Atomic Structure and Isotopes

Question 1

What is an Ion?

Answer 1

A negatively/positively charged atom or group of atoms, where the number of electrons is different from the number of protons

Question 2

What is the unit of atomic mass?

Answer 2

unified atomic mass unit ( u )

Question 3

What is the relative isotopic mass?

Answer 3

The mass of an isotope compared with 1/12th of the mass of an atom of carbon 12

Question 4

What are 2 assumptions we make when calculating the relative isotopic mass?

Answer 4

• ignore the contribution of electrons within atom
• we take the mass of both protons and neutrons to be exactly 1u

Question 5

What is relative atomic mass?

Answer 5

The weighted mean mass of an atom of an element compared with 1/12th of the mass of an atom of carbon 12

Question 6

What is relative molecular mass?

Answer 6

The weighted mean mass of a molecule compared with 1/12th of the mass of an atom of carbon 12

Question 7

What is relative formula mass? ( This is basically the relative molecular mass of ionic compounds as ionic compounds dont have molecules)

Answer 7

The weighted mean mass of a formula unit compared with 1/12 the of the mass of an atom of carbon 12.

Question 8

Polyatomic ions
- hydroxide
- nitrate
- carbonate
- sulfate
- ammonium

• phosphate
• hydrogencarbonate
• manganate (VII)
• dichromate
• sulfite
• nitrite

Answer 8

OH-
NO3-
CO3, 2-
SO4, 2-
NH4+

PO4, 3-
HCO3-
MNO4-
Cr2O7, 2-
SO3, 2-
NO2, -

Question 9

What is the empirical formula?

Answer 9

Simplest whole number ratio of atoms in a compound per element

Question 10

Why do isotopes have similar chemical properties?

Answer 10

Isotopes of the same element display the same chemical characteristics

This is because they have the same number of electrons in their outer shells

Electrons take part in chemical reactions and therefore determine the chemistry of an atom

Question 11

What are the differences between isotopes?

Answer 11

The only difference between isotopes is the number of neutrons
Since these are neutral subatomic particles, they only add mass to the atom
As a result of this, isotopes have different physical properties such as small differences in their mass and density

Question 12

Define atomic number

Answer 12

the number of protons in an atoms nucleus

Question 13

Define mass number

Answer 13

The number of protons and neutrons in an atoms nucleus

Question 14

What is the relative mass for a proton and neutron?

Answer 14

1

Question 15

What is the relative mass for an electron?

Answer 15

1/1836

Question 16

Define Isotope

Answer 16

Atoms of the same element with different numbers of neutrons and different masses but the same number of protons.

Question 17

Whats the formula to calculate relative atomic mass?

Answer 17

isotope mass x isotope abundance / 100.

if there are more than 1 isotopes we add them up and then divide.

Question 18

What is the structural difference between 2H and H2?

Answer 18

H2 is molecular hydrogen. It is bonded together by one single bond. Whereas, 2H denotes two moles of elemental hydrogen

Question 19

Iron (III) chloride vs Iron (II) chloride

What do the roman numerals tell us about this compound?

Answer 19

The charge i.e Iron (III) chloride has a 3+ charge but Iron (II) chloride has a 2+ charge

Question 20

When forming an ionic compound by combining ions, what must occur for the ionic compound to form?

Answer 20

We must cancel out the charges

Question 21

What is molecular formula?

Answer 21

The actual number of atoms (or ions) in a compound

Question 22

How do we find the empirical formula?

Answer 22

1) find number of moles by doing mass / mr.
2) divide each result by the smallest result to obtain the ratio for the empirical formula

Question 23

How do we find the molecular formula?

Answer 23

Method 1: Mr (compound) / Mr (empirical) = x
Then do x multiplied by the empirical formula

OR:

Method 2: Do the mass of each element divided by 100 and then multiplied individually by the mr of the compound. Then divide the answers by the smallest answer from the previous step to get the molecular formula

Question 24

Method 1:
Q) Chemical Analysis of a compound gave the percentage composition by mass whereby:

C= 40.0%
H= 6.67%
O= 53.33%

The relative molecular mass is 180. What is the molecular formula of this compound?

Answer 24

Method 1:

moles of C = 40/12= 3.33 /3.33= 1
moles of H= 6.67/1= 6.67 /3.33= 2
moles of O= 53.33/16= 3.33 /3.33= 1

Then divide each answer by the smallest (3.33)
This gives us the ratio of the empirical formula= CH2O

Mr of compound/ mr of empirical = 180/ (16+12+2)= 180/30 = 6
Now we do: 6 x CH20= C6H12O6

Question 25

Method 2
Q) Chemical Analysis of a compound gave the percentage composition by mass whereby:

C= 40.0%
H= 6.67%
O= 53.33%

The relative molecular mass is 180. What is the molecular formula of this compound?

Answer 25

C= 40/100 x 180 = 72
H= 6.67/100 x 180 = 12
O= 53.33/100 x 180 = 96

Divide by atomic mass
C= 72/12 = 6
H= 12/1 = 12
O= 96/16 = 6
Hence answer is C6H12O6

Question 26

What is an atom?

Answer 26

All matter is composed of atoms, which are the smallest parts of an element that can take place in chemical reactions

Question 27

Why is the mass of an atom concentrated at the nucleus?

Answer 27

The nucleus contains the heaviest subatomic particles (the neutral neutrons and positive protons)

Question 28

What holds the atom together?

Answer 28

The electrostatic attraction between the positive nucleus and negatively charged electrons orbiting around it.

Question 29

When can an element have different numbers of protons and electrons?

Answer 29

If its an ION
ons have a different number of electrons to their atomic number depending on their charge

A positively charged ion, or cation, has lost electrons and therefore has fewer electrons than protons
A negatively charged ion, or anion, has gained electrons and therefore has more electrons than protons

Question 30

Whats the fixed mass for carbon- 12?

Answer 30

fixed as exactly 12 atomic mass units / 12υ