# How do Atoms differ? Honors ch 4.3.

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How do Atoms differ? Honors ch 4.3

Henry Mosley An English scientist
Discovered each element contains a unique positive charge in the nucleus The number of protons in the nucleus of an atom identifies the element – atomic number

Because all atoms are neutral, the number of protons and electrons in an atom must be equal
Atomic number = # protons = # electrons

Practice Element Atomic # Protons Electrons Pb 82 8 30

Practice Element Atomic # Protons Electrons Pb 82 8 30

Practice Element Atomic # Protons Electrons Pb 82 O 8 30

Practice Element Atomic # Protons Electrons Pb 82 O 8 Zn 30

More practice: How many protons and electrons are in: a. radon
b. magnesium An atom of an element contains 66 electrons. What element is it? An atom of an element contains 14 protons. What element is it?

Isotopes and Mass number
Dalton was incorrect about atoms being indivisible and stating that all atoms of an element are identical. (#2 and #5) All atoms of a particular element have the same number of protons and electrons but the number of neutrons in the nucleus can differ. Isotopes – atoms with the same number of protons but different numbers of neutrons

Mass number = atomic # + # neutrons
Isotopes All Isotopes of a specific element have the same chemical behavior (determined by electrons) Isotopes are identified with a mass number Mass number = atomic # + # neutrons

Isotope of Potassium

Isotope notation In nature most elements are found as a mixture of isotopes Ex: copper – two types found naturally - 29 protons and 34 neutrons - 29 protons and 36 neutrons Cu – 63; Copper – 63, 63Cu, 6329Cu Cu – 65; Copper – 65, 65Cu, 6529Cu

Mass of atoms Atoms are measured in amu (atomic mass units). (based on carbon -12) 1 amu is almost equal to 1 proton or neutron Atomic mass – the weighted average mass of the isotopes of that element. Because isotopes have different masses, the weighted average mass is NOT a whole number.

For the following: Determine the number of protons, electrons, and neutrons. Name the isotope and give its symbol. Element Atomic # Mass # Neon 10 22 Calcium 20 46 Oxygen 8 17 Iron 26 57 Zinc 30 64 Mercury 80 204

Calculate the weighted mass of Chlorine

Practice Calculate the atomic mass of unknown Element X. Identify the unknown element. Isotope Mass (amu) Percent Abundance 6X 6.015 7.59% 7X 7.016 92.41%

Boron has two naturally occurring isotopes: boron-10 (abundance =19
Boron has two naturally occurring isotopes: boron-10 (abundance =19.8%; mass = amu) and boron-11 (abundance = 80.2% and mass = amu) . Calculate the atomic mass of boron.