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. Atoms bond to form molecules : Two or more atoms may bond with each other to form a molecule.
When two hydrogens and an oxygen share electrons via covalent bonds, a water molecule is formed. The substances used in the beginning of a chemical reaction are called the reactants usually found on the left side of a chemical equation , and the substances found at the end of the reaction are known as the products usually found on the right side of a chemical equation.
An arrow is typically drawn between the reactants and products to indicate the direction of the chemical reaction. For the creation of the water molecule shown above, the chemical equation would be:. An example of a simple chemical reaction is the breaking down of hydrogen peroxide molecules, each of which consists of two hydrogen atoms bonded to two oxygen atoms H 2 O 2.
The reactant hydrogen peroxide is broken down into water H 2 O , and oxygen, which consists of two bonded oxygen atoms O 2. In the equation below, the reaction includes two hydrogen peroxide molecules and two water molecules.
This is an example of a balanced chemical equation, wherein the number of atoms of each element is the same on each side of the equation.
According to the law of conservation of matter, the number of atoms before and after a chemical reaction should be equal, such that no atoms are, under normal circumstances, created or destroyed. Even though all of the reactants and products of this reaction are molecules each atom remains bonded to at least one other atom , in this reaction only hydrogen peroxide and water are representative of a subclass of molecules known as compounds: they contain atoms of more than one type of element.
Molecular oxygen, on the other hand, consists of two doubly bonded oxygen atoms and is not classified as a compound but as an element. Some chemical reactions, such as the one shown above, can proceed in one direction until the reactants are all used up.
The equations that describe these reactions contain a unidirectional arrow and are irreversible. Reversible reactions are those that can go in either direction. In reversible reactions, reactants are turned into products, but when the concentration of product goes beyond a certain threshold, some of these products will be converted back into reactants; at this point, the designations of products and reactants are reversed. This back and forth continues until a certain relative balance between reactants and products occurs: a state called equilibrium.
These situations of reversible reactions are often denoted by a chemical equation with a double headed arrow pointing towards both the reactants and products. If carbonic acid were added to this system, some of it would be converted to bicarbonate and hydrogen ions. Scientists determine the atomic mass by calculating the mean of the mass numbers for its naturally-occurring isotopes. Often, the resulting number contains a decimal. For example, the atomic mass of chlorine Cl is Given an atomic number Z and mass number A , you can find the number of protons, neutrons, and electrons in a neutral atom.
Isotopes are various forms of an element that have the same number of protons, but a different number of neutrons. Isotopes are various forms of an element that have the same number of protons but a different number of neutrons.
Some elements, such as carbon, potassium, and uranium, have multiple naturally-occurring isotopes. Isotopes are defined first by their element and then by the sum of the protons and neutrons present. While the mass of individual isotopes is different, their physical and chemical properties remain mostly unchanged. Isotopes do differ in their stability. Carbon 12 C is the most abundant of the carbon isotopes, accounting for Carbon 14 C is unstable and only occurs in trace amounts.
Neutrons, protons, and positrons can also be emitted and electrons can be captured to attain a more stable atomic configuration lower level of potential energy through a process called radioactive decay. The new atoms created may be in a high energy state and emit gamma rays which lowers the energy but alone does not change the atom into another isotope. These atoms are called radioactive isotopes or radioisotopes. 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. And you might guess, oh, you know, Sal already told me they're very small.
So maybe there's 1, carbon atoms there, or 10,, or , I would say, no. There are 1 million carbon atoms, or you could string 1 million carbon atoms across the width of the average human hair. That's obviously an approximation. It's not exactly 1 million. But that gives you a sense of how small an atom is.
You know, pluck a hair out of your head, and just imagine putting a million things next to each other, across the hair.
Not the length of the hair, the width of the hair. It's even hard to see the width of a hair, and there would be a million carbon atoms, just going along it. Now it would be pretty cool, in and of itself, that we do know that there is this most basic building block of carbon, this most basic building block of any element. But what's even neater is that, those basic building blocks are related to each other. That a carbon atom is made up of even more fundamental particles. A gold atom is made up even more fundamental particles.
And depending-- and they're actually defined by the arrangement of those fundamental particles. And if you were to change the number of fundamental particles you have, you could change the properties of the element, how it would react, or you could even change the element itself.
And just to understand it a little bit better, let's talk about those fundamental elements. So you have the proton. And the proton is actually the defining-- the number of protons in the nucleus of an atom, and I'll talk about the nucleus in a second-- that is what defines the element. So this is what defines an element. When you look at the periodic table right here, they're actually written in order of atomic number.
And the atomic number is, literally, just the number of protons in the element. So by definition, hydrogen has one proton, helium has two protons, carbon has six protons.
You cannot have carbon with seven protons. If you did, it would be nitrogen. It would not be carbon anymore. Oxygen has eight protons. If, somehow, you were to add another proton to there, it wouldn't be oxygen anymore. It would be fluorine. So it defines the element. And the atomic number, the number of protons-- and remember, that's the number that's written right at the top, here, for each of these elements in the periodic table-- the number of protons is equal to the atomic number.
And they put that number up here, because that is the defining characteristic of an element. The other two constituents of an atom-- I guess we could call it that way-- are the electron and the neutron.
And the model you can start to build in your head-- and this model, as we go through chemistry, it'll get a little bit more abstract and really hard to conceptualize. But one way to think about it is, you have the protons and the neutrons that are at the center of the atom.
They're the nucleus of the atom. So for example, carbon, we know, has six protons. So one, two, three, four, five, six. Carbon, which is a version of carbon, will also have six neutrons. You can have versions of carbon that have a different number of neutrons. So the neutrons can change, the electrons can change, you can still have the same element. The protons can't change. You change the protons, you've got a different element. So let me draw a carbon nucleus, one, two, three, four, five, six.
So this right here is the nucleus of carbon And sometimes, it'll be written like this. And sometimes, they'll actually write the number of protons, as well. And the reason why we write it carbon you know, I counted out six neutrons-- is that, this is the total, you could view this as the total number of-- one way to view it.
And we'll get a little bit nuance in the future-- is that this is the total number of protons and neutrons inside of its nucleus. And this carbon, by definition, has an atomic number of six, but we can rewrite it here, just so that we can remind ourselves. So at the center of a carbon atom, we have this nucleus. And carbon will have six protons and six neutrons.
Another version of carbon, carbon, will still have six protons, but then it would have eight neutrons. So the number of neutrons can change. But this is carbon, right over here. And if carbon is neutral-- and I'll give a little nuance on this word in a second as well-- if it is neutral, it'll also have six electrons. So let me draw those six electrons, one, two, three, four, five, six.
And one way-- and this is maybe the first-order way of thinking about the relationship between the electrons and the nucleus-- is that you can imagine the electrons are, kind of, moving around, buzzing around this nucleus.
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