Question

review: how to count atoms. the symbol of an element represents one atom of that element. a sub - script is written to the lower right of an element symbol to indicate that there is more than one atom of that element. no subscript present means that there is only 1 atom of that element. a sub - script outside the brackets multiplies all the elements inside the brackets (it does not multiply to any elements not inside the brackets). a coefficient is a number written in front of a chemical formula that indicates the number of molecules. it applies through to all of the elements in the molecule. use the tables below - the first two are done for you as an example. 2ca₃(po₄)₂: type of atoms: ca, # of atoms: 6; p, # of atoms: 4; o, # of atoms: 16; total: 26. 3ba₃cl₂: type of atoms: ba, # of atoms: 9; cl, # of atoms: 6; total: 15. 4al₂(co₃)₃: type of atoms: al, # of atoms: 8; c, # of atoms: 12; o, # of atoms: 36; total: 56. 2(nh₄)₂cr₂o₇: type of atoms: blank; # of atoms: blank; total: blank. remember to write hydrogen down once (add up the h’s).

Explanation:

Step1: Analyze $2(NH_4)_2Cr_2O_7$

For $N$: Inside the brackets, there is 1 $N$ atom in $NH_4$. The sub - script outside the brackets is 2 and the coefficient in front is 2. So the number of $N$ atoms is $2\times2\times1 = 4$.

Step2: Analyze $H$

Inside the brackets, there are 4 $H$ atoms in $NH_4$. The sub - script outside the brackets is 2 and the coefficient in front is 2. So the number of $H$ atoms is $2\times2\times4=16$.

Step3: Analyze $Cr$

There are 2 $Cr$ atoms in $Cr_2O_7$ and the coefficient in front is 2. So the number of $Cr$ atoms is $2\times2 = 4$.

Step4: Analyze $O$

There are 7 $O$ atoms in $Cr_2O_7$ and the coefficient in front is 2. So the number of $O$ atoms is $2\times7=14$.

Step5: Calculate total number of atoms

Add up the number of each type of atom: $4 + 16+4 + 14=38$.

Answer:

Type of Atoms	# of Atoms
H	16
Cr	4
O	14
Total	38