Atomic structure

All atoms have a central, positively charged nucleus, which is very dense, and makes up most of the mass of the atom. The nucleus is made up of two types of particle, protons and neutrons. Protons carry a positive charge, and neutrons are uncharged, hence the nucleus overall is positively charged. It is surrounded by much lighter, and rapidly orbiting, electrons (Figure 2.1). These are negatively charged, the charge being equal (but of course opposite) to that of the protons, but they have only 1/1840 of the mass of either protons or neutrons. The attractive force between the positively charged protons and the negatively charged electrons holds the atom together.

The number of protons in the nucleus is called the atomic number, and ranges from 1 to over 100. The combined total of protons and neutrons is known as the mass number. All atoms have an equal number of protons and electrons, so regardless of the atomic number, the overall charge on the atom will always be zero.

Atoms having the same atomic number have the same chemical properties; such atoms all belong to the same element. An element is made up of one type of atom only and cannot be chemically broken down into simpler substances; thus pure copper for example is made up entirely of copper atoms. There are 92 of these elements occurring naturally, 26 of which commonly occur in living things. Each element has been given a universally agreed symbol; examples which we shall encounter in biological macromolecules include carbon (C), hydrogen (H) and oxygen (O). The atomic numbers of selected elements are shown in Table 2.2.

The relationship between neutrons, protons, atomic number, and mass number is illustrated in Table 2.3, using carbon as an example, since all living matter is based

Table 2.1 Biological macromolecules

Proteins

Carbohydrates

Lipids

Nucleic acids

Enzymes

Sugars

Triacylglycerols

DNA

Receptors

Cellulose

(fats)

RNA

Antibodies

Starch

Phospholipids

Structural

Waxes

proteins

Sterols

Figure 2.1 Atomic structure. The nucleus of a carbon atom contains six protons and six neutrons, surrounded by six electrons. Note how these are distributed between inner (2) and outer (4) electron shells

Table 2.2 Symbols and atomic numbers of some elements occurring in living systems

Element

Symbol

Atomic no.

Hydrogen

H

1

Carbon

C

6

Nitrogen

N

7

Oxygen

O

8

Sodium

Na

11

Magnesium

Mg

12

Phosphorus

P

15

Sulphur

S

16

Chlorine

Cl

17

Potassium

K

19

Iron

Fe

26

Table 2.3 The vital statistics of carbon

No. of Protons

No. of Neutrons

Atomic number

Mass number

Atomic mass

6

6

6

12

12.011

upon this element. The carbon represented can be expressed in the form:

12 (mass number)

C = (Element symbol)

6 (atomic number)

The number of neutrons in an atom can be deduced by subtracting the atomic number from the mass number. In the case of carbon, this is the same as the number of protons (6), but this is not always so. Phosphorus for example has 15 protons and 16 neutrons, giving it an atomic number of 15 and a mass number of 31.

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