# How many protons, neutrons, and electrons are in a neutral atom of sodium?

Harris

Protons 11
Neutrons 12
Electrons 11

The subatomic particles (protons, neutrons and electrons) can be calculated using some basic information from the periodic table.

The Atomic Number of an element is always equal to the number of protons. The Mass Number of an element is equal to the protons plus neutrons.

The element Sodium has an atomic number of 11 and an average atomic mass of 22.98 which makes the mass number 23.

Therefore

An atomic number of 11 means this atom will have 11 protons.

A mass number of 23 means 23 minus 11, this atom will have 12 neutrons.

Since this atom is neutral the positive protons must be equal to the negative electrons. This atom will have Eleven electrons.

Example 1

The element sodium has 12 neutrons, 11 electrons and 11 protons. The number of electrons and protons come from the element's atomic number, which is same 11. The number of neutrons can be found by subtraction of the atomic number from sodium's atomic mass of twenty three.

Example 2

On the other hand Hydrogen has only one proton, no neutrons, and uncharged, has one electron. However note that in nature, free hydrogen is found as a diatomic molecule. Therefore, a hydrogen molecule has two protons, and 2 electrons (which form the bond between hydrogen atoms).

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##### 2. (11 marks) a) At 400 °C, ammonium iodide decomposes in the following endothermic reaction: NH41(s) + NH3(g) + HI(g) Kp = 0.215 at 400 °C. Write the equilibrium expression for this reaction. 15.0 g of NH4I was placed in an empty 3.0 L flask and heated to 400 °C. What is the partial pressure of NH3 in the flask once the reaction has reached equilibrium? How would the following changes to the equilibrium mixture affect the number of moles of NH3 in the flask and the value of K, once equilibrium is re-established? (Use increase, decrease or stay the same.) Change Effect on moles of NH3 Ke value Increase temperature Increase volume of flask Remove half of HI gas
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