Atoms and their Structure

Elements, Compounds & Mixtures We are surrounded by millions of different substances, some of which are naturally occurring like sand and water, and some of which are man-made like plastic and glass. The three main types are elements compounds mixtures

Elements Elements cannot be decomposed into smaller substances. They must contain one type of species of particle only. There are 105 elements at present of which eighty-four occur naturally They are classified within a table known as the Periodic Table

Compounds Elements can chemically combine with each other and form a compound. Compounds contain more than one type of particle. Compounds can only be broken down into smaller substances by chemical means. The elements present are present in fixed proportions. Eg water - H2O always 2H to 1O

Mixtures These substances are only physically mixed. No chemical combination has occurred. The substances can be separated the by physical means. The various substances in the mixture could be present in any proportion.

The Atom Greek philosophers – Democritus and Leucippus proposed original idea of the atom from particles of sand (~530 B.C) . Sand cut into smaller particles until – Atomos – an indivisible part – cannot be cut

John Dalton Scientist and English school teacher Early 1800s proposed modern day theory. Combined ideas of elements and atoms from experimental evidence. His theory is known as Dalton’s Atomic Theory.

Dalton’s Atomic Theory All matter is formed from tiny indivisible particles (atoms). All elements are made of atoms. All atoms from one element are identical but are different from atoms of a different element. Atoms from different elements can combine to form compounds but only in whole number ratios No new atoms are created or destroyed during these chemical reactions.

What makes up the Atom? Atoms are composed of small sub-atomic particles: protons (p) which are positively charged, neutrons (n) which are neutral and electrons (e) which are negatively charged. Nucleus – found in the centre of the atom Contains protons and neutrons Make up the dense core of the atom

Electrons Are extremely light, fast moving particles Travel around the nucleus. Impossible to specify the exact position of an electron relative to the nucleus The probability of finding the electrons is greatest within a certain volume around the nucleus (orbital). This volume is represented as an electron cloud.

Electrons Nucleus

Mass of Particles

Mass of Particles The neutron is slightly heavier than the proton which is much heavier than the electron. The mass of the atom is concentrated in the central core, the nucleus, which is small and dense. Hence the atom is mostly empty space.

Notations for the Elements Symbols are used to represent the element eg H for hydrogen, He for helium, S for sulphur and, Fe for iron (Latin is ferrum). Nucleur/nuclide notation may be used where m is the mass number and z the atomic number.

Atomic Number (z) Is the number of protons (p) in the nucleus and Every element will have its own unique atomic number e.g. Has 2 protons

No of Electrons Number of electrons is equal to the number of protons Hence the atomic number (z) is also the number of electrons. e.g. Has 2 protons Therefore has 2 electrons

Mass Number Is the mass of the element Given by the sum of the protons and the neutrons only An electron mass is negligible. eg Has a mass number of 4

No. of Neutrons To calculate the number of neutrons in a nucleus we subtract the number of protons from the mass number. e.g. Mass Number = 4 No of Protons = 2 No of Neutrons = Mass no. – No of Protons = 4 – 2 = 2

Isotopes Dalton was not quite accurate Isotopes are atoms of the same element which have different mass numbers. The different mass numbers result from different number of neutrons in the nucleus. Chlorine has two

Relative Atomic Mass Relative atomic mass is the average mass of one atom. It is not a whole number Hence the reason for decimal numbers on the periodic table Chlorine contains two isotopes 3517CI, 3717CI comprising 75% of the 3517CI version and 25% of the 3717CI isotope.

e.g. Chlorine contains two isotopes comprising Relative atomic mass (i) 75  100 x 35 = 26.25 (ii) 25  100 x 37 = 9.25 Average mass of one atom (Relative Atomic Mass) = 26.25 + 9.25 = 35.5 amu

Try These Find the Number of protons Number of neutrons Number of electrons Atomic Number Mass Number Name the element

Find the Number of protons Number of neutrons Number of electrons Atomic Number Mass Number Name the element

If an element has a atomic number of 26 and a mass number of 59, what is the Number of protons Number of neutrons Number of electrons Complete the symbol

If an element has 56 electrons and 81 neutrons what is the Atomic number Mass number Number of protons Complete the symbol

Calculating Relative Atomic Mass Calculate the relative atomic mass of copper if copper has two isotopes as follows: 69.1% has a mass of 62.9 amu and the remaining isotopes have a mass of 64.93 amu

The Periodic Table Vertical columns known as Groups. Horizontal rows of elements are known as Periods. Groups 1 – 7 are known as the ‘typical’ or representative elements. Group 8 are known as the ‘noble’ gases. Middle group referred to as the transition elements. The number of the group refers to the number of the electrons in the outermost filled or filling electronic shell.

Group 1 – The Alkali Metals Element State Type Reactivity Outer Electrons for Reaction* Li (3) solid metal very reactive 2, 1* Na (11) 2, 8, 1* K (19) 2, 8, 8, 1* Rb (37) 2, 8, 18, 8, 1* Cs (55) violently reactive 2, 8, 18, 18, 8, 1*

Outer Electrons for Reaction* Group 7 – The Halogens Element State Type Reactivity Outer Electrons for Reaction* F (9) yellow gas non metal violently reactive 2, 7* CI (17) green gas very reactive 2, 8, 7* Br (35) red liquid reactive 2, 8, 18, 7* I (53) violet solid 2, 8, 18, 18, 7*

Electronic Configuration Group 8 – The Noble Gases Noble Gas Electronic Configuration Helium He (2) 2 Neon Ne (10) 2, 8* Argon Ar (18) 2, 8, 8* Krypton Kr (36) 2, 8, 18, 8* Xenon Xe (54) 2, 8, 18, 18, 8*