Materials and Their Properties

Everything around us has to be made from something. The different types of stuff that make up the world around us are called "materials". Materials come in three main forms - Gases, Solids and Liquids. This is often refered to as the different states of matter. There is also a fourth phase, plasma, but it exists at very high temperatures, and molecules in that state can behave very differently. Plasma will not be discussed here.

Matter can exist in more than one state under different conditions.

For example, water can exist as ice (a solid), as water (a liquid) or as steam (a gas).

Physical properties:can be observed or measured without changing the chemical identity (composition) of the substance. For example, physical state, colour, temperature, melting point, solubility, odour etc.

Chemical properties: describe how substances react or change their chemical identity to form other substances. For example, flammability (e.g. hydrogen burns in the presence of oxygen to produce water).

Physical changes: involve a change in the physical appearance of matter, without altering its chemical identity. For example, a change of state.  Ice melts to form a liquid, however its chemical identity is still H₂O.

H₂O(s)     H₂O(l)      H₂O(g)

Note: the use of the subscripts (s), (l), and (g) denote the solid, liquid and gaseous states. The indicates the addition of heat to the system.

Chemical changes: involve a change in the chemical identity of matter into other substances that are chemically different.  i.e., a chemical reaction has taken place. For example, any chemical reaction.

2H₂(g)  +  O₂(g)    2H₂O(l)

Here is a summary of the modern view of an atoms structure.

• An atom is the smallest sample of an element because breaking it into its subatomic particles destroys its chemical identity.
• An atom consists of subatomic particles - protons, neutrons and electrons.
• The nucleus is in the centre of the atom.
  • Protons and neutrons are located in the nucleus.
  • Protons are positively charged (+1) and, therefore, the nucleus has a positive charge.
  • Neutrons have no charge.
  • The nucleus occupies a very small volume.
  • Most of the mass of the atom is in the nucleus.
  • Protons and neutrons have nearly the same mass (and are about 1800 times more massive than an electron).
• Electrons are located outside the nucleus.
• Electrons are negatively charged (-1).
• The largest volume is outside the nucleus in the region where the electrons exist.
• In a neutral atom, the number of electrons surrounding the nucleus equals the number of protons in the nucleus.
• Electrons are attracted to the protons in the nucleus by the law of electrostatic attraction (particles of opposite charge attract).
• All atoms of the same element have the same number of protons and electrons. Therefore, atoms have no net charge.
• Atoms are extremely small, and their diameters are measured in angstroms (Å ).  1Å = 10^-10
• Atoms have extremely small masses, so atomic mass units (amu) are used for convenience. 1 amu = 1.66054 x 10^-24 g.

Atomic Number - The number of protons in the atom; The number of electrons = the number of protons, hence the overall charge in an atom is 0.

Mass Number - The number of protons in the nucleus + the number of neutrons in the nucleus, ie the total number of particles in the nucleus.

Element - Consists of atoms all having the same atomic numbers.

Isotopes - Atoms which have the same atomic number but different mass numbers; ie, belong to the same element but have different number of neutrons. For example, there are three kinds of carbon atom ¹²C, ¹³C and ¹⁴C. They all have the same number of protons, but the number of neutrons varies.

These different atoms of carbon are called isotopes. The fact that they have varying numbers of neutrons makes no difference whatsoever to the chemical reactions of the carbon.

Elements, Compounds and Mixtures

All substances have mass and therefore must be composed of atoms. These atoms and how they assemble themselves in the substance determines their chemical and physical properties. Substances can be classified according to how these atoms are assembled and is known as Classification of Matter: All matter falls into one of three categroies: elements, compounds or mixtures. Furthermore, mixtures can be classified as homogeneous or inhomogeneous.

Elements

• Cannot be decomposed into simpler substances (by any chemical reaction).
• Are made up of only one kind of atom (i.e. atoms having the same atomic number).
• A molecule consists of two or more atoms of the same element, or different elements, that are chemically bound together. Note that the two nitrogen atoms which comprise a nitrogen molecule move as a unit.

Compounds

• Are made up of two or more elements
• Have a fixed composition, e.g., water (H₂O) has fixed proportions, by mass, of hydrogen (H) and oxygen (O).
  i.e. 2 H atoms for every one oxygen atom.
• The physical and chemical properties of compounds are different to those of the elements that make them, e.g. sodium chloride (NaCl) is common table salt and is relatively harmless and stable. The elements that combine to make NaCl are sodium (Na) and chlorine (Cl). The former is violently reactive, while the latter is a poisonous gas.

Mixtures

• Are combinations of two or more substances (elements or compounds).
• Mixtures can be homogeneous or heterogeneous (see below).


• Mixtures have variable composition.  i.e. they may be mixed in any proportion.


• Mixtures can be separated into substances by physical processes.
e.g.  filtration separates substances according to particle size.
•  Substances in the mixture retain their individual chemical and physical properties.

There are two types of mixtures:

• Homogeneous mixtures
  • Are also known as solutions
  • Are uniform throughout. i.e. if you take a sub-sample, it is representative of the whole mixture.
  • Consist of one phase.
  • Examples include: sodium chloride (table salt) dissolved in water.
• Heterogeneous mixtures
  • Are not uniform throughout. i.e. if you take a sub-sample, it is not representative of the whole mixture.
  • Consist of more than one phase
  • Suspension - a heterogeneous mixture in which the particles are large enough to be seen by a microscope or the unaided eye (eventually, they settle out of the mixture). Example: stirring a teaspoon of dirt in a glass of water.
• Colloid - a mixture where the size of particles in the mixture are between those of a solution and a suspension. NOTE: The particles appear evenly distributed. Examples: fog, cheese, butter, jellies, whipped cream.
• Examples include: beach sand, milk, granite

Most laboratory work in biology requires the use of techniques to separate the components of mixtures. This is done by exploiting some property that distinguishes the components, such as their relative

• size
  
• density
  
• solubility
  
• electrical charge

Dmitri Mendeleev

The arrangement of chemical elements started with Dimitri Mendeleev, a Russian chemist. In 1869 he arranged all the known chemical elements in the order of increasing atomic weights. He found that, for the first 20, each one resembled the eighth element following it in appearance, properties, and activity. Thus lithium, sodium, and potassium are related, as are beryllium, magnesium, and calcium. In the table these series in group Ia and IIa.

Atoms are built in an orderly, progressive manner. Hydrogen, the lightest atom, has a nucleus with one proton (positive electric charge) and one electron to match. adding one positive charge ( and two neutrons) to the nucleus of hydrogen and adding one electron to make an atom of helium. Other additions of protons and electrons in equal numbers (plus neutrons) build up the other kinds of atoms. since neutrons do not affect chemical activity, the number of positive charges identifies the kind of atom. This number is called the atomic number.

Group	IA	IIA	IIIB	IVB	VB	VIB	VIIB	VIIIB			IB	IIB	IIIA	IVA	VA	VIA	VIIA	VIIIA
Period
1	1 H																	2 He
2	3 Li	4 Be											5 B	6 C	7 N	8 O	9 F	10 Ne
3	11 Na	12 Mg											13 Al	14 Si	15 P	16 S	17 Cl	18 Ar
4	19 K	20 Ca	21 Sc	22 Ti	23 V	24 Cr	25 Mn	26 Fe	27 Co	28 Ni	29 Cu	30 Zn	31 Ga	32 Ge	33 As	34 Se	35 Br	36 Kr
5	37 Rb	38 Sr	39 Y	40 Zr	41 Nb	42 Mo	43 Tc	44 Ru	45 Rh	46 Pd	47 Ag	48 Cd	49 In	50 Sn	51 Sb	52 Te	53 I	54 Xe
6	55 Cs	56 Ba	57 to 71	72 Hf	73 Ta	1 W	75 Re	76 Os	77 Ir	78 Pt	79 Au	80 Hg	81 Tl	82 Pb	83 Bi	84 Po	85 At	86 Rn
7	87 Fr	88 Ra	89 to 103	104 Rf	105 Db	106 Sg	107 Bh	108 Hs	109 Mt	110 Uun	111 Uuu	112 Uub		114 Uuq		116 Uuh		118 Uuo
			57 La	58 Ce	59 Pr	60 Nd	61 Pm	62 Sm	63 Eu	64 Gd	65 Tb	66 Dy	67 Ho	68 Er	69 Tm	70 Yb	71 Lu
			89 Ac	90 Th	91 Pa	92 U	93 Np	94 Pu	95 Am	96 Cm	97 Bk	98 Cf	99 Es	100 Fm	101 Md	102 No	103 Lr