What do the letters in the boxes of the periodic table represent? apex

SYMBOLS

Explanation:

We see the SYMBOL of the element inside the boxes of the periodic table.

Element is represented by symbol since about 118 elements are known to us.

Some elements takes its first letter as its symbol

For example  

Hydrogen     H

Oxygen         O

Nitrogen        N

Phosphorus   P

Sulfur             S

Fluorine          F

Some elements takes the first and the second letter as its symbols

For example  

Calcium       Ca

Beryllium     Be

Lithium         Li

Argon           Ar

Radium        Ra

Bromine       Br

Cobalt          Co

Nickel           Ni

Some element have the first letter and one of the letters from the name of the element

For example

Chlorine        Cl

Zinc               Zn

Magnesium   Mg

Manganese   Mn

Astatine         At

Chromium     Cr

Some elements takes its symbol from its Latin word

For example  

Silver           Ag     The latin word is Argentum

Gold            Au      The latin word is Aurum

Sodium       Na       The latin word is Natrium

Copper       Cu      The latin word is Cuprum

Iron             Fe        The latin word is Ferrum  

We can also see atomic number and atomic mass of the elements given in the periodic table .

"record the time to complete a chemical reaction" would most likely be part of a lab procedure.

there are two significant figures.