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TRENDS in the PERIODIC TABLE. A trend is a pattern or a repetition of particular properties. Trends of the periodic table include Atomic Mass, Atomic Radius, Net Nuclear Charge, Ion Size, Metallic or Non-Metallic Property, Electro-negativity, 1st ionization energy.

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trends in the periodic table

A trend is a pattern or a repetition of particular properties.

Trends of theperiodic tableinclude

Atomic Mass, Atomic Radius, Net Nuclear Charge,Ion Size, Metallic or Non-Metallic Property, Electro-negativity,

1st ionization energy


The periodic table is arranged in a certain way to keep elements with similar properties close together

groups vs periods

Groups go up and down.

Periods go left and right.

Groups share many similarities.

Periods show periodically (regularly) changing properties.


The Periodic Table has a lot of information on it, available for you to use, if you know where to look and what it all means.

The Regents Reference Charts are your friends, play with them.


These Trends are…

  • Atomic Mass (in amu)
  • Atomic Radius (size in picometers)
  • Net Nuclear Charge (how many protons are in the nucleus of an atom?)
  • Ion Size (cations and anions)
  • Metal Properties and Non Metallic Properties
  • Electronegativity
  • 1st Ionization Energy levels
demetri mendeelev
Demetri Mendeelev
  • Developed the first real periodic table which ours is based upon.
  • He was able to predict the missing elements and their properties once the table was put together.
mendeelev was a genius
Mendeelev was a genius!

Many of his predictions came true, such as the existence of “eka-aluminum” which we know as gallium. A missing element in his table made him believe that an element with certain properties belonged there.

By looking specifically for this missing “eka-aluminum”, chemists were able to discover gallium.

His predictions were quite close to the actual values of the various properties he quoted, such as mass, density, ionic formula, and others.

our first trend
Our FIRST Trend

Atomic Mass

Atomic mass goes higher from element to element, with few exceptions.

The Group Trend is that atomic mass increases.

The Period Trend is that atomic mass increases too.

because of specific properties
Because of specific properties...

our Periodic Table has a non-regular shape.The elements are arranged by properties rather than a way just make the table be uniform in shape.



Atomic Radius or Atom Size

If you follow along Table S for atomic radius, you find:

The Group Trend for atomic radius is that it increases down a group.

The Period Trend for atomic radius is that it decreases left to right.


Period Trend for Atomic Radius

Atoms get smaller as you go across a period.

They gain electrons in the same energy levels, not getting bigger.

The additional positive charge from the additional protons pulls the electron orbital slightly tighter for each atom going across the period.


Net Nuclear Charge

This is too easy to even discuss much. Hold onto your hat, and be psyched that this is “REQUIRED” learning.

Each atom is neutral because it has the SAME number of protons & electrons.

Electrons fly around outside the nucleus in orbitals. Protons hang tight (no joke) with the neutrons in the nucleus. Since the neutrons are neutral (hence their name!), the only charge particles in the nucleus are the protons, which are ALL POSITIVE. So, the NET NUCLEAR CHARGE is how many protons are there in the nucleus, and since each is positively charged, that is the answer.

Example: Mercury is number 80, with 80 electrons and 80 protons, a neutral atom. It also has 121 neutral neutrons in the nucleus with the 80 protons. So, NET nuclear charge = +80.


Metallic or Non-Metallic Properties

Someone, somehow, determined that “the most metallic metal” was Francium, bottom left hand corner of the Periodic Table.

They also came to the conclusion that Helium was to become known as the “most non-metallic” element.

You have to remember that, and you have to be able to compare up to 4 elements at a time and see which is closer to either Fr or He, and decide which is the most metallic, or most non-metallic.

the next trend is 1 st ionization energy

[what the heck was that again?]


First Ionization Energy,

It is the amount of energy needed to remove a valence electron from an entire mole of atoms and make them all into +1 ions.

For example…

To turn a mole of Li atomsinto a mole of Li+1 ions, it would take (look at Table S now) 520 kJ/mole



the units are kilo-Joules per mole or kJ/mole

There is a whole list of themon your TABLE S,

Please get Table S out now.

look at table s find sodium
Look at Table S, find Sodium
  • Na has a first ionization energy of 496 kJ/mol
  • That means to take an outer electron from an entire mole of sodium atoms, making each one of those Avogadro’s Number of atoms into a Na+ ion, it would take 496 kJ for the mole.

Going across the 3rd period, the trend for1st Ionization Energyis to INCREASE.


Na 496 kJ/mol

Mg 736 kJ/mol

Al 578 kJ/mol

Si 787 kJ/mol

P 1012 kJ/mol

what about Mg to Al then???


The Mg - Al EXCEPTIONis one of those unusual places where the periodic table cannot manage to be perfect forall properties.

Still, “the trend” is that first ionization energy increases when going across any period.

It’s a trend to wear a tuxedo to the high school prom. But it’s an exception to wear a color like these guys!

ionization energy
Ionization Energy
  • just so you know, there are first, second and third ionization energy levels.
  • each is measured with the unit kJ/mol
  • the “first” is the energy required to remove the first electron
  • the “second” is to remove an additional electron from the mole of ions (+1 cations into +2 cations, this is not in Regents Chemistry.)
another trend ionic sizes
Atomic size

atoms get bigger going down a group

atoms get smaller going across a period

Cations are always smaller than atoms because the cations losea whole orbital when they form.

Anions are bigger than the atoms they started as, because by adding electrons into the outer orbital, they must stretch a bit larger to accommodate those extra negative charges that push against each other.

Another Trend – Ionic Sizes

Cs+1 is larger than K+1

Br-1 is bigger than Cl-1

when going down a groupthe ions get bigger

This is true for cations & anions


Cs +1







TABLE S in your reference charts

has all the data about 1st ionization energy levels, and about atomic sizes and about the last trend that we’ll cover - electro-negativity.

If you forget the trend, pick a few atoms and put the numbers onto your periodic tableand show yourself the trend.

Be sure to use a few though in case you pick a quirky exception (like the Mg to Al bump in 1st ionization energy.




Say it to yourself a few times in your head.

It’s a really cool word and you are going to know it real soon too. Impress your friends with new sayings like:

“Your electro-negativity is really getting on my nerves.”


“My goodness! I can feel your electro-negativity all the way over here!”

electro negativity

the measure of the attraction an atom has to gain an electronin a chemical reaction.

It’s measured on the

Linus Pauling electro-negativity scale.

fluorine and e n
Fluorine and E-N
  • Fluorine tops out the scale at 4.0
  • Pauling set this standard, because he could.
  • It’s a totally arbitrary scale, based upon Fluorine and at 4.0 just because. All other atoms are compared to that one.
  • All the other electro-negativity values are relative to Fluorine’s
dr linus pauling that s him at age 2
Dr. Linus PaulingThat’s him at age 2.

Because of his dynamic personality and his many accomplishments in widely diverse fields, it is hard to define Linus Pauling adequately. A remarkable man who insistently addressed certain crucial human problems while pursuing an amazing array of scientific interests, Dr. Pauling was almost as well known to the American public as he was to the world's scientific community. He is the only person ever to receive two unshared Nobel Prizes, one for Chemistry (1954) and for Peace (1962).


Linus Pauling always emphasized the importance of having a full and happy personal life.

To have met this man must have been quite an honor, he would have made a fine guest for dinner.

In addition to the general recognition as one of the two greatest scientists of the 20th century, he was usually acknowledged by his colleagues as the most influential chemist since Lavoisier, the 18th-century founder of the modern science of chemistry.

His introductory textbook General Chemistry, revised three times since its first printing in 1947.


Electro-negativity is the amount of pull that an atom has for another electron in a bonding situation. Fluorine has the greatest desire of all atoms for that electron gain. Fluorine is given the rating of 4.0 on the E-N scale, the highest Electronegativity of all elements.

Electro-negativity is on Table S. You don’t have to memorize the trend, you can look it up anytime you want to.

Going downa group the trend is towards LOWER E-N values.

Going across a period the trend is towards higher E-N values.

Trends in Electro-negativity

It is all about HOW CLOSE IS THE ATOM TO FLUORINE which determines the relative electro-negativity.



Trends are just trends, they do not ALWAYS hold true.

Use your Table S to determine trends, do not guess or foolishly try to memorizeso much material when the answers are right in your hand.