Atoms & Elements

Atoms, electrons, the periodic table and the elements — the building blocks of matter explained one fact at a time.

An atom consists of protons, neutrons, and electrons.
Protons carry a positive charge and reside in the atom's nucleus.
Electrons carry a negative charge and orbit the atomic nucleus.
Neutrons are electrically neutral particles found in the nucleus.
The atomic number equals the number of protons in an atom.
An element is a pure substance made of only one type of atom.
Hydrogen is the lightest and most abundant element in the universe.
The periodic table arranges elements by increasing atomic number.
Isotopes are atoms of the same element with different numbers of neutrons.
Oxygen makes up about twenty-one percent of Earth's atmosphere.
Carbon is the basis of all known organic life on Earth.
Noble gases like helium and neon are very chemically unreactive.
Metals are typically good conductors of heat and electricity.
Gold is a chemically stable metal that resists tarnishing and corrosion.
Electrons are arranged in energy levels or shells around the nucleus.
The mass of an atom is concentrated almost entirely in its nucleus.
The word atom derives from the Greek atomos, meaning indivisible or uncuttable.
John Dalton revived atomic theory in the early nineteenth century.
J.J. Thomson discovered the electron in 1897 using cathode ray tubes.
Ernest Rutherford discovered the atomic nucleus through his gold foil experiment.
Niels Bohr proposed that electrons occupy fixed energy levels in 1913.
A proton's mass is roughly 1,836 times that of an electron.
Atomic mass is commonly measured in unified atomic mass units.
A typical atom has a diameter of about one tenth of a nanometer.
Electrons were the first subatomic particles to be discovered.
Ions form when atoms gain or lose electrons.
An anion is a negatively charged ion that has gained electrons.
Mendeleev arranged elements by atomic weight and recurring properties.
Henry Moseley reordered the periodic table by atomic number in 1913.
Horizontal rows in the periodic table are called periods.
Group 1 elements are known as the alkali metals.
Group 17 elements are known as the halogens.
The alkali metals are highly reactive and stored under oil.
Fluorine, chlorine, bromine, and iodine are common halogens.
There are 118 confirmed elements in the periodic table.
Most transuranium elements are produced artificially in laboratories.
Oganesson, element 118, was officially named in 2016.
The lanthanides and actinides are placed below the main table.
Sodium's chemical symbol is Na, from the Latin natrium.
Iron's chemical symbol is Fe, from the Latin ferrum.
Silver's chemical symbol is Ag, from the Latin argentum.
Mercury's chemical symbol is Hg, from the Latin hydrargyrum.
Tungsten's chemical symbol is W, from the German wolfram.
Chemical symbols use one or two letters to represent elements.
Carbon-14 is radioactive and used in radiocarbon dating.
Tritium is a radioactive hydrogen isotope with two neutrons.
Uranium-235 is the isotope used in nuclear fission reactors.
Relative atomic mass is a weighted average of an element's isotopes.
Alpha decay releases a helium nucleus from a radioactive atom.
Half-life is the time for half of a radioactive sample to decay.
Marie Curie discovered the elements polonium and radium.
Henri Becquerel discovered radioactivity in uranium in 1896.
Each orbital can hold a maximum of two electrons.
The second electron shell holds a maximum of eight electrons.
Valence electrons largely determine an element's chemical behavior.
The octet rule states atoms favor eight electrons in their outer shell.
Electron configuration describes how electrons fill an atom's orbitals.
Hund's rule states electrons fill orbitals singly before pairing.
An ionic bond forms through the transfer of electrons between atoms.
Sodium chloride is held together by ionic bonds.
Electronegativity measures an atom's tendency to attract electrons.
A compound contains atoms of two or more different elements.
Oxygen naturally exists as a diatomic molecule, O2.
Ozone is a molecule made of three oxygen atoms.
Graphene is a single layer of carbon atoms in a hexagonal lattice.
Nitrogen makes up about seventy-eight percent of Earth's atmosphere.
Oxygen is the most abundant element in Earth's crust.
Iron is the most abundant element by mass in the whole Earth.
Most helium on Earth is produced by radioactive decay underground.
Heavier elements are forged inside stars through nuclear fusion.
Nuclear fusion combines light nuclei to release energy.
The Sun produces energy by fusing hydrogen into helium.
Metals are generally ductile and can be drawn into wires.
Nonmetals are generally poor conductors of heat and electricity.
Boron, silicon, arsenic, and tellurium are classified as metalloids.
Ionization energy is the energy needed to remove an electron from an atom.
The density of an element relates to its mass and volume.
Melting point is the temperature at which a solid becomes liquid.
Carbon has one of the highest melting points among the elements.
Plasma is an ionized state of matter found in stars.
Avogadro's number is approximately 6.022 times ten to the twenty-third.
Amedeo Avogadro proposed that equal gas volumes contain equal molecules.
Lavoisier helped establish the law of conservation of mass.
Joseph Proust formulated the law of definite proportions.
A chemical equation represents a chemical reaction with formulas.
Hydrogen gas is highly flammable and the lightest gas.
Carbon dioxide is produced by respiration and combustion.
Neon glows reddish-orange when an electric current passes through it.
Chlorine is a toxic greenish-yellow gas at room temperature.
Sulfur is a yellow nonmetal known since ancient times.
Sodium reacts vigorously with water, releasing hydrogen gas.
Iron is essential for carrying oxygen in human blood.
Copper is widely used in electrical wiring for its conductivity.
Titanium is valued for its strength and low density.
Zinc is used to galvanize and protect steel from rust.
Nickel is used in coins and rechargeable batteries.
Tin has long been used to coat steel cans.
Carbon dating helps determine the age of ancient organic materials.
Bronze is an alloy of copper and tin.
Steel is an alloy of iron and carbon.
A catalyst speeds up a chemical reaction without being consumed.
Reduction involves the gain of electrons by an atom.
An acid is a substance that donates hydrogen ions in solution.
The pH scale measures the acidity or alkalinity of a solution.
Hydrochloric acid is a strong acid found in the stomach.
Carbon's ability to form four bonds enables vast organic chemistry.
Erwin Schrodinger formulated a wave equation for electrons in 1926.
The uncertainty principle limits knowing position and momentum simultaneously.
Each element produces a unique emission spectrum of light.
Helium was first detected in the Sun's spectrum before being found on Earth.
Hydrogen was named for producing water when it burns.
Curium is named after Marie and Pierre Curie.
Polonium is named after Marie Curie's homeland of Poland.
The number of electron shells equals the period number for an atom.
Plutonium is a key fuel used in nuclear weapons and reactors.
Astatine is among the rarest naturally occurring elements on Earth.
A nuclide is an atom characterized by its specific protons and neutrons.
The strong nuclear force binds protons and neutrons in the nucleus.
Quarks are the fundamental particles that make up protons and neutrons.
A neutron is composed of one up quark and two down quarks.
Electrons belong to the family of particles called leptons.
Copper produces a blue-green color in flame tests.
Lithium produces a crimson color in flame tests.
Atoms are conserved in number during a chemical reaction.
The seventh period was completed with elements 113, 115, 117, and 118.
Hydrogen is often placed above group one despite being a nonmetal.
The electron's charge was measured by Robert Millikan's oil drop experiment.
Most of an atom's volume is occupied by its electron cloud.
Isotopes of an element have identical chemical properties.
The most stable elements have nuclei near iron in mass.
Atoms can be imaged using scanning tunneling microscopes.
Electron microscopes use electron beams to image tiny structures.
A helium nucleus is identical to an alpha particle.
Each element occupies a unique square in the periodic table.
Gallium was discovered in 1875, confirming Mendeleev's predictions.
Cesium and gallium are metals with very low melting points.
Hydrogen comprises about ninety percent of atoms in the universe.
Phosphorus and sulfur are also essential elements for life.
Fluoride is added to water to help prevent tooth decay.
Sodium bicarbonate is commonly known as baking soda.
Common salt provides essential sodium and chloride to the body.
Changing the number of neutrons changes only the isotope.
The periodic table's modern form is the long-form arrangement.
Elements in a group often have the same number of valence electrons.
Xenon can form compounds despite being a noble gas.
Krypton and xenon are used in certain specialized lamps.
The discovery of argon earned a Nobel Prize in 1904.
Boron is essential for plant growth in trace amounts.
Vanadium is added to steel to increase its hardness.
Rhodium is one of the rarest and most valuable precious metals.
Beryllium is a lightweight metal used in aerospace applications.
Arsenic is a toxic metalloid known since antiquity.
Germanium was used in early transistors before silicon.
Gallium arsenide is used in high-speed electronic devices.
Atomic weights listed on the periodic table reflect isotope averages.
Compounds can be separated into elements only by chemical means.
The periodic law states properties recur periodically with atomic number.
Johann Dobereiner grouped elements into triads in the 1820s.
When electrons drop to lower levels, they emit photons of light.
The energy of an emitted photon equals the gap between electron levels.
Albert Einstein explained the photoelectric effect using light quanta.
Modern atomic theory uses probability clouds rather than fixed orbits.
The d subshell can hold a maximum of ten electrons.
The p subshell can hold a maximum of six electrons.
Sodium has the electron configuration of neon plus one electron.
Valence indicates the combining capacity of an atom.
Gilbert Lewis introduced the electron-pair theory of bonding.
Water is a polar molecule due to its bent shape.
Carbon dioxide is a nonpolar molecule despite polar bonds.
VSEPR theory states electron pairs arrange to minimize repulsion.
A triple bond is stronger and shorter than a single bond.
Bond energy is the energy required to break a chemical bond.
Standard temperature and pressure define reference conditions for gases.
The kinetic theory describes gas particles in constant random motion.
Einstein's 1905 paper explained Brownian motion using atomic theory.
A single drop of water contains an enormous number of molecules.
Reactivity of metals generally increases down group one.
Fluorine is the most reactive of all the halogens.
Elements are sometimes synthesized by colliding lighter nuclei.
The International Union of Pure and Applied Chemistry approves element names.
Temporary placeholder names are used before elements are officially named.
Atomic clocks use the precise vibrations of cesium atoms.
Graphite conducts electricity because of its delocalized electrons.
Carbon nanotubes are cylindrical structures of carbon atoms.
Quartz is a crystalline form of silicon dioxide.
Water always contains two hydrogen atoms per oxygen atom.
The smallest unit of a compound is often a molecule.
Salt crystals are arranged in a repeating cubic lattice.
Humphry Davy isolated sodium and potassium using electrolysis.
The electron volt is a common unit of energy in atomic physics.
An excited state has electrons in higher energy levels than the ground state.
Periodic properties repeat because of recurring electron configurations.
Many transition metals can exhibit multiple oxidation states.
Atoms are mostly empty space between the nucleus and electrons.
The proton number is also called the atomic number.
Hydrogen can behave as both a metal and a nonmetal under conditions.
The carbon cycle moves carbon through the atmosphere and living things.
Nitrogen is a key component of proteins and DNA.
The periodic table organizes knowledge of all known elements.
Atomic theory unifies chemistry, physics, and materials science.

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Atoms & Elements

Atoms, electrons, the periodic table and the elements — the building blocks of matter explained one fact at a time.

An atom consists of protons, neutrons, and electrons.
Protons carry a positive charge and reside in the atom's nucleus.
Electrons carry a negative charge and orbit the atomic nucleus.
Neutrons are electrically neutral particles found in the nucleus.
The atomic number equals the number of protons in an atom.
An element is a pure substance made of only one type of atom.
Hydrogen is the lightest and most abundant element in the universe.
The periodic table arranges elements by increasing atomic number.
Isotopes are atoms of the same element with different numbers of neutrons.
Oxygen makes up about twenty-one percent of Earth's atmosphere.
Carbon is the basis of all known organic life on Earth.
Noble gases like helium and neon are very chemically unreactive.
Metals are typically good conductors of heat and electricity.
Gold is a chemically stable metal that resists tarnishing and corrosion.
Electrons are arranged in energy levels or shells around the nucleus.
The mass of an atom is concentrated almost entirely in its nucleus.
The word atom derives from the Greek atomos, meaning indivisible or uncuttable.
John Dalton revived atomic theory in the early nineteenth century.
J.J. Thomson discovered the electron in 1897 using cathode ray tubes.
Ernest Rutherford discovered the atomic nucleus through his gold foil experiment.
Niels Bohr proposed that electrons occupy fixed energy levels in 1913.
A proton's mass is roughly 1,836 times that of an electron.
Atomic mass is commonly measured in unified atomic mass units.
A typical atom has a diameter of about one tenth of a nanometer.
Electrons were the first subatomic particles to be discovered.
Ions form when atoms gain or lose electrons.
An anion is a negatively charged ion that has gained electrons.
Mendeleev arranged elements by atomic weight and recurring properties.
Henry Moseley reordered the periodic table by atomic number in 1913.
Horizontal rows in the periodic table are called periods.
Group 1 elements are known as the alkali metals.
Group 17 elements are known as the halogens.
The alkali metals are highly reactive and stored under oil.
Fluorine, chlorine, bromine, and iodine are common halogens.
There are 118 confirmed elements in the periodic table.
Most transuranium elements are produced artificially in laboratories.
Oganesson, element 118, was officially named in 2016.
The lanthanides and actinides are placed below the main table.
Sodium's chemical symbol is Na, from the Latin natrium.
Iron's chemical symbol is Fe, from the Latin ferrum.
Silver's chemical symbol is Ag, from the Latin argentum.
Mercury's chemical symbol is Hg, from the Latin hydrargyrum.
Tungsten's chemical symbol is W, from the German wolfram.
Chemical symbols use one or two letters to represent elements.
Carbon-14 is radioactive and used in radiocarbon dating.
Tritium is a radioactive hydrogen isotope with two neutrons.
Uranium-235 is the isotope used in nuclear fission reactors.
Relative atomic mass is a weighted average of an element's isotopes.
Alpha decay releases a helium nucleus from a radioactive atom.
Half-life is the time for half of a radioactive sample to decay.
Marie Curie discovered the elements polonium and radium.
Henri Becquerel discovered radioactivity in uranium in 1896.
Each orbital can hold a maximum of two electrons.
The second electron shell holds a maximum of eight electrons.
Valence electrons largely determine an element's chemical behavior.
The octet rule states atoms favor eight electrons in their outer shell.
Electron configuration describes how electrons fill an atom's orbitals.
Hund's rule states electrons fill orbitals singly before pairing.
An ionic bond forms through the transfer of electrons between atoms.
Sodium chloride is held together by ionic bonds.
Electronegativity measures an atom's tendency to attract electrons.
A compound contains atoms of two or more different elements.
Oxygen naturally exists as a diatomic molecule, O2.
Ozone is a molecule made of three oxygen atoms.
Graphene is a single layer of carbon atoms in a hexagonal lattice.
Nitrogen makes up about seventy-eight percent of Earth's atmosphere.
Oxygen is the most abundant element in Earth's crust.
Iron is the most abundant element by mass in the whole Earth.
Most helium on Earth is produced by radioactive decay underground.
Heavier elements are forged inside stars through nuclear fusion.
Nuclear fusion combines light nuclei to release energy.
The Sun produces energy by fusing hydrogen into helium.
Metals are generally ductile and can be drawn into wires.
Nonmetals are generally poor conductors of heat and electricity.
Boron, silicon, arsenic, and tellurium are classified as metalloids.
Ionization energy is the energy needed to remove an electron from an atom.
The density of an element relates to its mass and volume.
Melting point is the temperature at which a solid becomes liquid.
Carbon has one of the highest melting points among the elements.
Plasma is an ionized state of matter found in stars.
Avogadro's number is approximately 6.022 times ten to the twenty-third.
Amedeo Avogadro proposed that equal gas volumes contain equal molecules.
Lavoisier helped establish the law of conservation of mass.
Joseph Proust formulated the law of definite proportions.
A chemical equation represents a chemical reaction with formulas.
Hydrogen gas is highly flammable and the lightest gas.
Carbon dioxide is produced by respiration and combustion.
Neon glows reddish-orange when an electric current passes through it.
Chlorine is a toxic greenish-yellow gas at room temperature.
Sulfur is a yellow nonmetal known since ancient times.
Sodium reacts vigorously with water, releasing hydrogen gas.
Iron is essential for carrying oxygen in human blood.
Copper is widely used in electrical wiring for its conductivity.
Titanium is valued for its strength and low density.
Zinc is used to galvanize and protect steel from rust.
Nickel is used in coins and rechargeable batteries.
Tin has long been used to coat steel cans.
Carbon dating helps determine the age of ancient organic materials.
Bronze is an alloy of copper and tin.
Steel is an alloy of iron and carbon.
A catalyst speeds up a chemical reaction without being consumed.
Reduction involves the gain of electrons by an atom.
An acid is a substance that donates hydrogen ions in solution.
The pH scale measures the acidity or alkalinity of a solution.
Hydrochloric acid is a strong acid found in the stomach.
Carbon's ability to form four bonds enables vast organic chemistry.
Erwin Schrodinger formulated a wave equation for electrons in 1926.
The uncertainty principle limits knowing position and momentum simultaneously.
Each element produces a unique emission spectrum of light.
Helium was first detected in the Sun's spectrum before being found on Earth.
Hydrogen was named for producing water when it burns.
Curium is named after Marie and Pierre Curie.
Polonium is named after Marie Curie's homeland of Poland.
The number of electron shells equals the period number for an atom.
Plutonium is a key fuel used in nuclear weapons and reactors.
Astatine is among the rarest naturally occurring elements on Earth.
A nuclide is an atom characterized by its specific protons and neutrons.
The strong nuclear force binds protons and neutrons in the nucleus.
Quarks are the fundamental particles that make up protons and neutrons.
A neutron is composed of one up quark and two down quarks.
Electrons belong to the family of particles called leptons.
Copper produces a blue-green color in flame tests.
Lithium produces a crimson color in flame tests.
Atoms are conserved in number during a chemical reaction.
The seventh period was completed with elements 113, 115, 117, and 118.
Hydrogen is often placed above group one despite being a nonmetal.
The electron's charge was measured by Robert Millikan's oil drop experiment.
Most of an atom's volume is occupied by its electron cloud.
Isotopes of an element have identical chemical properties.
The most stable elements have nuclei near iron in mass.
Atoms can be imaged using scanning tunneling microscopes.
Electron microscopes use electron beams to image tiny structures.
A helium nucleus is identical to an alpha particle.
Each element occupies a unique square in the periodic table.
Gallium was discovered in 1875, confirming Mendeleev's predictions.
Cesium and gallium are metals with very low melting points.
Hydrogen comprises about ninety percent of atoms in the universe.
Phosphorus and sulfur are also essential elements for life.
Fluoride is added to water to help prevent tooth decay.
Sodium bicarbonate is commonly known as baking soda.
Common salt provides essential sodium and chloride to the body.
Changing the number of neutrons changes only the isotope.
The periodic table's modern form is the long-form arrangement.
Elements in a group often have the same number of valence electrons.
Xenon can form compounds despite being a noble gas.
Krypton and xenon are used in certain specialized lamps.
The discovery of argon earned a Nobel Prize in 1904.
Boron is essential for plant growth in trace amounts.
Vanadium is added to steel to increase its hardness.
Rhodium is one of the rarest and most valuable precious metals.
Beryllium is a lightweight metal used in aerospace applications.
Arsenic is a toxic metalloid known since antiquity.
Germanium was used in early transistors before silicon.
Gallium arsenide is used in high-speed electronic devices.
Atomic weights listed on the periodic table reflect isotope averages.
Compounds can be separated into elements only by chemical means.
The periodic law states properties recur periodically with atomic number.
Johann Dobereiner grouped elements into triads in the 1820s.
When electrons drop to lower levels, they emit photons of light.
The energy of an emitted photon equals the gap between electron levels.
Albert Einstein explained the photoelectric effect using light quanta.
Modern atomic theory uses probability clouds rather than fixed orbits.
The d subshell can hold a maximum of ten electrons.
The p subshell can hold a maximum of six electrons.
Sodium has the electron configuration of neon plus one electron.
Valence indicates the combining capacity of an atom.
Gilbert Lewis introduced the electron-pair theory of bonding.
Water is a polar molecule due to its bent shape.
Carbon dioxide is a nonpolar molecule despite polar bonds.
VSEPR theory states electron pairs arrange to minimize repulsion.
A triple bond is stronger and shorter than a single bond.
Bond energy is the energy required to break a chemical bond.
Standard temperature and pressure define reference conditions for gases.
The kinetic theory describes gas particles in constant random motion.
Einstein's 1905 paper explained Brownian motion using atomic theory.
A single drop of water contains an enormous number of molecules.
Reactivity of metals generally increases down group one.
Fluorine is the most reactive of all the halogens.
Elements are sometimes synthesized by colliding lighter nuclei.
The International Union of Pure and Applied Chemistry approves element names.
Temporary placeholder names are used before elements are officially named.
Atomic clocks use the precise vibrations of cesium atoms.
Graphite conducts electricity because of its delocalized electrons.
Carbon nanotubes are cylindrical structures of carbon atoms.
Quartz is a crystalline form of silicon dioxide.
Water always contains two hydrogen atoms per oxygen atom.
The smallest unit of a compound is often a molecule.
Salt crystals are arranged in a repeating cubic lattice.
Humphry Davy isolated sodium and potassium using electrolysis.
The electron volt is a common unit of energy in atomic physics.
An excited state has electrons in higher energy levels than the ground state.
Periodic properties repeat because of recurring electron configurations.
Many transition metals can exhibit multiple oxidation states.
Atoms are mostly empty space between the nucleus and electrons.
The proton number is also called the atomic number.
Hydrogen can behave as both a metal and a nonmetal under conditions.
The carbon cycle moves carbon through the atmosphere and living things.
Nitrogen is a key component of proteins and DNA.
The periodic table organizes knowledge of all known elements.
Atomic theory unifies chemistry, physics, and materials science.

Learn this lesson, not just read it

Load these facts straight into the free Class Coupon Speed Learning app — they become fast, playful study activities with audio and translation in 100+ languages.

Get the Speed Learning app →