How to Find an Atom’s Mass and Size.

In the previous post, we were discussing “What gives at atom its mass and size”. Today, we’ll look at how to find an individual atom’s mass and size.

 

How to find the atom’s mass.

 

This one is easy!

Grab a periodic table and take a look at it. Next to each the elements’ symbol you will find the atomic mass.

For example, let’s look at the element nitrogen.

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- On the periodic table nitrogen’s atomic mass is shown below the symbol as 14.007.

- If your periodic table doesn’t show the atomic mass on it, it would be fairly easy to estimate what the atomic mass would be.

- Remember the protons + neutrons in the nucleus make up the mass of an atom.

- To estimate the atomic mass of an element – add the number of protons and neutrons together.

- On the periodic table, the atomic number above the symbol tells you how many protons there are in an element.

- For nitrogen there are 7 protons.

- You could estimate there will be the same number of neutrons in the nucleus of a nitrogen atom (i.e. 7)

- In total, the atomic mass of nitrogen (i.e. the number of protons + neutrons in the nucleus of a nitrogen atom) could be estimated to be: 7 + 7 = 14.

- Ah, you might say, but why is the actual atomic mass on the periodic table showing up as 14.007 and not 14.Where did the extra .007 come from?

- The answer is isotopes!

- Remember that isotopes are when an element can have a different number of neutrons in its nucleus.

- For example, nitrogen has two stable isotopes: nitrogen-14 (i.e. 7 protons + 7 neutrons in the nucleus of 1 nitrogen atom) and nitrogen – 15 (i.e. 7 protons + 8 neutrons in the nucleus of 1 nitrogen atom).

- However, nitrogen 14 other types of isotopes (i.e. atoms with different number of neutrons in the nucleus). They can have from 3 – 18 neutrons in the nitrogen atom’s nucleus.

- These other isotopes of nitrogen are unstable (radioactive isotopes) and the atomic mass ranges from 10 to 25.

- The reason why the atomic mass of nitrogen is shown as 14.007 is because the atomic mass is: the weighted average mass of all the naturally occurring isotopes of an element.

- Natural nitrogen consists of two stable isotopes: nitrogen-14 + nitrogen-15. Nitrogen-14 makes up the majority of naturally occurring nitrogen.

- The weighted average mass of all the naturally occurring isotopes of nitrogen (i.e. nitrogen-14 + nitrogen-15) is 14.007.

 

How to find the atom’s size.

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The atom’s size is caused by its electrons whizzing around the nucleus.

The electrons in the shells of an atom will determine the size of an atom (or the amount of space the electrons take up in an atom, determine the atom’s size).

The atomic size is the atomic radius of an element. The atomic radius is usually the distance from its nucleus to the boundary of its cloud of electrons.

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If the outermost electrons (the valence electrons) are far away from the nucleus, you would expect size of the atom to be larger. If the valence electrons are close to the nucleus, the size of the atom should be smaller.

However the size is not always so easy to measure, as the outer boundary of the electron cloud might not be clearly defined and different conditions can affect the atom’s size.

Depending on the methods used to determine size and atom’s conditions (i.e. free or bonded to other atoms), different values can be obtained for the size of the atom.

It can get quite complex, but remember:

- As you go down a group in the periodic table, the atomic size increases (i.e. going down the column).

- As you go across a period in the periodic table, the atomic size tends to decrease (i.e. going across the row).

- Why does this happen?

- As you go down the group (or column) on a periodic table the number of electron shells (or energy levels) increase  – so the atom gets bigger.

- As you go across the period (i.e. along the row) on the periodic table, the number of electron shells (or energy levels) stay the same BUT the effective nuclear charge increases and this pulls the electrons closer to the nucleus. As these valence electrons are pulled closer to the nucleus – the atomic size decreases.

This is a simplified explanation of the measurement of the atomic size.

 

Summary: You now know how to find:

- The atomic mass of an element on the periodic table. The atomic mass is the weighted average mass of all the naturally occurring isotopes of an element.

- The size of an atom:

-increases as you go down a group (i.e. column) in the periodic table (because the number of electron shells / energy levels increase going down), but

-tends to decrease across a period (i.e. row) in the periodic table (because the number of electron shells / energy levels stay the same going across the row but the effective nuclear charge increases and pulls the valence electrons closer to the nucleus).

 

 

Don’t miss our next post on KiDzUcation

If you can’t wait, or want more detailed information see:
“The Periodic Table of Elements”
(contains 58 PowerPoint slides with teaching notes on each slide)

 



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