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How would you define and explain periodic trends?

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Hint:Electronegativity, ionization enthalpy, electron affinity parameters are some of the periodic trends in the periodic trends.
Every group and period in a periodic table are considered while considering the periodic trends.

Complete answer:
So in the question it’s asked to comment on what is a periodic trend and explain various periodic trends.
From the lower chemistry classes itself we are familiar with the term modern periodic table, in which the elements are arranged according to the increasing atomic number. There are 18 groups and 7 periods in a periodic table.
Now let’s discuss what periodic trends mean?
Periodic trends are the special pattern of the properties of the chemical elements shown in the periodic table.
The major periodic trends that we consider are the electronegativity, ionization enthalpy, atomic size, ionic radii, metallic character etc.
The periodic trends are considered in accordance with the periodic law. We consider the periodic trends along the horizontals rows i.e. periods and along down the vertical columns i.e. Groups.
Now let’s discuss few periodic trends:
Atomic radii: We know that the elements are arranged according to the increasing order of atomic number. As the atomic number increases the number of electrons and the number of shells are also increasing.
Now let’s see the atomic radii trend along a period and group:
In a period, all the elements have the same number of shells in and moving from left to the right of the periodic table as the atomic number is getting increased that means new electrons are added on to the shell which decreases the size of an atom as the nuclear charge is also increasing with atomic number. The attraction between the outer electrons and the nucleus increases, as a result size decreases.
Down the group the new shells are introduced in the element and hence the atomic radii increases down the group.
Ionization energy: It is the minimum amount of energy which is required to remove an electron from the atom.
The ionization energy decreases down the group, since we know that the down the group the size of the atom is increasing and the valence electrons are far from the nucleus, hence the attraction on the valence electrons will be less and it easy to remove an electron while we move down the group.
Along a period as the size is decreasing, hence the ionization energy will increase while moving along a period i.e. from left to right of a periodic table.
Electron affinity: It can be explained as the energy released during the gain of an electron or addition of one electron to the element to obtain stable configuration.
Electron affinity is opposite to the trend of ionization energy. The valence shell with possess more electrons and need few electrons to complete its octet configuration are said to have more electron affinity,
In that sense, if we move along a period the electron affinity increases as the elements, toward the right side of the periodic table, accept electrons to form the complete stable octet configuration.
If we move down the group the electron affinity decreases.

Note:
The periodic trends have some exceptions like electron affinity of Cl is more than F since the F experiences much repulsion force to accommodate an electron due to its small size.
The periodic trends are helpful to trace the properties of the elements which are unknown.