I. STRUCTURE OF SUBSTANCES

1. I. STRUCTURE OF SUBSTANCES Atomic number, mass number and chemical element • Atomic number Z = the number of protons in the nucleus • Mass number A = the total number of protons and neutrons • Number of neutrons = N; N = A – Z • Because the atom electrically neutral, the number of electrons is also equal to Z. • The three subatomic particles considered, the electron, proton and neutron, are the only ones involved in chemical phenomena.

2. I. STRUCTURE OF SUBSTANCES Examples 1. Find the number of protons, neutrons and electrons in the atom of Cu (Z = 29, A = 64) p+ = e- = no = 2. Find the number of protons, neutrons and electrons in the atom of Hg (Z = 80, A = 200) p+ = e- = no = 3. Find the number of protons, neutrons and electrons in the atom of Cl (Z = 17, A = 35) p+ = e- = no =

3. I. STRUCTURE OF SUBSTANCES All atoms with the same number of protons (same atomic number Z) reprezent a chemical element. Each element has a name and a distinctive symbol. Chemical symbols are one or two letter abbreviations of the elements name (usually the Latin name). The first letter, but never the second is capitalized. For example :

4. I. STRUCTURE OF SUBSTANCES

5. mass number symbol of the element atomic number I. STRUCTURE OF SUBSTANCES To represent the composition of any particular atom, we need to specify its number of protons, neutrons and electrons. We can do this with the symbol : Atoms that have the same atomic number Z, but different mass numbers A are called isotopes.

6. - Light hydrogen (protium): symbol (1p + 0n) - Heavy hydrogen (deuterium): symbol or D (1p + 1n) - super-heavy hydrogen (tritium): symbol or T (1p + 2n) - 0.015% - 0.015% - 99.985% - 99.985% - insignificant percent - insignificant percent I. STRUCTURE OF SUBSTANCES Hydrogen is the first element of the periodic table. The nucleus of hydrogen consists of one proton, so the hydrogen atom has just one electron. Hydrogen exists in nature as 3 isotopes: Natural abundance of hydrogen isotopes is :

7. 1 amu is the 12th part of the mass of the isotope I. STRUCTURE OF SUBSTANCES Atomic mass unit Because the mass of protons, neutrons and electrons is very small and it is difficult to operate with such small values, the atomic mass unit (amu) has been introduced. 1 amu = 1.6605 · 10-27 kg The relative atomic mass of an element is the ratio of the average mass of atoms of an element to 1/12 of the mass of an atom of 12C.

8. I. STRUCTURE OF SUBSTANCES Relative atomic masses are fractional number because the natural elements are a mixture of two or more isotopes. For example, magnesium has 3 stable isotopes: (78,70 %), exact atomic mass: 23,98504 (10,13 %), exact atomic mass: 24,98384 (11,17 %), exact atomic mass : 25,98259 Knowing the abundance of the stable isotopes one may calculate the atomic weight of magnesium: AMg=0.787 x 23.98504 + 0.1013 x 24.98384 + 0.1117 x 25.98259 = = 24.30934

9. Abundance of the elements What is the most abundant element? This simple question does not have a simple answer. If we consider the entire Universe, hydrogen accounts for about 90% of all the atoms and 75% of the mass, and helium accounts for most of the rest. If we consider only the elements present on Earth, iron is probably the most abundant element. However, most of the iron is in Earth’s core. The currently accessible elements are those present in Earth’s atmosphere, oceans and solid continental crust up to 16 km depth. The relative abundance in these parts of the Earth are called Clark parameters.

11. Not all the known elements exist in Earth’s crust. There are only 88 natural elements. The rest of known elements can be produced only artificially by nuclear processes. Moreover, most of the elements do not occur free in nature, that is, as uncombined element. Only about 20% of them do. The remaining elements occur in chemical combinations with other elements. We can see in the last table that oxygen is the most abundant element in the Earth’s crust (49.4%).

12. - 99.759% - 0.037% - 0.204% There are 3 natural isotopes of oxygen: Large amount of oxygen exists in water and rocks as well in free state like molecular oxygen (O2) and ozone (O3). Molecular oxygen (O2) and ozone (O3) are allotropes of the element oxygen. The second element in Clark’s table is silicon (Si – 25.75%), but silicon occurs only in chemical combinations.