# Chem topic 1 - atomic structure and periodic table

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• Created by: BIS
• Created on: 05-08-17 12:17

## The atom

Isotopes of an element are atoms with same number of protons but different number of neutrons.

The no. and arrangement of electrons decides chemical properties of element.
Isotopes have same configuration of electrons = same chemical properties.

Isotopes of an element have slightly different physical properties e.g. Density + rate of diffusion = physical properties depend on mass of atom.

Atomic (proton) number is no. Protons in nucleus - identifies element.
All atoms of same element have same no. protons

Mass number - total no. of protons + neutrons in nucleus.

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## Relative mass

The actual mass of an atom is very small. So the mass is compared to an atom of carbon-12.

The relative atomic mass,( Ar) is the weighted mean mass of an atom of an element compared to 1/12th the mass of an atom of carbon-12

Relative isotopic mass is the mass of an atom of an isotope compared with 1/12th the mass of carbon-12

The relative molecular mass is the average mass of a molecule or formula unit, compared to 1/12th if the mass of an atom of carbon-12.

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## Ar can be worked from isotopic abundance.

Step 1: multiply each relative isotopic mass by its % relative isotopic abundance and add the results

Step 2 divide by 100

E.g lithium has two isotopes of relative isotopic mass of 6.015 and 7.016. The % abundance of each isotope is 7.59 and 92.1.

Stage 1: (6.015 x 7.59) + (7.016 x 92.41) = 694.00241

Stage 2: 694.00241/100 = 6.94000241

Give answer to three s.f = 6.94

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## How a mass spec work - not in spec

Atoms can be deflected by magnetic fields - provided the atom is first turned into ion. Only electrically charged particles are affected by magnetic fields.

Stage1: Ionisation
The atom is ionised by knocking one or more electrons off to give + charge.

Stage 2: acceleration
Ions are accelerated so they have the same speed

Stage 3: deflection
Different ions are deflected by magnetic field by different amounts. The amount of deflection depends on:

• the mass of an ion. Lighter ions are deflected more than heavier ones.
the charge of the ion. Ions with 2 (or more) positive charges are deflected more
These two factors can combine + give mass/charge ratio (m/z) e.g ion with mass of 24 and charge of 2+ (m/z=12)

Stage 4: detection
Detector recognises the ions presence + gives measure of no.ions arriving

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## Mass spec can tell us about isotopes

The y axis gives the abundance of ions, as a percentage. For an element the height of the peak gives the relative isotopic abundance.

The x axis is called the mass/charge ratio. This is the relative isotopic mass.

Working out relative atomic mass by relative isotopic abundance.

Step 1: Multiply each relative isotopic mass by relative isotopic abundance and add up results p.
Step 2: divide by the sum of the isotopic abundance p.

• rv
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## Calculating isotopic masses from relative atomic m

silicin exists in three isotopes. 92.23% silicon is 28Si and 4.67% is 29Si. Given that Ar of silicon is 28.1, calculate the abundance & isotopic mass of third isotope.

Step 1: find abundance of third isotope
100%-92.23-4.67= 3.10%

Step 2: formulate an equation to work out the Ar from relative abundances + isotopic masses. Rearrange it to work out the final isotopic mass, X

28.1=(28 x 92.23) + (29 x 4.67) + (X x 3.1) / 100
28.1 = (2717.87 + (X x 3.1) / 100
2810 - 2717.87 = X x 3.1
29.719 = X .

So isotopic mass of third isotope is 30 - 30Si

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## Mass spec can identify compounds

When molecules in a sample are bombarded with electrons, an electron is removed from molecules to form a molecular ion M+

To find molecular mass of compound look at molecular ion peak (M peak) on mass spec. This is the peak with highest m/z value. The m/z value of molecular ion peak is molecular mass.

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## Mass spec can identify compounds

When molecules in a sample are bombarded with electrons, an electron is removed from molecules to form a molecular ion M+

To find molecular mass of compound look at molecular ion peak (M peak) on mass spec. This is the peak with highest m/z value. The m/z value of molecular ion peak is molecular mass.

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## Electronic structure -subs hells and orbitals

Electrons move around in quantum shells. These shells are given numbers known as principal quantum numbers.
Shells further from nucleus have greater energy level than shells closer to nucleus.
The shells contain different types of subshell. The subshells have different no. orbitals, which can hold up to 2 electrons.

Subshell. No.orbitals. Maximum electrons
S 1. 1 x 2 = 2
p. 3. 3 x 2 =6
d. 5. 5 x 2 = 10
f. 7. 7 x 2 = 14

An orbital is space where an electron can move in. Orbitals within the same subshell have same energy. They spin in opposite directions called spin - pairing.

• rev for shapes
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