12 the element x occurs naturally to the extent of 20.0% x and 80.0% 13 x. the atomic mass of x is ( a ) 17 (b) a n argon atom is isoelectronic 2 0 with (c) 21 ( d ) 3 7 ( a ) c l (b) which pair of particles has the same number of electrons? t i 4+ nj- (a) r ,mg2+ (b) (c) br, se (d) ne, ar al3+, p3- (a) the nucleus of an atom contains neutrons and electrons. c a (c) ( d ) (b) the atomic number of an element is the number of protons in one atom. (c) the mass number of an atom is the number of protons in the nucleus plus the number of electrons outside. (d) the number of electrons outside the nucleus is the same as the number of neutrons in the nucleus. (a) the ion has an inert gas electron configuration; the atom does not. (b) the positive charge on the nucleus of the ion is two units greater than the nuclear charge on the atom. (c) the ion has two more protons than the atom. (d) the ion has two more planetary electrons than the atom. (a) has fewer electrons. (b) is an isotope of sodium. (c) exists only in solution. (d) has a negative charge on its nucleus. neares