Nuclear Equations | CAIE 9702 Paper 4 Notes

Paper 4

Physics9702

Read the two numbers before balancing

A nucleus contains protons and neutrons. Both are called nucleons. The nuclide symbol records the total number of nucleons and the number that are protons.

Swipe left or right to see the whole diagram →

Read a nuclide symbol before balancing a reaction. A counts all protons and neutrons; Z counts only protons; the neutron number is A − Z.

$A$ is the nucleon number: protons plus neutrons.
$Z$ is the proton number. It identifies the element.
The number of neutrons is found by subtracting the two numbers.

\text{neutron number}=A-Z

Your turnfoundation check

How many protons and neutrons are in ${}^{23}_{11}\mathrm{Na}$ ?

Show worked answer

There are 11 protons. The neutron number is 23 − 11 = 12.

Conserve A and Z separately

A nuclear equation is a bookkeeping statement. The total nucleon number and the total proton number must each be the same before and after the reaction.

{}^{14}_{7}\mathrm{N}+{}^{4}_{2}\mathrm{He}\rightarrow{}^{17}_{8}\mathrm{O}+{}^{1}_{1}\mathrm{H}

Add the upper numbers.
$14+4=17+1$
Add the lower numbers.
$7+2=8+1$
Solve the missing upper and lower values independently. Do not use the chemical symbols to guess the answer.

Your turn9702/42/F/M/24 Q8(b)(i)

Find $x$ in the fission equation below.

\begin{aligned}{}^{235}_{92}\mathrm{U}+{}^{1}_{0}\mathrm{n}&\rightarrow{}^{142}_{54}\mathrm{Xe}\\&\quad+{}^{90}_{38}\mathrm{Sr}+x{}^{1}_{0}\mathrm{n}\end{aligned}

Show worked answer

The proton numbers already balance. For nucleon number,

236=142+90+x

Therefore, four neutrons are produced.

Common mistake

A

is not the neutron number. It counts both protons and neutrons. Balance

A

and

Z

; do not try to conserve the number of each element symbol.

Read the two numbers before balancing

A nucleus contains protons and neutrons. Both are called nucleons. The nuclide symbol records the total number of nucleons and the number that are protons.

Swipe left or right to see the whole diagram →

Read a nuclide symbol before balancing a reaction. A counts all protons and neutrons; Z counts only protons; the neutron number is A − Z.

$A$ is the nucleon number: protons plus neutrons.
$Z$ is the proton number. It identifies the element.
The number of neutrons is found by subtracting the two numbers.

\text{neutron number}=A-Z

Your turnfoundation check

How many protons and neutrons are in ${}^{23}_{11}\mathrm{Na}$ ?

Show worked answer

There are 11 protons. The neutron number is 23 − 11 = 12.

Conserve A and Z separately

A nuclear equation is a bookkeeping statement. The total nucleon number and the total proton number must each be the same before and after the reaction.

{}^{14}_{7}\mathrm{N}+{}^{4}_{2}\mathrm{He}\rightarrow{}^{17}_{8}\mathrm{O}+{}^{1}_{1}\mathrm{H}

Add the upper numbers.
$14+4=17+1$
Add the lower numbers.
$7+2=8+1$
Solve the missing upper and lower values independently. Do not use the chemical symbols to guess the answer.

Your turn9702/42/F/M/24 Q8(b)(i)

Find $x$ in the fission equation below.

\begin{aligned}{}^{235}_{92}\mathrm{U}+{}^{1}_{0}\mathrm{n}&\rightarrow{}^{142}_{54}\mathrm{Xe}\\&\quad+{}^{90}_{38}\mathrm{Sr}+x{}^{1}_{0}\mathrm{n}\end{aligned}

Show worked answer

The proton numbers already balance. For nucleon number,

236=142+90+x

Therefore, four neutrons are produced.

Common mistake

A

is not the neutron number. It counts both protons and neutrons. Balance

A

and

Z

; do not try to conserve the number of each element symbol.