AS ChemistryLesson 1 atomic structure

Friday 21 April 2023

Lesson objectivesAtomic Structure• (a) describe protons, neutrons and electrons in terms

of relative charge and relative mass; • (b) describe the distribution of mass and charge within

an atom; • (c) describe the contribution of protons and neutrons to

the nucleus of an atom, in terms of atomic (proton) number and mass (nucleon) number;

• (d) deduce the numbers of protons, neutrons and electrons in: – (i) an atom given its atomic and mass number, – (ii) an ion given its atomic number, mass number and ionic

charge;

• (e) explain the term isotopes as atoms of an element with different numbers of neutrons and different masses;

ELECTRON – negative (-1), mass 1/2000 (so small compared to

the nucleus we say it is negligible).

Occupy shells around the nucleus

PROTON – positive (+1), same mass as neutron

(“1”)

NEUTRON – neutral (0), same mass as proton

(“1”)

Nucleus contains

protons and neutrons

Structure of the atom

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-

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Draw a labelled diagram for the structure of the atom. This can be any atom or just a generic diagram.

Remember this is a model. It is the nuclear model of an atom.

Essential information

Sub-atomic particle

Mass Charge Location

Subatomic particle

Relative mass Relative charge

Proton 1 +1

Neutron 1 0

electron, e- 1/2000 -1

Some GCSE revision questions1. The number of protons that an element has

determines what atoms it contains. For example, all carbon atoms have six protons. In any atom, which particle is present in the same number as the number of protons?  

2. Using the properties of the subatomic particles explain why is there no overall charge on an atom? 

3. Using the properties of the subatomic particles justify where most of the mass of an atom is found. 

4. Using the structure of an atom determine where most of the matter (material) in an atom is distributed. 

5. Why is the overall charge of the nucleus positive?

Most of the atom is empty space!

If you imagine an atom being the size of Wembley stadium, the nucleus would be about the size of a football on the centre spot.

The electrons would be two peas flying around the whole stadium. The rest of it: emptiness.

What is the structure of an atom?Protons, neutrons and electrons are not evenly distributed in an atom.

The electrons are spread out around the edge of the atom. They orbit the nucleus in layers called shells.

The protons and neutrons exist in a dense core at the centre of the atom. This is called the nucleus.

In an atom the number of positive protons is the same as the number of negative electrons.

Nucleus contains protons and neutrons, overall charge is positive.Most of an atoms mass is found here.

Most of the atom is empty space.Electrons give the atom it’s overall size and shape.An atom has no overall charge because the number of positive protons is the same as the number of negative electrons.

Distribution of mass in an atom

• The nucleus of an atom takes up almost no space

• It is tiny but it makes up most of the mass of the atom

• This is because it contains protons and neutrons which both have a mass of 1, the electrons orbit the nucleus and have negligible mass

• It contains positive protons and neutral neutrons – so it overall it is positive

Distribution of matter in atoms

• The rest of the atom is mostly empty space

• There are negative electrons going around the nucleus really fast. They give the atom its overall size

• The mass of an electron is negligible compared to a proton or neutron, this means you can usually ignore it.

IsotopesThe number of protons is what

determines which element it is.8 protons = oxygen2 protons = heliumBut the number of neutrons can vary.If the neutrons vary then what else will

vary?If neutrons vary then so does the

atoms….….mass!

IsotopesDefinition:• Isotopes are atoms of the same element

with different numbers of neutrons.

• Isotopes of an element will have:– different masses– the same number of protons and electrons– different numbers of neutrons

• Isotopes of an element react in exactly the same way because reactions involve electrons, neutrons have no effect on chemical reactions.

XAZ

Element symbol

Mass number (nucleon number)total number of protons and neutronsin the nucleus

Atomic (proton) number1)Number of protons in the nucleus – defines the

element2)All atoms of the same element have the same

number of protons

Symbol for an isotope

Cl35

17 Cl37

17

Chlorine -35 and -37 are examples of isotopes

Different mass numbers mean different numbers of neutrons

The atomic numbers are the same. Both isotopes have 17 electrons and 17 protons

Remember Number of neutrons = mass number – atomic number

18 neutrons 20 neutrons

(35 – 17) (37 – 17)

1. It is the number and arrangement of electrons that decides the chemical properties of an element. Isotopes have the same configuration of electrons, so they have the same chemical properties.

2. Isotopes of an element do have slightly different physical properties such as density and rates of diffusion, which is dependant on the mass of particles.

Isotopes of Carbon

Carbon is a mixture of three isotopes, for each of them complete the table:

Isotope 12 6

13 6

14 6

Mass number

Atomic Number

No. of neutrons

C C C

Isotope questions (actual exam ones)

1. Hydrogen, deuterium and tritium are all isotopes of each other.

a) Identify one similarity and one difference between these isotopes.

b) Deuterium can be written as 2H. Determine the number of protons, neutrons and electrons in a neutral deuterium atom.

c) Write a nuclear symbol for tritium, given that it has 2 neutrons.

2. A certain atom X has one less proton and two more neutrons than K. What are its atomic number and mass number?

3. Explain in terms of sub atom particles, why C-12 and C-14 are isotopes

IonsWhat is an ion?It is an atom that is charged.How does this happen?Either by gaining or losing one or more

electrons.

DefinitionAn ion is a positively or negatively charged atom or (covalently bonded) group of atoms (a molecular ion)

Ions

• Ions have different numbers of protons and electrons

• Negative ions have more electrons than protons

e.g. Br- the negative charge means that there’s one more electron than there are protons. Br has 35 protons, so it must have 36 electrons. The overall charge is +35 – 36 = -1

• Positive ions have fewer electrons than protons

e.g. Mg 2+ charge means that there’s 2 fewer electrons than there are protons. Mg has 12 protons, so Mg 2+ must have 10 electrons. The overall charge = +12 – 10 = +2

IonsComplete the table:

Ion 23 11

35 17

Mass number, A

Atomic Number, Z

No. of protons

No. of neutrons

No. of electrons

Overall charge

Na+ Cl-

Ions questions

a) How many protons, neutrons and electrons are in the following atoms and ions?

b) Which are atoms and ions? How do you know?

1)173Li 2) 24

11Na 3) 199F 4) 55

26Fe

5) 3919 K+ 6) 19

9 F- 7) 3920 Ca 2+ 8) 17

8 O 2-