Atomic Structure — Isotopes & Atomic Mass

What are atoms?

the fundamental building blocks that make up all matter

What are the three subatomic particles in an atom?

protons neutrons and electrons

What is the charge of a proton?

positive

What is the charge of a neutron?

neutral (no charge)

What is the charge of an electron?

negative

Where are protons and neutrons found?

in the nucleus

Where are electrons found?

in the electron cloud around the nucleus

What is the nucleus?

the dense core of the atom that holds the protons and neutrons

What is the electron cloud?

the region around the nucleus where electrons are likely to be found

What does the number of protons determine?

the identity of the element

What determines how an atom bonds and reacts?

its electrons

Where is almost all of an atom's mass?

in the nucleus (its protons and neutrons)

Where is almost all of an atom's volume?

in the electron cloud

Why do electrons add almost no mass?

an electron is roughly 1/2000 the mass of a proton or neutron

What is the atomic number?

the number of protons in an atom

In a neutral atom what does the atomic number also equal?

the number of electrons

What makes an atom electrically neutral?

equal numbers of protons and electrons

What is the mass number?

the total of protons plus neutrons (mass number = protons + neutrons)

How do you find the number of neutrons?

subtract the atomic number from the mass number

How many neutrons are in platinum-195?

117 (195 minus 78 after looking up Pt as atomic number 78)

What are isotopes?

atoms of the same element with different numbers of neutrons

What is identical across all isotopes of an element?

the number of protons (its atomic number and identity)

What varies between isotopes and changes their mass?

the number of neutrons

In isotope symbol notation what does A stand for?

the mass number (the top-left number)

In isotope symbol notation what does Z stand for?

the atomic number (the bottom-left number)

In isotope symbol notation what does X stand for?

the element's chemical symbol

In hyphen notation like carbon-13 what does the number mean?

it is the mass number

What is the make-up of carbon-12?

6 protons and 6 neutrons

What is the make-up of carbon-13?

6 protons and 7 neutrons

What is the unified atomic mass unit (u)?

the standard unit for the mass of atoms and subatomic particles

What is the mass of a proton and of a neutron in u?

each is about 1 u

What is average atomic mass?

the weighted average mass of an element's naturally occurring isotopes

Why is a periodic table mass not a whole number?

it is a weighted average across several isotopes

What are the isotope masses weighted by?

the natural abundance of each isotope

How do you calculate average atomic mass?

multiply each isotope mass by its fraction then add the results

How do you turn a percentage abundance into a fraction?

divide the percentage by 100

What is a quick check on your abundance fractions?

they should add up to 1 (100%)

Where does a weighted average always lie?

closest to the mass of the most abundant isotope

What is the average mass when 80% is 5 u and 20% is 6 u?

5.2 u

What is relative atomic mass?

the unitless average atomic mass listed on the periodic table

What is the older term for average atomic mass?

atomic weight (though it really means mass not weight)

What was Rutherford's contribution to atomic theory?

he discovered the nucleus

What was Bohr's contribution to atomic theory?

electrons occupy specific energy levels or orbits

What is the nickname of Thomson's model?

the plum pudding model

What are the five atomic models in order?

Dalton then Thomson then Rutherford then Bohr then Schrodinger

What is the modern model of the atom?

the electron cloud model (Schrodinger)