How long is a piece of string?

How long is a piece of string?
What units of length you use to measure a piece of string will depend on how long the string is.
If you are putting up a string of Christmas lights you might measure the length of the string in metres or centimetres.
But if you are measuring the length of string, or ribbon, to tie back your hair you might measure the length of string in centimetres or millimetres.
1 centimetre is a hundredth of a metre: 1 cm = 10^-2 m
1 millimetre is a thousandth of a metre: 1mm = 10^-3 m
The metre is the SI base unit of length.
On the other hand, if your piece of string is holding 2 atoms together in a molecule you are going to need a very tiny piece of string, less than a billionth of a metre. Which is why chemists use units of length like the nanometre (1nm = 10^-9 m), the Ångstrom (1 Å = 10^-10 m), and the picometre (1 pm = 10^-12 m).
You will need to be familiar with these units and be able to convert one unit of length into another, so AUS-e-TUTE has just added new tutorials, games, tests to help our members do this.
If you are not a member, you can access a "free-to-view" tutorial at https://www.ausetute.com.au/lengthconv.html

Which Element has the Smallest Atomic Radius?

Question: Write the symbol for the element which has the smallest atomic radius Na, Si, Mg, P.

Answer: In general, atomic radius decreases going across a period of the Periodic Table.
Sodium (Na), magnesium (Mg), silicon (Si) and phosphorus (P) all belong to the third period.
In order of increasing atomic number they are: Na (Z=11), Mg (Z=12), Si (Z=14), P(Z=15)
Therefore, in order of decreasing atomic radius they are: Na > Mg > Si > P
So, phosphorus, symbol P, has the smallest atomic radius.

Read more about trends in atomic radius at AUS-e-TUTE: http://www.ausetute.com.au/trendar.html