atoms

Periodic Table

A chart of the elements showing the repeating pattern of their properties

Electron Cloud

in the electron cloud model of the atom, region around the nucleus where an electron may be found

electron

A subatomic particle that has a negative charge

proton

A subatomic particle that has a positive charge and that is found in the nucleus of an atom

neutron

A subatomic particle that has no charge and that is found in the nucleus of an atom

Chemical formula

A way of describing the number of atoms that makes up one molecule of a compound

Atomic Number (Jersey Number)

Number of protons in an atom

Chemical symbol

One or two letter code that stands for an element; many symbols are abbreviations of the elements named, which may be English, Latin or Greek in origin

Atoms are the basic building blocks of ordinary matter. Atoms can join together to form molecules, which in turn form most of the objects around you. Atoms are composed of particles called protons, electrons and neutrons. Protons carry a positive electrical charge, electrons carry a negative electrical charge and neutrons carry no electrical charge at all. The protons and neutrons cluster together in the central part of the atom, called the nucleus, and the electrons 'orbit' the nucleus. A particular atom will have the same number of protons and electrons and most atoms have at least as many neutrons as protons. Protons and neutrons are both composed of other particles called quarks and gluons. Protons contain two 'up' quarks and one 'down' quark while neutrons contain one 'up' quark and two 'down' quarks. The gluons are responsible for binding the quarks to one another.

What is an atom? What are atoms made of?

An element is a substance that is made entirely from one type of atom. For example, the element hydrogen is made from atoms containing a single proton and a single electron. If you change the number of protons an atom has, you change the type of element it is. If you had very, very good eyes and could look at the atoms in a sample of hydrogen, you would notice that most of the hydrogen atoms would have no neutrons, some of them would have one neutron and a few of them would have two neutrons. These different versions of hydrogen are called isotopes. All isotopes of a particular element have the same number of protons, but have a different number of neutrons. If you change the number of neutrons an atom has, you make an isotope of that element. Currently, scientists know of 118 different elements. Some, like gold, silver, copper and carbon, have been known for thousands of years. Others, such as meitnerium, darmstadtium and ununquadium, have only recently been created by scientists. All known elements are arranged on a chart called the Periodic Table of Elements.

What is an element? How many elements are there?

Get ready for an imperfect analogy. Imagine going to an ice cream store. Let's say that they have 30 different flavors of ice cream. Those are elements, the things that I have available to build my dessert from. The smallest amount of ice cream that the store will sell to me is a scoop. This is an atom. If I want, I can put two or more scoops of ice cream together. This is a molecule. If my molecule has more than one flavor of ice cream, I can call it a compound. So, in summary: element - a basic substance that can't be simplified (hydrogen, oxygen, gold, etc...) atom - the smallest amount of an element molecule - two or more atoms that are chemically joined together (H2, O2, H2O, etc...) compound - a molecule that contains more than one element (H2O, C6H12O6, etc...) What's wrong with the ice cream analogy? Splitting an atom creates different elements (split an oxygen atom and you don't have oxygen any longer). Splitting a scoop of ice cream results in smaller blobs of the same flavor. For the analogy to hold true, the flavor of the ice cream would have to change when you split a scoop (the chocolate 'element' would have to change into some other 'element' (flavor)).

What is the difference between atoms and elements?

NOUN the basic unit of a chemical element. synonyms: grain · iota · jot · whit · mite · scrap · shred · ounce · scintilla · trace · smidgen · modicum atoms as a source of nuclear energy: "the power of the atom" an extremely small amount of a thing or quality: "I shall not have one atom of strength left" synonyms: grain · iota · jot · whit · mite · scrap · shred · ounce · scintilla · trace · smidgen · modicum.

atom

atom

smallest unit of matter

nucleus of atom

the positively charged dense center of an atom

Atomic Mass

total weight of protons and neutrons

Most matter consists of an agglomeration of molecules, which can be separated relatively easily. Molecules, in turn, are composed of atoms joined by chemical bonds that are more difficult to break. Each individual atom consists of smaller particles—namely, electrons and nuclei. These particles are electrically charged, and the electric forces on the charge are responsible for holding the atom together. Attempts to separate these smaller constituent particles require ever-increasing amounts of energy and result in the creation of new subatomic particles, many of which are charged. SIMILAR TOPICS energy transport phenomenon principles of physical science physical constant metrology momentum chemical reaction light friction absorption As noted in the introduction to this article, an atom consists largely of empty space. The nucleus is the positively charged centre of an atom and contains most of its mass. It is composed of protons, which have a positive charge, and neutrons, which have no charge. Protons, neutrons, and the electrons surrounding them are long-lived particles present in all ordinary, naturally occurring atoms. Other subatomic particles may be found in association with these three types of particles. They can be created only with the addition of enormous amounts of energy, however, and are very short-lived. All atoms are roughly the same size, whether they have 3 or 90 electrons. Approximately 50 million atoms of solid matter lined up in a row would measure 1 cm (0.4 inch). A convenient unit of length for measuring atomic sizes is the angstrom (Å), defined as 10−10 metre. The radius of an atom measures 1-2 Å. Compared with the overall size of the atom, the nucleus is even more minute. It is in the same proportion to the atom as a marble is to a football field. In volume the nucleus takes up only 10−14 metres of the space in the atom—i.e., 1 part in 100,000. A convenient unit of length for measuring nuclear sizes is the femtometre (fm), which equals 10−15 metre. The diameter of a nucleus depends on the number of particles it contains and ranges from about 4 fm for a light nucleus such as carbon to 15 fm for a heavy nucleus such as lead. In spite of the small size of the nucleus, virtually all the mass of the atom is concentrated there. The protons are massive, positively charged particles, whereas the neutrons have no charge and are slightly more massive than the protons. The fact that nuclei can have anywhere from 1 to nearly 300 protons and neutrons accounts for their wide variation in mass. The lightest nucleus, that of hydrogen, is 1,836 times more massive than an electron, while heavy nuclei are nearly 500,000 times more massive. Basic properties BRITANNICA STORIES DEMYSTIFIED / MUSIC Does Listening to Mozart Make Kids Smarter? Pregnant woman holding headphones on her belly SPOTLIGHT / HISTORY The Mechanical Turk: AI Marvel or Parlor Trick? game and gambling, gaming machines, chess playing Turk, design by Wolfgang von Kempelen (1734 - 1804), built by Christoph Mechel, mechanical turk SPOTLIGHT / ARTS & CULTURE Normality Is a Paved Road: The Tumultuous Life of Vincent van Gogh Self Portrait by Vincent Van Gogh, dated around 1887. Oil on canvas 42x34cm DEMYSTIFIED / SCIENCE Do Lie Detectors Actually Work? Polygraph Lie Detector Machine Atomic number The single most important characteristic of an atom is its atomic number (usually denoted by the letter Z), which is defined as the number of units of positive charge (protons) in the nucleus. For example, if an atom has a Z of 6, it is carbon, while a Z of 92 corresponds to uranium. A neutral atom has an equal number of protons and electrons so that the positive and negative charges exactly balance. Since it is the electrons that determine how one atom interacts with another, in the end it is the number of protons in the nucleus that determines the chemical properties of an atom. Atomic mass and isotopes TEST YOUR KNOWLEDGE Camel. Close-up of a head of a camel in an Egyptian desert. The Animals of Asia The number of neutrons in a nucleus affects the mass of the atom but not its chemical properties. Thus, a nucleus with six protons and six neutrons will have the same chemical properties as a nucleus with six protons and eight neutrons, although the two masses will be different. Nuclei with the same number of protons but different numbers of neutrons are said to be isotopes of each other. All chemical elements have many isotopes. The image shows the three isotopes of the element hydrogen. All three forms have one proton (pink) and one electron (dark green) but differ in the number of neutrons (gray) in the nucleus. Protium, or ordinary hydrogen (top), has no neutrons. Deuterium, or hydrogen-2 (bottom left), has one neutron. Tritium, or hydrogen-3 (bottom right), has two neutrons. The image shows the three isotopes of the element hydrogen. All three forms have one proton (pink) ... © designua/Fotolia It is usual to characterize different isotopes by giving the sum of the number of protons and neutrons in the nucleus—a quantity called the atomic mass number. In the above example, the first atom would be called carbon-12 or 12C (because it has six protons and six neutrons), while the second would be carbon-14 or 14C. The mass of atoms is measured in terms of the atomic mass unit, which is defined to be 1/12 of the mass of an atom of carbon-12, or 1.660538921 × 10−24 gram. The mass of an atom consists of the mass of the nucleus plus that of the electrons, so the atomic mass unit is not exactly the same as the mass of the proton or neutron. The electron BRITANNICA LISTS & QUIZZES MRI Image Of Head Showing Brain HEALTH & MEDICINE QUIZ The Human Brain Urtica dioica, often called common nettle or stinging nettle SCIENCE LIST 7 Plants You Can't Even Touch The Pulitzer wall at the New York Times, celebrating the journalistic awards received by the newspaper and its journalists. Pulitzer prize ARTS & CULTURE QUIZ Pulitzer Prize Entrants falling and tumbling over while chasing the cheese at the 2016 'Cheese Rolling' held at Cooper's Hill, in the Cotswolds, Brockworth England SOCIETY LIST 7 of the World's Most Dangerous Festivals Charge, mass, and spin Scientists have known since the late 19th century that the electron has a negative electric charge. The value of this charge was first measured by the American physicist Robert Millikan between 1909 and 1910. In Millikan's oil-drop experiment, he suspended tiny oil drops in a chamber containing an oil mist. By measuring the rate of fall of the oil drops, he was able to determine their weight. Oil drops that had an electric charge (acquired, for example, by friction when moving through the air) could then be slowed down or stopped by applying an electric force. By comparing applied electric force with changes in motion, Millikan was able to determine the electric charge on each drop. After he had measured many drops, he found that the charges on all of them were simple multiples of a single number. This basic unit of charge was the charge on the electron, and the different charges on the oil drops corresponded to those having 2, 3, 4,... extra electrons on them. The charge on the electron is now accepted to be 1.602176565 × 10−19 coulomb. For this work Millikan was awarded the Nobel Prize for Physics in 1923. Millikan oil-drop experimentBetween 1909 and 1910 the American physicist Robert Millikan conducted a series of oil-drop experiments. By comparing applied electric force with changes in the motion of the oil drops, he was able to determine the electric charge on each drop. He found that all of the drops had charges that were simple multiples of a single number, the fundamental charge of the electron. Millikan oil-drop experimentBetween 1909 and 1910 the American physicist Robert Millikan conducted a series of oil-drop experiments. By comparing applied electric force with changes in the motion of the oil drops, he was able to determine the electric charge on each drop. He found that all of the drops had charges that were simple multiples of a single number, the fundamental charge of the electron. Millikan oil-drop experiment Encyclopædia Britannica, Inc. Millikan oil-drop experiment. Encyclopædia Britannica, Inc. The charge on the proton is equal in magnitude to that on the electron but opposite in sign—that is, the proton has a positive charge. Because opposite electric charges attract each other, there is an attractive force between electrons and protons. This force is what keeps electrons in orbit around the nucleus, something like the way that gravity keeps Earth in orbit around the Sun. The electron has a mass of about 9.109382911 × 10−28 gram. The mass of a proton or neutron is about 1,836 times larger. This explains why the mass of an atom is primarily determined by the mass of the protons and neutrons in the nucleus. CONNECT WITH BRITANNICA The electron has other intrinsic properties. One of these is called spin. The electron can be pictured as being something like Earth, spinning around an axis of rotation. In fact, most elementary particles have this property. Unlike Earth, however, they exist in the subatomic world and are governed by the laws of quantum mechanics. Therefore, these particles cannot spin in any arbitrary way, but only at certain specific rates. These rates can be 1/2, 1, 3/2, 2,... times a basic unit of rotation. Like protons and neutrons, electrons have spin 1/2. Particles with half-integer spin are called fermions, for the Italian American physicist Enrico Fermi, who investigated their properties in the first half of the 20th century. Fermions have one important property that will help explain both the way that electrons are arranged in their orbits and the way that protons and neutrons are arranged inside the nucleus. They are subject to the Pauli exclusion principle (named for the Austrian physicist Wolfgang Pauli), which states that no two fermions can occupy the same state—for example, the two electrons in a helium atom must have different spin directions if they occupy the same orbit. Because a spinning electron can be thought of as a moving electric charge, electrons can be thought of as tiny electromagnets. This means that, like any other magnet, an electron will respond to the presence of a magnetic field by twisting. (Think of a compass needle pointing north under the influence of Earth's magnetic field.) This fact is usually expressed by saying that electrons have a magnetic moment. In physics, magnetic moment relates the strength of a magnetic field to the torque experienced by a magnetic object. Because of their intrinsic spin, electrons have a magnetic moment given by −9.28 × 10−24 joule per tesla. Page 1 of 8 NEXT PAGE Orbits and energy levels ADDITIONAL MEDIA VIEW ALL MEDIA John Dalton and the development of the atomic theory. Bohr atomic model of a nitrogen atom. Each element has an atomic size, which is a measure of how far away the outermost, or valence, electrons are from the atomic nucleus. Images of man-made elliptic arrangements of atoms on a metallic surface. Figure 5: Electron density functions of a few hydrogen atom states. Plots of electron density in the xz plane of atomic hydrogen are shown for the n = 8, ml = 0, l = 0, 2, 6, and 7 states. The l = 0 state, for example, should be visualized as a spherically symmetric standing wave, and the l = 7 state as having the electron density localized into two blobs near the two poles of the atom. MORE ABOUT atom 51 REFERENCES FOUND IN BRITANNICA ARTICLES Assorted References history of science (in history of science: The Romantic revolt) atom counting (in spectroscopy: Atom counting) atomic physics (in atomic physics) Auger effect (in Auger effect) band theory (in band theory) electromagnetic radiation (in electromagnetic radiation: Discrete-frequency sources and absorbers of electromagnetic radiation) (in electromagnetic radiation: Resonance absorption and recoil) electromagnetism (in electromagnetism: Interaction of a magnetic field with a charge) electronics (in electronics: Valence electrons) visual images (in chemical bonding: Visual images of atoms) VIEW MORE ADDITIONAL READING EXTERNAL LINKS Boston University - Physics Department - Understanding the Atom British Broadcasting Corporation - Bitesize - Atoms Chemguide - A Simple View Of Atomic Structure Hyperphysics - Atoms and Elements Iowa State University - Atoms and Spectral Lines Kidipede Science for Kids - Atom NeoK12 - Educational Videos, Lessons and Games - Atom Rader's Chem4Kids.com - Atom The Shodor Education Foundation - Atomic Structure BRITANNICA WEB SITES Articles from Britannica encyclopedias for elementary and high school students. atom - Children's Encyclopedia (Ages 8-11) The tiny particles called atoms are the basic building blocks of all matter. Atoms can be combined with other atoms to form molecules, but they cannot be divided into smaller parts by ordinary means. atom - Student Encyclopedia (Ages 11 and up) The tiny units of matter known as atoms are the basic building blocks of chemistry. An atom is the smallest piece of matter that has the characteristic properties of a chemical element, such as hydrogen, oxygen, calcium, iron, gold, and neon. More than 90 types of atom exist in nature, and each one forms a different element. Elements are made up of only one type of atom-gold contains only gold atoms, and neon contains only neon atoms-but other substances are mixtures of different kinds of atoms. Atoms also join together chemically to form molecules. Matter is made up of molecules, atoms, and ions (electrically charged atoms or groups of atoms), so atoms are basic components of matter. Since atoms, ions, and molecules are very small, the bulk matter of everyday life consists of large amounts of these components. A glass of water, for instance, contains an extremely large amount (about 8 1024) of water molecules. Each water molecule is in turn made up of two hydrogen atoms and one oxygen atom that have been combined chemically. ARTICLE CONTRIBUTORS ARTICLE HISTORY FEEDBACK Corrections? Updates? Help us improve this article! Contact our editors with your feedback.

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