Elements and Substances

This chapter asks what type of particles are found in gold, how the elements are arranged in the Modern Periodic Table of Elements, how positive and negative ions are formed from atoms, and what an isotope is p. 167.

8.1 Matter

Matter consists of tiny and discrete particles, and it is formed by three types of particles namely atoms, molecules or ions p. 168. The atom is the smallest particle of an element, a molecule consists of two or more atoms which are chemically combined, and an ion is a charged particle formed when an atom loses or gains electrons p. 168. An atom consists of subatomic particles, which are protons, neutrons and electrons p. 168.

Atomic substances contain only atoms, and all metals and inert gases are made up of atoms p. 168. Atoms in metals are arranged in a neat and orderly manner, while atoms in inert gases are spread far apart with no regular arrangement p. 168. Molecular substances contain molecules made up of two or more atoms that combined chemically, and most of the non-metal substances exist in the form of molecules p. 169. The reaction between non-metallic elements forms a compound which exists in the form of molecules, such as water p. 169. Ionic substances are formed by the reaction between metal elements and non-metal elements, contain positive ions and negative ions, and an example is sodium chloride p. 169.

Elements can exist in the form of atoms, molecules or ions, and the same element can exist in different forms p. 170. Iron exists in the form of atoms in the head of a hammer but in the form of ions in rust p. 170. Hydrogen exists in the form of ions in lime juice but in the form of molecules in the air p. 170.

8.2 Modern Periodic Table of Elements

There are 118 types of elements listed in the Modern Periodic Table of Elements p. 171. Each vertical column is called a group, and there are 18 groups p. 171. Each horizontal row of elements is called a period, and there are 7 periods p. 171. Group 1 is the alkali metals, Group 2 the alkaline earth metals, Group 17 the halogens and Group 18 the inert gases p. 171. The two series with proton numbers 57 to 71 and 89 to 103 are placed separately at the bottom and are known as lanthanoids and actinoids p. 171.

Moving from left to right across a period, metallic properties decrease and non-metallic properties increase, the properties of oxides change from basic to acidic, and electrical conductivity decreases p. 171. The elements are arranged from left to right and top to bottom in the order of their increasing proton number p. 171. Proton number is defined as the number of protons in the nucleus of an atom p. 171.

The proton number of oxygen is 8 and the proton number of magnesium is 12, and for a neutral atom the number of protons is equal to the number of electrons p. 172. The electrons are arranged in shells around the nucleus and occupy the shells closest to the nucleus first p. 172. For atoms with proton numbers 1 to 20, the first shell can accommodate up to two electrons, and the second and third shells up to eight electrons each p. 172. Sulphur has a proton number of 16, so its 16 electrons are arranged as 2.8.6 p. 172.

Most atoms have an unstable electron arrangement, so they tend to form a stable electron arrangement p. 173. Neutral atoms form ions by donating or accepting outermost electrons to achieve a stable duplet or octet electron arrangement p. 173. A duplet electron arrangement has 2 electrons in the first shell, and an octet electron arrangement has 8 electrons in the outermost shell p. 173. Positive ions are formed when the atoms of metal elements donate electrons, and negative ions are formed when the atoms of non-metallic elements accept electrons p. 173. A magnesium atom has the electron arrangement 2.8.2 and donates two electrons to form a magnesium ion, Mg²⁺ p. 173.

A chlorine atom has 17 electrons with the electron arrangement 2.8.7, and it accepts one electron to achieve a stable octet electron arrangement as a chloride ion (2.8.8) p. 174. Metal corrosion occurs when the metal atoms form positive ions, and rust is one of the main problems faced by vehicles and bridges p. 174.

8.3 Isotope

Some elements have atoms with the same number of protons but a different number of neutrons, and these atoms are known as isotopes p. 175. Hydrogen-1, hydrogen-2 and hydrogen-3 each have 1 proton but 0, 1 and 2 neutrons p. 175. Carbon-12, carbon-13 and carbon-14 each have 6 protons but 6, 7 and 8 neutrons p. 175. Isotopes are usually written only by their nucleon number, which is the total number of protons and neutrons in the nucleus p. 175. The isotope of hydrogen with 1 proton and 2 neutrons is written as hydrogen-3 p. 175.

Radioisotopes are unstable isotopes that emit radioactive radiation, and they have important uses if handled correctly p. 176. Radioisotopes are used in the medical field, agriculture, industry, power generation and research p. 176. Gamma radiation from cobalt-60 is used to kill cancer cells, and to destroy the microorganisms on vegetables without changing the quality of the food p. 176. Phosphorus-32 is injected into the root of plants to study the rate of absorption of phosphorus fertiliser p. 176. Other isotopes used in various fields include sodium-24, iodine-131, carbon-14, americium-241 and uranium-235 p. 177.