Chemguide: Support for CIE A level Chemistry

Learning outcome 1.1

Particles in the atom and atomic radius

Learning outcome 1.1.1

This statement is a simple overall description of the arrangement of the particles in an atom. There is nothing much to add to it, and you should find and read the statement in your copy of the syllabus.

Learning outcome 1.1.2

This statement deals with the relative charges and masses of protons, neutrons and electrons.

Before you go on, you should find and read the statement in your copy of the syllabus.

You will find a table of these values at the beginning of the page which looks at a simple view of atomic structure.

Learning outcome 1.1.3

This statement expects you to know the terms atomic number (also known as proton number) and mass number (also known as nucleon number.

Atomic number (or proton number) counts the number of protons in an atom, and defines what an atom is. So, for example, every atom of fluorine has 9 protons in its nucleus. If an atom has 9 protons, then it is fluorine, and nothing else.

Mass number (or nucleon number) counts the number of protons plus neutrons in the atom. The number of neutrons can vary within small limits for most elements. So although the atomic number of an element is always the same for an element, its mass number may vary.

For example, all carbon atoms have an atomic number of 6, because they always have 6 protons. But the mass number can vary. There are versions of carbon (called isotopes) which have 6, 7 or 8 neutrons, and so have mass numbers of 12, 13 and 14.

There is more about isotopes in learning outcome 1.2.

Learning outcome 1.1.4

This statment deals with the distribution of mass and charge in the atom

Distribution of mass

The masses of the electrons are negligible compared with the masses of protons and neutrons. So almost all the mass is concentrated in the nucleus.

Distribution of charge

The protons are positively charged (each with a 1+ charge) and the neutrons are neutral. The nucleus is therefore positively charged.

The electrons around the outside of the nucleus are negatively charged (each with a 1- charge).

In an electrically neutral atom, there must be the same numbers of protons and electrons

Learning outcome 1.1.5

This statement deals with what happens to protons, neutrons and electrons in electric fields. Read the statement before you go on.

You will find this discussed near the beginning of the page which explores a simple view of atomic structure.

On that page, you will find that I have given two different versions of what happens depending on whether the beams of the different particles have the same energy or the same speed (velocity).

Be careful! The syllabus statement wants you to describe the behaviour of the particles in an electric field when they are moving at the same velocity.

Totally ignore the section where the particles have the same energy - don't even read it!

Learning outcome 1.1.6

This section wants you to be able to work our how many of each sort of particle there are in an atom or an ion given the proton number (atomic number), nucleon number (mass number) and charge (if any).

The important thing to notice is that the proton number (atomic number) counts the number of protons in the nucleus. In a neutral atom that also tells you the number of electrons. That isn't as simple in an ion, though. We will look at a couple of examples of ions later on.

The nucleon number (mass number) counts the total number of protons and neutrons. You can work out the number of neutrons present by subtracting the proton number from the nucleon number.

Some examples:

Fe: proton number 26; nucleon number 56

There are 26 protons (from the proton number).

This is a neutral atom (not an ion) and so there are 26 electrons.

There are a total of 56 protons and neutrons (from the nucleon number), and so there must be 30 neutrons (56 - 26).

S2-: proton number 16; nucleon number 32

This is an ion - take care!

There are 16 protons (from the proton number).

If it was a neutral atom, there would be 16 electrons. But this is an ion with 2 negative charges. That's because it has 2 extra electrons. There are 18 electrons.

There are a total of 32 protons and neutrons (from the nucleon number), and so there must be 16 neutrons (32 - 16).

Notice that the fact that you have an ion rather than a neutral atom makes no difference whatsoever to the number of protons or neutrons.

Al3+: proton number 13; nucleon number 27

Another ion - take care with the electrons.

There are 13 protons (from the proton number).

If it was a neutral atom, there would be 13 electrons. However, this is a 3+ ion. That's because it has lost 3 electrons. There are 10 electrons.

And you can work out the number of neutrons from the nucleon number. In this case there are 14 neutrons (27 - 13).

Note:  This all assumes that you know from earlier work what an ion is. If you are starting A level without any chemistry background, an ion is an electrically charged atom or group of atoms. Negative ions are negative because they have one or more extra electrons. Positive ions are positive because they have lost one or more electrons. The ion is negative or positive because there are no longer equal numbers of protons and electrons.

Learning outcome 1.1.7

This statement wants you to be able to state and explain the way atomic radius and ionic radius vary as you move around the Periodic Table.

At this stage of a course, it is only possible to talk about this in a superficial way, because you might well not have enough detailed knowledge of the arrangement of electrons in atoms and ions.

However, the questions that CIE ask are mainly pretty trivial as far as the facts go. They mostly deal with atoms in periods 2 (Li to F) and 3 (Na to Cl).

Atomic radius

Start by reading the first section (titled "Measures of atomic radius") on the Chemguide page Atomic and Ionic Radius.

If you haven't come across van der Waals forces yet, don't worry. These are just weak attractions between atoms or molecules - weaker than covalent or metallic bonds.

Now read the next section Trends in atomic radius in the Periodic Table on the same page, but don't worry about the trends in the transition elements. You can't possibly understand this unless you know about the detailed electronic structure of the transition metals, and I haven't been able to find a single question in an AS level paper which asks about it.

If you are confused about the mention of 1s2 electrons, this is just a sophisticated way of talking about the 2 electrons in the inner energy level of an atom.

So in summary . . .

  • Atoms get bigger as you go down a group, because there are extra layers of electrons being added.

  • Atoms get smaller across a period (ignoring the noble gas at the end), because you are adding additional protons to the nucleus. The greater attractions pull the outer electrons closer to the nucleus.

    The screening of the outer electrons from the nucleus by inner electrons is the same for all the atoms in the period - only the extra protons matter.

Ionic radius

On the same page, read the section "Trends in ionic radius down a group", ignoring the green box at the end of it.

Now, still on the same page, read the section "Trends in ionic radius across a period".

It is important to notice that you have to consider positive ions and negative ions separately, because the negative ions in a period always have an additional layer of electrons. But within both sets of ions, the radius falls as you add extra protons.

The relative sizes of ions and atoms

The normal description and explanation goes like this:

Ions aren't the same size as the atoms they come from. Compare the sizes of sodium and chloride ions with the sizes of sodium and chlorine atoms.

Positive ions

Positive ions are smaller than the atoms they come from. Sodium is 2,8,1; Na+ is 2,8. You've lost a whole layer of electrons, and the remaining 10 electrons are being pulled in by the full force of 11 protons.

Negative ions

Negative ions are bigger than the atoms they come from. Chlorine is 2,8,7; Cl- is 2,8,8. Although the electrons are still all in the 3-level, the extra repulsion produced by the incoming electron causes the atom to expand. There are still only 17 protons, but they are now having to hold 18 electrons.

This description is flawed, but is the one usually quoted at this level, and will be what CIE will probably expect if they ask it. The current syllabus doesn't suggest that they might.

There is a lengthy discussion about this at the bottom of the page you have been looking at, but it is more suitable for teachers than students just starting the course.

Go to the Section 1 Menu . . .

To return to the list of learning outcomes in Section 1

Go to the CIE Main Menu . . .

To return to the list of all the CIE sections

Go to Chemguide Main Menu . . .

This will take you to the main part of Chemguide.

© Jim Clark 2019