Question

What is the difference between the number of electrons in an atom of selenium,se, and the number of electrons in an atom of aluminum, al

Answer 1

Selenium has 36 electrons34(atomic number) + 2(amount of electrons gained in the ion, has a 2- charge) = 36
To identify the number of electrons an element has, all you need is a periodic table. Identify the atomic number (the smallest of the 2 numbers in the box). That number will be the number of protons which is also the number of electrons. 
*number Atoms of all elements are made up of three primary particles: protons, neutrons, and electrons. The "sub-atomic" particles each have specific properties that such as size, electrical charge, etc. that are crucial for the stability of the atom. The particles break down like this: Particle Relative size Charge Location How to determine

Proton 1 + in the nucleus # of protons= element's atomic # (use periodic table)

Electron 0 - outside nucleus # of electrons=# of protons(in neutral, "normal" atom)

Neutron 1 0 in the nucleus Must be told or given (ie. can vary--isotopes--

for small elements most common form often

consists of isotope where #protons=#neutrons) Aluminum has 13 electrons because it needs 13 negative charges to balance the 13 postively-charged protons (which is what the periodic table told us it contains). This will make the Aluminum atom electrically neutral.