Why do group 1 elements get less reactive as you go down?
The reactivity of Group 1 elements increases as you go down the group because: the atoms get larger. the outer electron gets further from the nucleus. the attraction between the nucleus and outer electron gets weaker – so the electron is more easily lost.
Reactivity decreases down the group.
This is because group 7 elements react by gaining an electron. As you move down the group, the amount of electron shielding increases, meaning that the electron is less attracted to the nucleus.
Because the halogen elements have seven valence electrons, they only require one additional electron to form a full octet. This characteristic makes them more reactive than other non-metal groups.
The non-metal elements in Group 7 – known as the halogens – get less reactive as you go down the group. This is the opposite trend to that seen in the alkali metals in Group 1 of the periodic table . Fluorine is the most reactive element of all in Group 7.
Reactivity of Group 1 Elements
Hydrogen is a very reactive gas, and the alkali metals are even more reactive. In fact, they are the most reactive metals and, along with the elements in group 17, are the most reactive of all elements.
Group 17 elements are halogens. They are highly reactive because, they have seven valence electrons in their outer shell and hence, they only require one additional electron to form a full octet.
As you go down the group the reactivity of the halogens decreases because: The atomic mass of the halogens increases. They increase in electron shells; so the atoms are larger as you go down the group. Therefore, the attraction of the outer electron to the nucleus decreases as you go down group 7.
Reactivity of elements increases down the group as down the group number of shells increases and thus nuclear pull on the outermost electrons decreases.
As we descend Group 7, the reactivity decreases. For stability, the atom needs to have a full outer shell. Group 7 elements need to gain 1 electron to have a full shell.
(b) In group 17 of the periodic table, the tendency to gain electrons decreases on moving from top to bottom. On moving down in a group, an electron shell gets added at every stage. As a result, the atomic radius increases.
Is the least reactive element of group 17?
Iodine is the least reactive element belonging to group 17. The 5 non-metallic, toxic elements that form group 17 of the periodic table are iodine, chlorine, fluorine, bromine, and astatine. These are known as halogens.
Fluorine is most reactive element of group 17 due to its small size, electro negativity, extremely high oxidizing power and low dissociation enthalpy of F-F bond.

It is because the number of valence shells is increasing, and the attraction between valence electrons and the nucleus decreases. Therefore, the tendency to pull electrons from other atoms decreases. On moving across a period, the electronegativity of atoms increases.
Why are the elements in Group 17 categorised together? Group 17 is the halogens, they have 7 electrons in their outermost shell. They are all non-metals, and form anions with a -1 charge. Reactivity decreases down the group, with fluorine being the most reactive.
The elements in this group are all non-metals. The term 'halogen' is a Greek word which means 'salt former' because they react with metals to form compounds called salts. For example, fluorine reacts explosively with sodium to form sodium fluoride. Hence group 17 are called halogens.
Metal reactivity increases down a group because as nuclear shielding increases and the nucleus' hold on the valence electron weakens, therefore it is easier to remove valence electrons.
The reactivity of group 1 elements increases as you go down the group because: the atoms become larger. the outer electron becomes further from the nucleus. the force of attraction between the nucleus and the outer electron decreases.
The metallic character increases as you go down a group. Since the ionization energy decreases going down a group (or increases going up a group), the increased ability for metals lower in a group to lose electrons makes them more reactive.
Chemical reactivity of the elements
Period - reactivity decreases as you go from left to right across a period. The farther to the left and down the periodic chart you go, the easier it is for electrons to be given or taken away, resulting in higher reactivity.
The electronegativity decreases as we move down group 17 because of the increase in nuclear radii. In group 17, fluorine is the most electronegative element.
Which of the following properties increases going down in group 17 elements?
Ionic radius increases on going down the group because number of shells increases.
On moving down the group, the reducing power increases because from top to bottom atomic size increases, due to this attraction between the nucleus and valence electron increases that allowing the element to lose electrons more easily and therefore increasing its reducing power.
The reactivity of group 1 elements increases as you go down the group because: the atoms become larger. the outer electron becomes further from the nucleus. the force of attraction between the nucleus and the outer electron decreases.
The answer to your question is quite simple. The alkali metals of group 1 grow larger in size downwards and since the effective nuclear charge remains the same as we go down group 1 the number of shells also increases and so it is easy to lose an electron and therefore as we move downwards the reactivity increases.
Down a group, the number of energy levels (n) increases, so there is a greater distance between the nucleus and the outermost orbital. This results in a larger atomic radius. Ionic radius is the distance from the nucleus to the outer edge of the electron cloud of an ion.
As we go down the group, the atom gets bigger. The bigger the atom, the further away the last electron. Therefore, the attraction between the nucleus and the last electron gets weaker. This makes it easier for the atom to give up the electron which increases its reactivity.
The group 7 elements want to gain one more electron so that they have a stable electronic structure. The smaller the atom, the easier it is to grab an electron from another atom, making the atom more reactive.
As we descend Group 7, the reactivity decreases. For stability, the atom needs to have a full outer shell. Group 7 elements need to gain 1 electron to have a full shell.
Effective nuclear charge decreases, there is an overall weaker attraction on the valence electrons and less energy is required to remove the electrons. Therefore ease of oxidation and chemical reactivity increases down group 2.