There are several trends within the periodic table which are good to know about. These are atomic radius, electronegativity, and metallic properties. Interestingly, these are all directly connected to how hard the elements are able to hold their outermost electrons. Below, you see an image of the trends in the periodic table. After the picture, we go through them one by one.
The variation in atomic radius in the periodic table is dependent of two factors.
The first factor is how many electron shells that contains electrons. Electron shells further away from the nuclei will increase the radius of the atom. This is the reason why atomic radius increases the further down we go within a group, since we add an electron shell with each period.
The second factor is how strongly the nuclei pulls the electrons. Electrons in the same shell can be said to be on the same distance from the nuclei in the base state. The bigger the charge of the nuclei is compared to the electron shell, the closer to the nuclei the electrons will be. In the beginning of a period, (to the left), we just started a new electron shell. The nuclear charge is then relatively weak, meaning the electrons in the outer electron shell can exist further out, giving the atom a large atomic radius. As we move to the right within a period, the nuclear charge will increase, but the outer electron shell remains the same. The increased nuclear charge will pull the electrons harder, making the atomic radius decrease.
Electronegativity is the ability of an atom to attract electrons in a bond. Read more about this in our other articles.
According to the same reasoning as under the title "atomic radius", the atomic nuclei pulls the electrons harder when the nuclear charge is bigger in comparison to the outermost electron shell. Thus, electronegativity increases the further to the right we go in the periodic table.
The closer to the nuclei the valence electrons are, the more strongly they are attracted. Thus, electronegativity increases the further up we go in the periodic table.
For a substance to work as a metal, it needs to donate an electron to the shared electron cloud. Read more about this in our other articles.
For an element to have metallic properties, it needs to easily let go of electrons. That is the exact opposite of the concept of electronegativity. The more metallic an element is, the less electronegative it is. Metallic properties increases with a weaker pull from the nuclei (metallic properties increases the further to the left we go), and valence electrons in shells far away from the nuclei (metallic properties increases the further down we go).