# The Atom & Subatomic Particles

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The Atom & Subatomic Particles

Atomic Number atomic number: For neutral atoms:
= 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?

Number of Protons = Number of Electrons
Number of Protons = Atomic Number Atomic Mass = Protons + Neutrons

What we knew by 1932: 1.0087 amu (or 1 amu)‏ Neutron, 1n 1.0073 amu +1
Neutron, 1n amu +1 Proton, 1p or 1H amu (or 0 amu)‏ -1 Electron, 0e Relative Mass Relative Charge -1 1 1

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

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

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

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

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

Other Notations Might also see: C-12 Need to look up atomic number
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 1 proton, 0 neutrons H-2: mass # = 2 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? How many neutrons in U-234? How many neutrons in U-235? How many neutrons in U-238? How many electrons in U? 92 92 234 – 92 = 142 235 – 92 = 143 238 – 92 = 146 92

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

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: = # protons minus # electrons
(subtract the electrons since are negative)‏

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

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 23Na+1 Got 2 Lost 1 34S-2 1 16 11 p, 12 n, 10 e 16 p, 18 n, 18 e Lost 2 Lost 2 25Mg+2 64Zn+2 12 30 p, 34 n, 28 e 30 12 p, 13 n, 10 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
1st 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