welcome to this key Concepts video on periodic trends what are periodic trends well periodic trends are the specific patterns we find on the periodic table patterns which describe changing properties of the chemical elements using the table we can predict how certain properties change down a group or across a period more specifically we'll first start with a general overview of the periodic table next we'll talk about groups followed by a discussion of periods and blocks after that we'll discuss the concepts of ionization energy electron affinity and electro negativity and how these change across a period or down a group so let's jump straight into it the elements on the periodic table are arranged according to their atomic numbers the atomic number of an atom indicates the number of protons in the nucleus changing the atomic number changes the element the periodic table shows the elements in order of increasing atomic number each element is represented by its symbol with the atomic number on the top the relative atomic mass is shown below the symbol this is approximately equal to the number of protons plus the average number of neutrons Row in the periodic table table are called periods while columns are called groups elements are organized into periods and groups based on their electron configuration let's start by talking about groups elements in the same group have the same number of electrons in their outermost shell elements in the same group therefore have similar chemical and physical properties groups are numbered from left to right and the group number indicates the number of outer shell electrons these numbers range from 1 to 18 group one elements are the alkal metals these all have one veence electron in their outermost shell they are soft highly reactive due to only having one veence electron and have low melting points and form soluble salts group two elements which all have two veence electrons R are called the alkaline earth metals these are harder and have higher melting points than the alcali metals and also are slightly less reactive and form less soluble salts groups 3 to 12 form the d block or transition metals these contain partially or completely filled D orbitals and therefore exhibit many unique properties groups 13 to 18 are mostly non-metals though elements near the bottom of groups 13 to 16 may be metals or semimetals also known as metalloids Group 17 elements all have seven veence electrons and are called the halogens at room temperature Florine and chlorine are gases bromine is a liquid and iodine is a solid that readily sublimes because these elements only need one more electron to form a full octet these elements react readily to form haly salts group 18 contains elements which all have a full octet of outermost electrons these are called the noble gases and are all highly unreactive gases some elements also contain occupied F orbitals these elements are positioned at the start of the D Block in periods 5 and six and are called the lanthanides and the actinides to make the periodic table easier to draw out they are usually listed below the main table now that we've talked about groups let's talk about periods elements in the same period have the same number of electron shells sometimes represented by the letter N so all of these elements here in Period 2 have two occupied electron shells the periodic table is also separated into blocks depending on what kind of orbital is occupied by the outermost electron of an element for groups 1 and two the highest energy occupied orbitals are the S orbitals the these elements are therefore referred to as the S block the D Block metals are socalled because the highest energy occupied orbitals are D orbitals after the D Block is the P block comprising of elements for which the highest energy electrons are found in the P orbitals before we go on to discuss other Trends let's quickly mention a very important Trend in the periodic table and that is as the atomic number increases the charge of the nucleus increases and as the charge of the nucleus increases the electrons in the outer shell are more strongly attracted to the nucleus and therefore the atomic radius decreases let's also mention how metals and non-metals are arranged on the periodic table atoms form bonds to gain complete outer shells of elect Rons atoms on the left hand side of the period do this by losing electrons forming cations these elements are the metals towards the right hand side of a period atoms gain electrons to form annion these elements are non-metals now that we have a good understanding of the periodic table and some of the trends scen let's talk about how the properties of ionization energy Elon affinity and electro negativity change when moving across the table we'll start with the ionization energy we've already seen that atoms get smaller across a period because the nuclear charge increases while the number of occupied shells stays the same this means outer electrons become more strongly attracted to the nucleus across a period the strength of this attraction between the electron and the nucleus can be Quantified by measuring the ionization energy this is the energy needed to remove an electron from an atom or an ion we usually refer to the molar ionization energy which is the energy needed to remove a mole of electrons from a mole of gaseous atoms or ions the first ionization energy is the energy needed to convert one Mo of gas gous atoms to one Mo of gaseous ions with a 1 plus charge for example the equation for the first ionization energy of sodium is sodium going to a sodium 1l ion plus an electron The energy needed to convert 1 plus ions to 2+ ions is the second ionization energy we can similarly describe a third ionization energy for producing 3 plus ions a fourth for producing four plus ions and so on these energies increase as more electrons are removed because there are progressively fewer electrons there is less electron electron repulsion so remaining electrons can lie closer to the nucleus and are attracted to it more strongly the successive ionization energies can tell us something about an atom's electronic structure for example there is a sharp increase in ionization energy once the outermost shell is empty and further electrons must be removed from a more inner shell this is because the inner shells are much lower in energy and closer to the nucleus so electrons in these shells are more strongly Bound for example in aluminum the first second and third ionization energies see a modest increase as these these electrons are all being removed from the 3s or 3p orbitals but the fourth ionization energy is much higher at about 11,000 K per mole as this represents a 2p electron being removed from al3+ so that's how the successive ionization energies change for the same atom but how do ionization energies for different elements change when moving across the periodic table the ionization energy depends on how strongly the outermost electron is attracted to the nucleus as the atomic radius increases the attraction between the nucleus and outermost electron becomes weaker so the ionization energy decreases down a group the number of shells increases and the atoms become larger and larger therefore the outermost electron is further from the nucleus and the electrostatic attraction is weaker hence the ionization energy decreases down a group when going across a period the number of veence electrons increases but the number of shells stays the same therefore the electrons in the outer shell are attracted by a nucleus with a greater positive charge the atom becomes smaller and the attraction of the outermost electron becomes stronger and the ionization energy increases a related but different concept to the ionization energy is the electron affinity we can also think about how easily atoms or ions gain electrons the energy change when one mole of gaseous atoms or ions gains one mole of electrons is called the molar electron affinity the electron Affinity of an atom can be represented with an equation as for the ionization energy here we have a suur atom gaining an electron to become a sulfur annion the electron affinity of an atom is negative because the additional electron is attracted to the positively charged nucleus the process releases energy however the electron affinity of metals is usually small because the atomic radius is relatively large and the outer shell is far from the nucleus further electrons can be added after the first but now the electrons are approaching an annion the electron and ion are both negatively charged and repel each other electrostatically so after the first electron affinity further electron affinity values are positive or endothermic for different elements across a period the electron affinity becomes more negative because the atom becomes smaller the added electron is closer to the nucleus so it's more strongly attracted down a group the electron affinity becomes less negative because the number of shells increases and the atom becomes larger the added electron is therefore further from the nucleus so less strongly attracted let's end this video with a discussion on electro negativity atoms form bonds by sharing electrons with other atoms or donating or accepting electrons to form ions the ability of an atom in a bond to attract a bonding pair of electrons towards itself is called the electro negativity it is sometimes given the symbol X electro negativity is usually measured using the 4o poing scale where four is the most electronegative and 0 is the least Electro negative an atom with low electro negativity can also be called electropositive in this example here the oxygen in water is more Electro negative than hydrogen so has an electro negativity value closer to four the trends in electro negativity are similar to the trends in ionization energy electro negativity decreases down a group because there is an increased number of shells and the bonding pair of electrons is further away from the nucleus electro negativity increases across a period because an increasing nuclear charge causes the atoms to become smaller the bonding pair of electrons is therefore closer of the nucleus increasing the strength of Attraction and that's that hopefully you learned something new about periodic trends to summarize the periodic table is organized into groups and periods elements in the same group have the same number of veence electrons but a different number of electron shells elements in the same period have the same number of electron shells but a different number of veence el electrons importantly elements get larger down a group but smaller across a period and this change in atomic radius can explain the trends in ionization energy electro negativity and electron affinity