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tht means the max no. of electrons in the first shell is 2. the max no. of electrons in the second shell is 8. and the max number of electrons in the third shell is 8....it goes on as the no. of electron in the atom increases. so the electron configuration for magnesium is 2.8.2. this shows tht the first and second shells are filled to the max. the third shell isn't. it has a 2+ charge because it has lost 2 electrons( which carries a negative charge), hence the atom is more positive than normal.
Look at the periodic table. Mg is number 12, so it has 12 protons and 12 electrons. The +2 charge of a magnesium ion means the number of protons is 2 more than the number of electrons. To become a magnesium ion, 2 electrons are removed from the outer shell of the magnesium atom. The Mg +2 ion contains 12 protons and 10 electrons = +12 + -10 = +2 charge. The number of protons cannot change, unless the atom is radioactive. The nuclei of radioactive elements emit an alpha particle, which is composed of 2 protons and 2 neutrons. This process is called transmutation.