Carbon is normally present in the atmosphere in the form of gaseous compounds like carbon dioxide and methane. Carbon 14 C is a naturally-occurring radioisotope that is created from atmospheric 14 N nitrogen by the addition of a neutron and the loss of a proton, which is caused by cosmic rays. This is a continuous process so more 14 C is always being created in the atmosphere.
Once produced, the 14 C often combines with the oxygen in the atmosphere to form carbon dioxide. Carbon dioxide produced in this way diffuses in the atmosphere, is dissolved in the ocean, and is incorporated by plants via photosynthesis. Animals eat the plants and, ultimately, the radiocarbon is distributed throughout the biosphere. In living organisms, the relative amount of 14 C in their body is approximately equal to the concentration of 14 C in the atmosphere.
When an organism dies, it is no longer ingesting 14 C, so the ratio between 14 C and 12 C will decline as 14 C gradually decays back to 14 N. This slow process, which is called beta decay, releases energy through the emission of electrons from the nucleus or positrons. After approximately 5, years, half of the starting concentration of 14 C will have been converted back to 14 N. This is referred to as its half-life, or the time it takes for half of the original concentration of an isotope to decay back to its more stable form.
Because the half-life of 14 C is long, it is used to date formerly-living objects such as old bones or wood. Comparing the ratio of the 14 C concentration found in an object to the amount of 14 C in the atmosphere, the amount of the isotope that has not yet decayed can be determined. On the basis of this amount, the age of the material can be accurately calculated, as long as the material is believed to be less than 50, years old.
This technique is called radiocarbon dating, or carbon dating for short. Application of carbon dating : The age of carbon-containing remains less than 50, years old, such as this pygmy mammoth, can be determined using carbon dating.
Other elements have isotopes with different half lives. For example, 40 K potassium has a half-life of 1. Scientists often use these other radioactive elements to date objects that are older than 50, years the limit of carbon dating. Through the use of radiometric dating, scientists can study the age of fossils or other remains of extinct organisms. Everything in the universe is made of one or more elements.
The periodic table is a means of organizing the various elements according to similar physical and chemical properties. Matter comprises all of the physical objects in the universe, those that take up space and have mass. All matter is composed of atoms of one or more elements, pure substances with specific chemical and physical properties. There are 98 elements that naturally occur on earth, yet living systems use a relatively small number of these. Living creatures are composed mainly of just four elements: carbon, hydrogen, oxygen, and nitrogen often remembered by the acronym CHON.
As elements are bonded together they form compounds that often have new emergent properties that are different from the properties of the individual elements. Life is an example of an emergent property that arises from the specific collection of molecules found in cells.
Elements of the human body arranged by percent of total mass : There are 25 elements believed to play an active role in human health. The different elements are organized and displayed in the periodic table. Devised by Russian chemist Dmitri Mendeleev — in , the table groups elements that, although unique, share certain chemical properties with other elements. In the periodic table the elements are organized and displayed according to their atomic number and are arranged in a series of rows periods and columns groups based on shared chemical and physical properties.
If you look at a periodic table, you will see the groups numbered at the top of each column from left to right starting with 1 and ending with Looking at carbon, for example, its symbol C and name appear, as well as its atomic number of six in the upper left-hand corner and its atomic mass of The periodic table : The periodic table shows the atomic mass and atomic number of each element.
The atomic number appears above the symbol for the element and the approximate atomic mass appears below it. The arrangement of the periodic table allows the elements to be grouped according to their chemical properties. Within the main group elements Groups , , there are some general trends that we can observe. The further down a given group, the elements have an increased metallic character: they are good conductors of both heat and electricity, solids at room temperature, and shiny in appearance.
Moving from left to right across a period, the elements have greater non-metallic character. These elements are insulators, poor heat conductors, and can exist in different phases at room temperature brittle solid, liquid, or gas. The elements at the boundary between the metallic elements grey elements and nonmetal elements green elements are metalloid in character pink elements.
They have low electrical conductivity that increases with temperature. They also share properties with both the metals and the nonmetals. The main group elements : Within the p-block at the boundary between the metallic elements grey elements and nonmetal elements green elements there is positioned boron and silicon that are metalloid in character pink elements , i.
Today, the periodic table continues to expand as heavier and heavier elements are synthesized in laboratories. These large elements are extremely unstable and, as such, are very difficult to detect; but their continued creation is an ongoing challenge undertaken by scientists around the world.
Niels Bohr proposed an early model of the atom as a central nucleus containing protons and neutrons being orbited by electrons in shells. In this model, electrons exist within principal shells. An electron normally exists in the lowest energy shell available, which is the one closest to the nucleus. Energy from a photon of light can bump it up to a higher energy shell, but this situation is unstable and the electron quickly decays back to the ground state.
In the process, a photon of light is released. As previously discussed, there is a connection between the number of protons in an element, the atomic number that distinguishes one element from another, and the number of electrons it has. In all electrically-neutral atoms, the number of electrons is the same as the number of protons.
Each element, when electrically neutral, has a number of electrons equal to its atomic number. An early model of the atom was developed in by Danish scientist Niels Bohr — The Bohr model shows the atom as a central nucleus containing protons and neutrons with the electrons in circular orbitals at specific distances from the nucleus.
These orbits form electron shells or energy levels, which are a way of visualizing the number of electrons in the various shells. Electrons fill orbit shells in a consistent order. Under standard conditions, atoms fill the inner shells closer to the nucleus first, often resulting in a variable number of electrons in the outermost shell. The innermost shell has a maximum of two electrons, but the next two electron shells can each have a maximum of eight electrons.
This is known as the octet rule which states that, with the exception of the innermost shell, atoms are more stable energetically when they have eight electrons in their valence shell, the outermost electron shell. Examples of some neutral atoms and their electron configurations are shown in. As shown, helium has a complete outer electron shell, with two electrons filling its first and only shell. Similarly, neon has a complete outer 2n shell containing eight electrons.
In contrast, chlorine and sodium have seven and one electrons in their outer shells, respectively. Theoretically, they would be more energetically stable if they followed the octet rule and had eight.
Bohr diagrams : Bohr diagrams indicate how many electrons fill each principal shell. Group 18 elements helium, neon, and argon are shown have a full outer, or valence, shell. A full valence shell is the most stable electron configuration. Elements in other groups have partially-filled valence shells and gain or lose electrons to achieve a stable electron configuration. An atom may gain or lose electrons to achieve a full valence shell, the most stable electron configuration. The periodic table is arranged in columns and rows based on the number of electrons and where these electrons are located, providing a tool to understand how electrons are distributed in the outer shell of an atom.
As shown in, the group 18 atoms helium He , neon Ne , and argon Ar all have filled outer electron shells, making it unnecessary for them to gain or lose electrons to attain stability; they are highly stable as single atoms. Their non-reactivity has resulted in their being named the inert gases or noble gases. In comparison, the group 1 elements, including hydrogen H , lithium Li , and sodium Na , all have one electron in their outermost shells.
This means that they can achieve a stable configuration and a filled outer shell by donating or losing an electron.
As a result of losing a negatively-charged electron, they become positively-charged ions. Group 17 elements, including fluorine and chlorine, have seven electrons in their outermost shells; they tend to fill this shell by gaining an electron from other atoms, making them negatively-charged ions.
When an atom gains an electron to become a negatively-charged ion this is indicated by a minus sign after the element symbol; for example, F-.
Electron orbitals are three-dimensional representations of the space in which an electron is likely to be found. Although useful to explain the reactivity and chemical bonding of certain elements, the Bohr model of the atom does not accurately reflect how electrons are spatially distributed surrounding the nucleus. They do not circle the nucleus like the earth orbits the sun, but are rather found in electron orbitals.
These relatively complex shapes result from the fact that electrons behave not just like particles, but also like waves. Mathematical equations from quantum mechanics known as wave functions can predict within a certain level of probability where an electron might be at any given time.
The area where an electron is most likely to be found is called its orbital. The closest orbital to the nucleus, called the 1s orbital, can hold up to two electrons. This orbital is equivalent to the innermost electron shell of the Bohr model of the atom.
It is called the 1s orbital because it is spherical around the nucleus. The 1s orbital is always filled before any other orbital. Hydrogen has one electron; therefore, it has only one spot within the 1s orbital occupied.
This is designated as 1s 1 , where the superscripted 1 refers to the one electron within the 1s orbital. Helium has two electrons; therefore, it can completely fill the 1s orbital with its two electrons. This is designated as 1s 2 , referring to the two electrons of helium in the 1s orbital. On the periodic table, hydrogen and helium are the only two elements in the first row period ; this is because they are the sole elements to have electrons only in their first shell, the 1s orbital.
The second electron shell may contain eight electrons. After the 1s orbital is filled, the second electron shell is filled, first filling its 2s orbital and then its three p orbitals. When filling the p orbitals, each takes a single electron; once each p orbital has an electron, a second may be added. Lithium Li contains three electrons that occupy the first and second shells. Two electrons fill the 1s orbital, and the third electron then fills the 2s orbital.
Its electron configuration is 1s 2 2s 1. Neon Ne , on the other hand, has a total of ten electrons: two are in its innermost 1s orbital, and eight fill its second shell two each in the 2s and three p orbitals. Thus, it is an inert gas and energetically stable: it rarely forms a chemical bond with other atoms.
Diagram of the S and P orbitals : The s subshells are shaped like spheres. Both the 1n and 2n principal shells have an s orbital, but the size of the sphere is larger in the 2n orbital. Each sphere is a single orbital. Principal shell 2n has a p subshell, but shell 1 does not. Larger elements have additional orbitals, making up the third electron shell. Subshells d and f have more complex shapes and contain five and seven orbitals, respectively. Principal shell 3n has s, p, and d subshells and can hold 18 electrons.
Principal shell 4n has s, p, d, and f orbitals and can hold 32 electrons. Moving away from the nucleus, the number of electrons and orbitals found in the energy levels increases. Progressing from one atom to the next in the periodic table, the electron structure can be worked out by fitting an extra electron into the next available orbital.
While the concepts of electron shells and orbitals are closely related, orbitals provide a more accurate depiction of the electron configuration of an atom because the orbital model specifies the different shapes and special orientations of all the places that electrons may occupy.
Chemical reactions occur when two or more atoms bond together to form molecules or when bonded atoms are broken apart.
According to the octet rule, elements are most stable when their outermost shell is filled with electrons. This is because it is energetically favorable for atoms to be in that configuration. However, since not all elements have enough electrons to fill their outermost shells, atoms form chemical bonds with other atoms, which helps them obtain the electrons they need to attain a stable electron configuration.
When two or more atoms chemically bond with each other, the resultant chemical structure is a molecule. The familiar water molecule, H 2 O, consists of two hydrogen atoms and one oxygen atom, which bond together to form water. Atoms can form molecules by donating, accepting, or sharing electrons to fill their outer shells. Why is the atomic number used to identify elements? Why is the atomic number a whole number?
Why is atomic number represented by Z? How did Dmitri Mendeleev contribute to the atomic number? How is the atomic number affected by gamma decay? See all questions in Atomic Number. Impact of this question views around the world. Elements are pure substances that make up all other matter, so each one is given a unique name. However, it would more powerful if these names could be used to identify the numbers of protons and neutrons in the atoms. That's where atomic number and mass number are useful.
If an atom has only one proton, we know that it's a hydrogen atom. An atom with two protons is always a helium atom. If scientists count four protons in an atom, they know it's a beryllium atom. An atom with three protons is a lithium atom, an atom with five protons is a boron atom, an atom with six protons is a carbon atom.
Since an atom of one element can be distinguished from an atom of another element by the number of protons in its nucleus, scientists are always interested in this number, and how this number differs between different elements. This number is very important because it is unique for atoms of a given element. All atoms of an element have the same number of protons, and every element has a different number of protons in its atoms.
For example, all helium atoms have two protons, and no other elements have atoms with two protons. Of course, since neutral atoms have to have one electron for every proton, an element's atomic number also tells you how many electrons are in a neutral atom of that element. For example, hydrogen has an atomic number of 1. This means that an atom of hydrogen has one proton, and, if it's neutral, one electron as well.
Gold, on the other hand, has an atomic number of 79, which means that an atom of gold has 79 protons, and, if it's neutral, 79 electrons as well.
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