This configuration of electron shells is the same for all atoms. The precise number of shells depends on the number of electrons in the atom. Hydrogen and helium have just one and two electrons, respectively. If you take a look at the periodic table of the elements, you will notice that hydrogen and helium are placed alone on either sides of the top row; they are the only elements that have just one electron shell Figure.
A second shell is necessary to hold the electrons in all elements larger than hydrogen and helium. Lithium Li , whose atomic number is 3, has three electrons. Two of these fill the first electron shell, and the third spills over into a second shell. The second electron shell can accommodate as many as eight electrons. Carbon, with its six electrons, entirely fills its first shell, and half-fills its second. With ten electrons, neon Ne entirely fills its two electron shells.
Again, a look at the periodic table reveals that all of the elements in the second row, from lithium to neon, have just two electron shells. Atoms with more than ten electrons require more than two shells.
These elements occupy the third and subsequent rows of the periodic table. The factor that most strongly governs the tendency of an atom to participate in chemical reactions is the number of electrons in its valence shell.
If the valence shell is full, the atom is stable; meaning its electrons are unlikely to be pulled away from the nucleus by the electrical charge of other atoms. If the valence shell is not full, the atom is reactive; meaning it will tend to react with other atoms in ways that make the valence shell full.
Consider hydrogen, with its one electron only half-filling its valence shell. All atoms except hydrogen and helium with their single electron shells are most stable when there are exactly eight electrons in their valence shell. This principle is referred to as the octet rule, and it states that an atom will give up, gain, or share electrons with another atom so that it ends up with eight electrons in its own valence shell.
When two hydrogen atoms each share their single electron with oxygen, covalent bonds are formed, resulting in a molecule of water, H 2 O. In nature, atoms of one element tend to join with atoms of other elements in characteristic ways. For example, carbon commonly fills its valence shell by linking up with four atoms of hydrogen. In so doing, the two elements form the simplest of organic molecules, methane, which also is one of the most abundant and stable carbon-containing compounds on Earth.
As stated above, another example is water; oxygen needs two electrons to fill its valence shell. It commonly interacts with two atoms of hydrogen, forming H 2 O. The human body is composed of elements, the most abundant of which are oxygen O , carbon C , hydrogen H and nitrogen N.
You obtain these elements from the foods you eat and the air you breathe. The smallest unit of an element that retains all of the properties of that element is an atom.
But, atoms themselves contain many subatomic particles, the three most important of which are protons, neutrons, and electrons. These particles do not vary in quality from one element to another; rather, what gives an element its distinctive identification is the quantity of its protons, called its atomic number.
Heavier and lighter versions of the same element can occur in nature because these versions have different numbers of neutrons. Different versions of an element are called isotopes.
Helium, as well as larger atoms with eight electrons in their valence shell, is unlikely to participate in chemical reactions because they are stable.
All other atoms tend to accept, donate, or share electrons in a process that brings the electrons in their valence shell to eight or in the case of hydrogen, to two. The mass number is the total number of protons and neutrons in the nucleus of an atom. On the periodic table of the elements, mercury Hg has an atomic number of 80 and a mass number of It has seven stable isotopes. Nitrogen has an atomic number of seven. How many electron shells does it likely have?
The most abundant elements in the foods and beverages you consume are oxygen, carbon, hydrogen, and nitrogen. Why might having these elements in consumables be useful? These four elements—oxygen, carbon, hydrogen, and nitrogen—together make up more than 95 percent of the mass of the human body, and the body cannot make elements, so it is helpful to have them in consumables. Oxygen, whose atomic number is eight, has three stable isotopes: 16 O, 17 O, and 18 O.
Explain what this means in terms of the number of protons and neutrons. Oxygen has eight protons. In its most abundant stable form, it has eight neutrons, too, for a mass number of In contrast, 17 O has nine neutrons, and 18 O has 10 neutrons.
Magnesium is an important element in the human body, especially in bones. Is it stable or reactive? If it were to react with another atom, would it be more likely to accept or to donate one or more electrons? According to the octet rule, magnesium is unstable reactive because its valence shell has just two electrons. It is therefore likely to participate in chemical reactions in which it donates two electrons.
Skip to content The Chemical Level of Organization. Elements and Compounds All matter in the natural world is composed of one or more of the 92 fundamental substances called elements. Elements of the Human Body. The main elements that compose the human body are shown from most abundant to least abundant.
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Jump to main content. Periodic Table. Glossary Allotropes Some elements exist in several different structural forms, called allotropes. Discovery date Discovered by Gustav Kirchhoff and Robert Bunsen Origin of the name The name comes from the Latin 'caesius', meaning sky blue, and derived from its flame colour. Glossary Group A vertical column in the periodic table.
Fact box. Group 1 Melting point Glossary Image explanation Murray Robertson is the artist behind the images which make up Visual Elements. Appearance The description of the element in its natural form. Biological role The role of the element in humans, animals and plants.
Natural abundance Where the element is most commonly found in nature, and how it is sourced commercially. Uses and properties. Image explanation. The symbol reflects the use of the element in highly accurate atomic clocks. Caesium is a soft, gold-coloured metal that is quickly attacked by air and reacts explosively in water. The most common use for caesium compounds is as a drilling fluid.
They are also used to make special optical glass, as a catalyst promoter, in vacuum tubes and in radiation monitoring equipment. These clocks are a vital part of the internetand mobile phone networks, as well as Global Positioning System GPS satellites. They give the standard measure of time: the electron resonance frequency of the caesium atom is 9,,, cycles per second. Some caesium clocks are accurate to 1 second in 15 million years.
Biological role. Caesium has no known biological role. Caesium compounds, such as caesium chloride, are low hazard. Natural abundance. Caesium is found in the minerals pollucite and lepidolite. Pollucite is found in great quantities at Bernic Lake,Manitoba, Canada and in the USA, and from this source the element can be prepared. However, most commercialproduction is as a by-product of lithium production. Help text not available for this section currently. Elements and Periodic Table History.
Caesium was almost discovered by Carl Plattner in when he investigated the mineral pollucite caesium aluminium silicate. It was later realised that he mistook the caesium for sodium and potassium. They examined mineral water from Durkheim and observed lines in the spectrum which they did not recognise, and that meant a new element was present.
They produced around 7 grams of caesium chloride from this source, but were unable to produce a sample of the new metal itself. Atomic data. Glossary Common oxidation states The oxidation state of an atom is a measure of the degree of oxidation of an atom.
Oxidation states and isotopes. Glossary Data for this section been provided by the British Geological Survey. Relative supply risk An integrated supply risk index from 1 very low risk to 10 very high risk. Recycling rate The percentage of a commodity which is recycled. Substitutability The availability of suitable substitutes for a given commodity. Reserve distribution The percentage of the world reserves located in the country with the largest reserves. Political stability of top producer A percentile rank for the political stability of the top producing country, derived from World Bank governance indicators.
Political stability of top reserve holder A percentile rank for the political stability of the country with the largest reserves, derived from World Bank governance indicators. Supply risk. Young's modulus A measure of the stiffness of a substance.
Shear modulus A measure of how difficult it is to deform a material. Bulk modulus A measure of how difficult it is to compress a substance. Vapour pressure A measure of the propensity of a substance to evaporate. Pressure and temperature data — advanced. Listen to Caesium Podcast Transcript :. You're listening to Chemistry in its element brought to you by Chemistry World , the magazine of the Royal Society of Chemistry.
I've been asked on a number of occasions what my favourite element is. I used to think either oxygen or hydrogen - both so much fun - but that was until my sample of caesium arrived, when it was love at first sight.
Now many people think it's slightly odd having a favourite element, but when they too see my caesium, they understand why it's so special. Who wouldn't be attracted to this beautiful element? For starters, there are only three metallic elements that are not silver-coloured.
Two are well-known and fairly obvious - gold and copper. The third most people would never guess, it's caesium.
Apparently, the beautiful gold diminishes if the sample is extremely pure since tiny traces of captured oxygen give it the colour. This is a little disappointing - its colour is quite stunning and I would be sad if it really did disappear when purified. The next exciting thing about caesium is that my love is not unrequited, it responds to my touch. Strictly speaking, it's the warmth from the hand that melts it, given that its melting point is only So just holding its container converts the crystalline solid into liquid gold.
Liquid metals are always fascinating - everyone loves mercury; just imagine playing with liquid gold! But here's the snag that adds to my fascination with this metal - it has a rather fiery temper. In fact, you can't actually touch the metal itself since it spontaneously bursts into flames in the presence of air and reacts explosively with water. Awkward indeed. My caesium is sealed inside a glass tube under an atmosphere of the chemically inert gas argon.
So to play with it, you have to hold the glass tube, knowing that if you accidentally crushed it, or dropped it, all hell would break loose. Caesium gets its name from the Greek for heavenly blue. Not for its eyes it's only an element! Caesium was discovered in by Robert Bunsen he of the burner fame and physicist Gustav Kirchhoff. The previous year they had invented an instrument known as a spectroscope to help in chemical analysis.
When atoms are energetically excited, for instance when a compound is introduced into a flame, electrons can temporarily be promoted to higher energy levels. When they return to their lower energy states, energy is released in the form of light.
The spectroscope splits up the light with a prism and reveals a spectrum consisting of series of sharp coloured lines. Each element has its own unique spectrum of lines, like a rainbow barcode. When examining the spectrum of the residue from some spa mineral water, Bunsen and Kirchhoff found a series of lines that did not correspond to any known element. They named the new element caesium because of the distinct blue lines in the spectrum. Because hydrogen is so light, the pure element isn't commonly found on the Earth.
It would just float away. The prime components of air, nitrogen and oxygen, are fourteen and sixteen times heavier, giving hydrogen dramatic buoyancy. This lightness of hydrogen made it a natural for one of its first practical uses - filling balloons.
No balloon soars as well as a hydrogen balloon. The first such aerial vessel was the creation of French scientist Jacques Charles in , who was inspired by the Montgolfier brothers' hot air success a couple of months before to use hydrogen in a balloon of silk impregnated with rubber. Hydrogen seemed to have a guaranteed future in flying machines, reinforced by the invention of airships built on a rigid frame, called dirigibles in the UK but better known by their German nickname of Zeppelins, after their enthusiastic promoter Graf Ferdinand von Zeppelin.
These airships were soon the liners of the sky, carrying passengers safely and smoothly across the Atlantic. But despite the ultimate lightness of hydrogen it has another property that killed off airships - hydrogen is highly flammable.
The destruction of the vast zeppelin the Hindenburg, probably by fire caused by static electricity, was seen on film by shocked audiences around the world. The hydrogen airship was doomed. Yet hydrogen has remained a player in the field of transport because of the raw efficiency of its combustion. Many of NASA's rockets, including the second and third stages of the Apollo Program's Saturn V and the Space Shuttle main engines, are powered by burning liquid hydrogen with pure oxygen. More recently still, hydrogen has been proposed as a replacement for fossil fuels in cars.
Here it has the big advantage over petrol of burning to provide only water. No greenhouse gasses are emitted. The most likely way to employ hydrogen is not to burn it explosively, but to use it in a fuel cell, where an electrochemical reaction is used to produce electricity to power the vehicle. Not everyone is convinced that hydrogen fuelled cars are the future, though.
We would need a network of hydrogen fuel stations, and it remains a dangerous, explosive substance. At the same time, it is less efficient than petrol, because a litre of petrol has about three times more useful energy in it than a litre of liquid hydrogen if you use compressed hydrogen gas that can go up to ten times more.
The other problem is obtaining the hydrogen. It either comes from hydrocarbons, potentially leaving a residue of greenhouse gasses, or from electrolysing water, using electricity that may not be cleanly generated.
But even if we don't get hydrogen fuelled cars, hydrogen still has a future in a more dramatic energy source - nuclear fusion, the power source of the sun. Fusion power stations are tens of years away from being practical, but hold out the hope of clean, plentiful energy. However we use hydrogen, though, we can't take away its prime position.
It is numero uno, the ultimate, the king of the elements. So it's the most abundant element, is essential for life on earth, fuels space rockets and could resolve our fossil fuel dependents.
You can see why Brian Clegg classes hydrogen as number one. Now next week we meet the time keeper of the periodic table. One current use is in atomic clocks, though rubidium is considered less accurate than caesium.
The rubidium version of the atomic clock employs the transition between two hyperfine energy states of the rubidium isotope. These clocks use microwave radiation which is tuned until it matches the hyperfine transition, at which point the interval between wave crests of the radiation can be used to calibrate time itself. Until then I'm Meera Senthilingam, thanks for listening and goodbye. Chemistry in its element is brought to you by the Royal Society of Chemistry and produced by thenakedscientists.
There's more information and other episodes of Chemistry in its element on our website at chemistryworld. Click here to view videos about Hydrogen. View videos about. Help Text. Learn Chemistry : Your single route to hundreds of free-to-access chemistry teaching resources. We hope that you enjoy your visit to this Site. We welcome your feedback. Data W. Haynes, ed. Version 1. Coursey, D. Schwab, J. Tsai, and R. Dragoset, Atomic Weights and Isotopic Compositions version 4.
Periodic Table of Videos , accessed December Podcasts Produced by The Naked Scientists. Download our free Periodic Table app for mobile phones and tablets. Explore all elements.
D Dysprosium Dubnium Darmstadtium. E Europium Erbium Einsteinium. F Fluorine Francium Fermium Flerovium. G Gallium Germanium Gadolinium Gold. I Iron Indium Iodine Iridium.
K Krypton. O Oxygen Osmium Oganesson. U Uranium. V Vanadium. X Xenon. Y Yttrium Ytterbium. Z Zinc Zirconium. Membership Become a member Connect with others Supporting individuals Supporting organisations Manage my membership. Facebook Twitter LinkedIn Youtube.
Discovery date. Discovered by. Henry Cavendish. Origin of the name. The name is derived from the Greek 'hydro' and 'genes' meaning water forming. Melting point. Boiling point. Atomic number. Relative atomic mass. Key isotopes. Electron configuration.
CAS number. ChemSpider ID. ChemSpider is a free chemical structure database. Electronegativity Pauling scale. Covalent bond. Found in. SiH 4. AsH 3. Common oxidation states. Atomic mass. Half life.
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