with a fraudulent 1998 study linking MMR vaccine to autism) has led to outbreaks of preventable diseases. Vaccines are safe and effective. The discovery of vaccination is one of the greatest medical breakthroughs in history.

Mendeleev created the periodic table! He wrote the properties of each element on cards and arranged them in a "patience" (solitaire) layout. He noticed that properties repeated periodically. He left blank spaces for elements that had not yet been discovered and predicted their properties. When gallium, scandium, and germanium were discovered later, they matched his predictions. Mendeleev is often called the "father of the periodic table." The modern periodic table is ordered by atomic number (number of protons), not atomic mass (Mendeleev used atomic mass, but some elements were out of order). The International Union of Pure and Applied Chemistry (IUPAC) is responsible for naming new elements. The most recent elements added are nihonium (Nh, 113), moscovium (Mc, 115), tennessine (Ts, 117), and oganesson (Og, 118), officially named in 2016. The periodic table has 118 confirmed elements. Elements are arranged in rows (periods) and columns (groups). Elements in the same group have similar chemical properties (same number of valence electrons). The periodic table is a powerful tool for understanding chemistry. It predicts how elements will react, what compounds they will form, and what their properties are. Mendeleev was a professor at the University of Saint Petersburg. He also made contributions to thermochemistry (the study of heat in chemical reactions). He died in 1907. The element mendelevium (Md, 101) is named after him. The periodic table is one of the most iconic images in science. It is displayed in every chemistry classroom. The discovery of new elements is ongoing; elements 119 and 120 are being sought. The "island of stability" (superheavy elements that may be stable) is a goal of nuclear physics. The periodic table is a testament to the power of scientific classification.

X-rays are the correct answer! Other major discoveries: Penicillin (first antibiotic), DNA structure (double helix), atomic nucleus (Rutherford), evolution by natural selection (Darwin), electricity (Franklin's kite experiment), theory of relativity (Einstein), smallpox vaccine (Jenner), and the periodic table (Mendeleev). Scientific discoveries have transformed our understanding of the world and our ability to treat disease, generate energy, and communicate. The scientific method (observation, hypothesis, experimentation, conclusion) is the foundation of scientific discovery. Many discoveries were accidental (penicillin, X-rays). Others were the result of years of dedicated research. The pace of discovery has accelerated in recent centuries. The 20th century saw more scientific progress than all previous centuries combined. The 21st century will see even more. Scientific discoveries have improved life expectancy, reduced poverty, and increased our understanding of the universe. However, they also pose ethical challenges (nuclear weapons, genetic engineering, AI). It is important to use science responsibly. The history of science is the story of human curiosity and ingenuity. You have learned about some of the greatest scientific discoveries of all time. Keep exploring, and who knows – you might make the next great discovery!