Isotopes and Ions

Quotes about atomsQuotes about atomsFrom From A Short History of Nearly A Short History of Nearly Everything Everything by Bill Bryson by Bill Bryson

““protons give an atom its protons give an atom its identity, electrons its identity, electrons its personality” p. 140personality” p. 140““if an atom were expanded if an atom were expanded to the size of a cathedral, to the size of a cathedral, the nucleus would be only the nucleus would be only about the size of a fly- but about the size of a fly- but a fly many thousands of a fly many thousands of times heavier than the times heavier than the cathedral” p. 141cathedral” p. 141

Parts of the atom:Proton:– + charge– Located in nucleus– Role: identifies the atom

Neutron:– no charge– Located in nucleus– Role: space out nucleus and add mass (stabilize the

nucleus)

Electron:– - charge– Located in orbital's outside of nucleus (energy levels)– Role: perform all bonding of atom, give it charge

Atom Model

Atomic Number

Found at top of each element square

Represents the number of protons in atom

If atom is “neutral” number of protons = number of electrons

Atomic Mass# at bottom of each element box

= # of protons + # of neutrons

To find # neutrons, subtract atomic number from atomic mass

Ex) C: atomic mass = 12, atomic # = 6,– # of protons = 6,– # of neutrons = 6, (12-6)– # of electrons = 6 (= to # of protons)

Atomic Symbols

Shorthand way of writing information

Write symbol for element

Put atomic number in subscript in front of symbol

Put atomic mass in superscript in front of symbol

BASICALLY FLIPPED FROM P.T.

Atomic Symbol Example

What is an ion?Ion - an atom or group of atoms that has a positive or negative charge

An ion forms when an atom gains or loses electrons (protons identify atom)

How does an atom become an ion?

What is the charge on an electron?– negative

If you have more electrons than protons the charge would be?– negative

If you have more protons than electrons?– positive (extra protons)

CationPositive charge– Cat-ions are pawwwwwsitive

Formed by the atom losing electrons

Metals (left side of periodic table) form cations

Ca Ca2+ + 2e-

How many electrons did Ca start with?

How many did it end with?20

18

Anion

Negative charge– An-ion is a negative ion

Formed by the atom gaining electrons

Non-metals (right side of periodic table) form anions

O + 2e- O2-

How many electrons did O start with?

How many did it end with?

810

Key Point

If atom has more p+ than e-, it will be a cation, have a + charge

If atom has more e- than p+, it will be anion, have a - charge

Ion atomic Symbol

Can always tell p+, no, e- from atomic symbol,

Must include charge for ion!– Atomic # & atomic mass go on the LEFT– Charge goes on the RIGHT

Ex: Al+3

– How many protons, electrons and neutrons?

€

13

27

What is an isotope?

Isotopes are atoms of the same element with different atomic masses.

They have different masses because they have different numbers of neutrons in the nucleus (remember, neutrons add mass and stability to the atom)

How do you write isotopes?There are different types of isotope notation:

Carbon-14 C C

These all tell us that we are dealing with an isotope of carbon with an atomic mass of 14.

How many protons and neutrons are in these isotopes? – Protons = 6– Neutrons = 14 – 6 =8

€

14

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614

How many protons and neutrons in these?

O

N

Hydrogen-3

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816

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15

protons = 8, neutrons = 16 – 8 = 8

protons = 7, neutrons = 15 - 7 = 8

protons = 1, neutrons = 3 -1 = 2

Ions & Isotopes

Both ions and isotopes can be shown in the same symbol.– Atomic # & atomic mass go on the LEFT– Charge goes on the RIGHT

– How many protons, neutrons and electrons

S

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34

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−2 Protons= 16Neutrons = 34-16 = 18Electrons = 16 +2 = 18

Write your own

Write symbols for the following:

The ion with 17 protons, 18 electrons, 20 neutrons:

The ion with 92 protons, 88 electrons, 146 neutrons:

Cl-

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1737

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−

U+4

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92238

Key Points (atomic symbols)

Mass Number (# on top) – use to find neutrons

Atomic Number (# on bottom) – use to find protons

Charge Symbol (to right) – use to find electrons

Average atomic mass

So far all the atomic masses we have used have been whole numbers

BUT there are not whole number masses on the periodic table

WHY??? The numbers on the periodic table are AVERAGE atomic masses

Calculating average atomic mass

Average atomic mass is calculated by taking the average of all the different isotope masses

For example, if these were the only 5 atoms of carbon we had:Carbon-12, Carbon-12, Carbon-12, Carbon-14, Carbon-14

What would the average mass be?

– 12+12+12+14+14 / 5 = 12.8

We can’t possibly weigh every atom in the world though…So we have to average their abundances– How much of them exists in relation to total

Using our example from before:Carbon-12, Carbon-12, Carbon-12, Carbon-14, Carbon-14

Abundance of carbon-12 = 3/5 = (0.60 relative abundance) 60%

Abundance of carbon-14 = 2/5 = (0.40 relative abundance) 40%

Calculate average mass: 12g(0.60) + 14g(0.40) = 12.8g

SAME ANSWER!!! LESS WORK!!

Practice Problem #1Calculate the average atomic mass of magnesium using the following data for three magnesium isotopes.

Isotope mass (amu) % abundance

Mg-24 23.985 78.70

Mg-25 24.986 10.13

Mg-26 25.983 11.17

Practice Problem #2

Calculate the average atomic mass of lithium, which occurs as two isotopes that have the following atomic masses and abundances in nature:

Lithium-6, 6.017 amu, 7.30%

Lithium-7, 7.018 amu, 92.70%

Key Points

% of all isotopes has to add up to 100!

Make sure to take %’s and convert to relative abundance (move decimal 2 places to right)

What happens when you change…What happens when you change…Protons: Protons: number of protons is the same as the number of protons is the same as the atomic number, protons are what define an atomic number, protons are what define an element.element.– change the protons you change the elementchange the protons you change the elementNeutrons: Neutrons: The neutrons add to the mass and The neutrons add to the mass and stability to the element.stability to the element.– change the mass of the element (make an change the mass of the element (make an

isotope)isotope)Electrons: Electrons: The electrons balance the charge of The electrons balance the charge of the protons.the protons.– change the charge of the element (make an change the charge of the element (make an

ion)ion)

Marshmallium Lab

On the BACK:– Calculate average marshmallow mass

using the LONG way• Find the mass of each marshmallow, add them

all up and divide by the total number of marshmallows

– Write a sentence about why the way on the front (read directions) is easier.