WJEC Chemistry for AS Level Student Book: 2nd Edition (Draft)

WJEC Chemistry for AS Level

If a neutral atom loses one or more electrons it forms a positive ion or cation, e.g. Na →  Na + + e − If a neutral atom gains one or more electrons it forms a negative ion or anion, e.g. Cl + e − →  Cl − In both examples the number of protons has not changed but the number of electrons has.

Link

Ionic bonding page 50

Knowledge check

2

State the number of protons and electrons in (a) 131 I – (b) 25 Mg 2+ .

The number of electrons in Na + is 10 (atomic number − charge on ion). The number of electrons in Cl − is 18 (atomic number + charge on ion).

Radioactivity Types of radioactive emission and their behaviour Some isotopes are unstable and split up to form smaller atoms. The nucleus divides and sometimes protons, neutrons and electrons fly out. The process is called radioactive decay and the element is said to be radioactive. Radioactive isotopes have unstable nuclei and they give off three types of radiation: alpha (  ) , beta (  ) and gamma (  ) . Alpha particles consist of two protons and two neutrons and are therefore helium nuclei. They are the least penetrating of the three types of radiation and are stopped by a thin sheet of paper or even a few centimetres of air. Beta particles consist of streams of high-energy electrons and are more penetrating. They can travel through up to 1m of air but are stopped by a 5mm thick sheet of aluminium. Gamma rays are high-energy electromagnetic waves and are the most penetrating of the three radiations. They can pass through several centimetres of lead or more than a metre of concrete.

Stretch & challenge

Nuclei contain protons packed together in a very small space. Why do nuclei not fly apart?

Exam tip If you are given an element’s mass number and symbol, use the periodic table to find its atomic number. Remember it might be an isotope so the mass number might be different from that in the periodic table.

Lead

Key terms a -particles have a nucleus of 2 protons and 2 neutrons, therefore positively charged.  -particles are fast moving electrons, therefore negatively charged.  -rays are high energy electromagnetic radiation, therefore no charge.

Aluminium

Thin paper DRAFT α β γ ▲ The penetrating powers of radiation When alpha, beta and gamma radiation pass through matter they tend to knock electrons out of atoms, ionising them. Alpha particles are strongly ionising because they are large, relatively slow moving and carry two positive charges. On the other hand, gamma rays are only weakly ionising. Stretch & challenge β particles can be considered as being formed when a neutron changes into a proton, i.e. 1 0 n → 1 1 p + –1 β . 18