CCEA GCSE Chemistry

How the model of the atom changed

The idea of the atom has been revised several times as new evidence arrived. In the plum pudding model, an atom was a ball of positive charge with negative electrons embedded in it, rather like fruit in a pudding. Rutherford's model replaced this after experiments showed that most of an atom's mass and all of its positive charge are packed into a tiny central nucleus, with the electrons orbiting around it at a distance and the rest of the atom mostly empty space. That model could not account for all of an atom's mass, because protons alone were too light. Chadwick's discovery of the neutron, a particle in the nucleus with no charge, filled the gap and led to the model used today.

Particles in the atom

An atom has a central nucleus containing protons and neutrons, which together hold almost all of its mass, and electrons that occupy shells around the nucleus.

An atom has no overall charge because it has equal numbers of protons and electrons.

The atomic number is the number of protons in an atom, and it identifies the element. The mass number is the total number of protons and neutrons. The number of neutrons is therefore the mass number minus the atomic number.

Atoms are very small, with a radius of about 0.1 nm, which is $1 \times 10^{-10}$ m. The nucleus is less than one ten-thousandth of the size of the whole atom, so its radius is below $1 \times 10^{-14}$ m.

Counting particles in atoms and ions

An atom of sodium written as $^{23}_{11}\text{Na}$ has mass number 23 and atomic number 11. It has 11 protons, 11 electrons and $23 - 11 = 12$ neutrons.

An ion is an atom, or group of atoms, that has gained or lost electrons. The number of protons does not change, only the electrons. Take $^{27}_{13}\text{Al}^{3+}$: it has 13 protons and 14 neutrons, and because the charge is 3+ it has lost three electrons, so it has $13 - 3 = 10$ electrons. A sulfide ion $^{32}_{16}\text{S}^{2-}$ has 16 protons, 16 neutrons and $16 + 2 = 18$ electrons. To work backwards from numbers of particles, compare the protons with the electrons: 9 protons and 10 electrons means a charge of 1-.

Electronic configuration

Electrons are arranged in shells around the nucleus, filled from the innermost shell outwards. For the first twenty elements the shells hold 2, 8, 8 and then 2 electrons in the fourth shell (for potassium and calcium). The configuration is written as numbers separated by commas, or drawn as circles round a nucleus with dots or crosses for the electrons.

An ion is drawn in the same way, with the electrons it has after gaining or losing some. The sodium ion, $\ce{Na+}$, has 10 electrons and the configuration 2,8, the same as neon; the chloride ion, $\ce{Cl-}$, has 18 electrons and the configuration 2,8,8, the same as argon. The number of electrons in the outer shell decides how an atom reacts, and it is why the elements of a group behave alike.

Isotopes

Isotopes are atoms of the same element, so with the same atomic number, that have different mass numbers because they have different numbers of neutrons. Chlorine has two common isotopes, $^{35}\text{Cl}$ with 18 neutrons and $^{37}\text{Cl}$ with 20. Both have 17 protons and 17 electrons, and since their electrons are arranged identically their chemical properties are the same.

To identify isotopes from data, look for particles with the same number of protons but different numbers of neutrons. Given four species with (protons, neutrons, electrons) of (6, 6, 6), (6, 8, 6), (7, 7, 7) and (6, 7, 6), the first, second and fourth are isotopes of carbon and the third is nitrogen.

Relative atomic mass

The relative atomic mass, $A_r$, of an element compares the mass of its atoms with carbon-12, which is given a mass of exactly 12. Because an element is usually a mixture of isotopes, $A_r$ is a weighted mean of the mass numbers of the isotopes, taking account of how abundant each is. It need not be a whole number.

A sample of chlorine is 75% $^{35}\text{Cl}$ and 25% $^{37}\text{Cl}$:

The abundances can also be given as a ratio. Magnesium is found as $^{24}\text{Mg}$, $^{25}\text{Mg}$ and $^{26}\text{Mg}$ in the proportions 79%, 10% and 11%:

which rounds to 24.3. Most of the atoms are $^{24}\text{Mg}$, so the answer lies close to 24.

Elements and compounds

An element consists of one type of atom only, and cannot be broken down into anything simpler by chemical means. A compound is two or more elements chemically combined.