Helium - 4 isotope has 2 protons and 2 neutrons.
Its atomic number is 2 (the number of protons). Atomic number is the number of protons in the atom of an element and it identifies the element. All the isotopes of an element have the same atomic number (same number of protons). So, all the isotopes of hellium have 2 protons.
The number 4 that identifies the helium - 4 isotope is the mass number, i.e. the number of protons plus the number of neutron. So, helium-4 isotope has 2 protons and 2 neutrons. While, other isotopes of helium have 2 protons but different number of neutrons.
The atomic number enable us to deduce the number of electrons in an atom because the atoms are neutral, which implies that the positive and negative charge are equal. Given that the magnitude of the charge of electons and protons are equal but the sign is opposed (protons are positve and electrons are negative) the number of electrons in the atom equals the number of protonts. That is why the atomic number is also the number of electrons of the atom.
If the solute is properly distributed in the given volume, there are 2.642 g of (NH4)2SO4 per 10 mL. For the new solution, divide the 2.642 g by the molar mass of the compound. The answer is 0.02 moles. Then, divide this by the new volume, 50 mL or 0.05 L. The concentration of the new solution is 0.4 M.
Protons have a positive charge.
How I remember is Protons and Positive both start with the letter P.
Lead Oxide
Pb is Lead
O is Oxygen
I hope this helped