Significant Figures, Atomic # & Mass??Please help!?

"Is the answer 8.04 because 2.02 only has 3 significant digits?"

No. The rule for add/subtract is to round off to the least number of places in the decimal portion. 2.02 has 2 places in the decimal portion and it's the least (the others are 4 and 3), so 8.04 is the answer.

"How did they get that answer!? "

The answer has two sig figs because it is (1) a mult/divide problem and (2) 0.010 has 2 sig figs, which is less than the four that 6305 has.

"atomic number is the same amount of proton and electron. "

This is true only if the atom is neutral in charge. Only then does the atomic number equal the number of electrons.

The sum of the protons and neutrons is called the mass number and it is one of the values used to determine the number of neutrons (the other being the atomic number). Both numbers are integers.

The 78.96 is the atomic mass and is a weighted average of the weights of all the stable isotopes of, in this case, selenium. This number is always a decimal value.

What we can do is conclude that, most probably, the most common isotope of selenium is the Se-79 isotope, since 78.96 is quite close to 79.

We use the mass number of Se (the 79) and the atomic number (the 34) to do this:

79 - 34 = 45

There are 45 neutrons in the nucleus of the Se-79 isotope of selenium.

"tips on how to do the Orbital Notation"

There is a very common diagram for remembering the filling order of the orbitals:

https://www.google.com/search?q=orbital+filling+order&ie=utf-8&oe=utf-8&aq=t&rls=org.mozilla:en-US:official&client=firefox-a

This diagram is also called the Aufbau diagram.

I think the best way to help remember the filling order is to take it off a periodic table:

https://www.google.com/search?q=orbital+filling+order+periodic+table&ie=utf-8&oe=utf-8&aq=t&rls=org.mozilla:en-US:official&client=firefox-a

Significant digits are usually surmised as the lowest number digit placements right of the . (point) of the number.

Two digits right of the . ( point ) on 0.010 where the last number occurs

? isotopes

Orbitals: ( if I remember right ) s, p ,d ,f, g
S having 1 orbital: 2 electrons
P having 3 : 6 electrons
D having 5 : 10 electrons
F having 7 orbital: 14 electrons
G having 9 orbital: 18 electrons

Here comes the fun part: shells
s1
s1s2
s1s2p2
s1s2p2s3
s1s2p2s3p3
s1s2p2s3p3d3
and so forth

1) In addition and subtraction of significant
figures you look at the digits to the right of the
decimal rather than counting which values have
the least number of significant figures. In this case
the answer is 8.04 because 2.02 has two digits
to the right of the decimal which is the number with
the least.

2) With division you count the number with the least
significant digits. In 0.010, the right two digits are
significant so the answer has two significant figures.

3) The number of neutrons in an element is the same as
the number of protons and electron. The number of
neutrons change with isotopes.