The Atom Subatomic Particles Atomic Number atomic number

The Atom & Subatomic Particles

Atomic Number atomic number: = size of nuclear charge = number of protons in nucleus determines identity of element used to give order to periodic table For neutral atoms: atomic number = number of electrons

What is an atom?

What we knew by 1932: Relative Charge Relative Mass Electron, 0 e -1 . 0005486 amu (or 0 amu) Proton, 1 p or 1 H +1 1. 0073 amu (or 1 amu) Neutron, 1 n 0 0 1. 0087 amu (or 1 amu) -1 1 1 0

Location of Subatomic Particles • Electrons located outside nucleus • Protons & neutrons located inside nucleus • Protons & neutrons = nucleons

Isotopes atoms of same element with different # neutrons (mass # is different) # protons stays same! (atomic # is same) Dalton’s theory modified in number of important ways: the atom is divisible (protons, neutrons, electrons) atoms of same element can be different (different # neutrons)

Mass Number Mass number = # protons + # neutrons

Notation 12 C 6 • left superscript is mass number • left subscript is atomic number

Notation 12 C 6 12 = # protons + neutrons 6 = # protons How many neutrons in this atom? 12 – 6 = 6 neutrons

17 O 8 Mass number is ? 17 Atomic number is ? 8 # of neutrons is ? 17 – 8 = 9 # of electrons in atom is? 8

15 7 N Mass number is ? Atomic number is ? 15 7 # of neutrons is ? 15 – 7 = 8 # of electrons in atom is? 7

19 F Mass number is ? 19 Atomic number is ? 9 # of neutrons is ? 19 – 9 = 10 # of electrons in atom is? 9

Other Notations Might also see: C-12 number following symbol is mass number! Need to look up atomic number

What’s the difference between H-1, H-2, and H-3? All H’s have 1 proton H-1: mass # = 1 H-2: mass # = 2 1 proton, 0 neutrons 1 proton, so it must have 1 neutron H-3: mass # = 3 1 proton, so it must have 2 neutrons

Consider U-234, U-235, & U-238 What’s the atomic number of U? How many protons in U? 92 92 How many neutrons in U-234? 234 – 92 = 142 How many neutrons in U-235? 235 – 92 = 143 How many neutrons in U-238? 238 – 92 = 146 How many electrons in U? 92

How many neutrons in Po-217? What’s the atomic number? How many protons? 84 So the neutrons are … 217 – 84 = 133 neutrons! 84

IONS Atoms: electrically neutral so. . . # electrons = # protons IONS = atoms that gained or lost electrons protons and electrons aren't = anymore, so ions are not neutral they carry a charge (+/-)

IONS Charge of ion: electrons = # protons minus # (subtract the electrons since are negative)

IONS If atom loses electrons, it has more positive charge than negative What kind of ion is it? positive If atom gains electrons, it has more negative charge than positive negative What kind of ion is it?

Charge of Ions Ion charge written as right superscript Cl-1 a chloride ion with a charge of -1 Na+1 a sodium ion with a charge of +1 O-2 an oxygen ion with a charge of -2 If no right superscript, it’s understood to be zero and therefore a neutral atom

Putting it all together How many protons, neutrons, & electrons in each of the following 23 Na+1 1 Lost 1 11 p, 12 n, 10 e 25 Mg+2 12 Lost 2 12 p, 13 n, 10 e 34 S-2 16 Got 2 16 p, 18 n, 18 e Lost 2 64 Zn+2 30 30 p, 34 n, 28 e

What is an amu? equal to the atomic mass unit amu 1/12 the mass of the C-12 atom C-12 is used as the reference for atomic mass

Atomic Weight or Mass Atomic mass is relative 1 st H-1 used as standard: 1 atom of H = 1 amu next O-16 used as standard: O combines with many elements 1 atom of O = 16 amu's Now C-12 is the standard: 1 atom of C-12 has a mass of exactly 12 amu's