19 2.1 Electrons, Protons, Neutrons, and Atoms — Physical Geology – 2nd Edition

The element hydrogen has the simplest atoms, each with just one proton and one electron. The proton forms the nucleus, while the electron orbits around it. All other elements have neutrons as well as protons in their nucleus, such as helium, which is depicted in Figure 2.1.1. The positively charged protons tend to repel each other, but the neutrons help to hold the nucleus together. The number of protons is the atomic number, and the number of protons plus neutrons is the atomic mass. For hydrogen, the atomic mass is 1 because there is one proton and no neutrons. For helium, it is 4: two protons and two neutrons.

For most of the 16 lightest elements (up to oxygen) the number of neutrons is equal to the number of protons. For most of the remaining elements there are more neutrons than protons because extra neutrons are needed to keep the nucleus together by overcoming the mutual repulsion of the increasing numbers of protons concentrated in a very small space. For example, silicon has 14 protons and 14 neutrons. Its atomic number is 14 and its atomic mass is 28. The most common isotope of uranium has 92 protons and 146 neutrons. Its atomic number is 92 and its atomic mass is 238 (92 + 146).

A helium atom is depicted on Figure 2.1.1. The dot in the middle is the nucleus, and the surrounding cloud represents where the two electrons might be at any time. The darker the shade, the more likely that an electron will be there. The helium atom is about 1 angstrom across.  An angstrom (Å) is 10⁻¹⁰ metres (m). The helium nucleus is about 1 femtometre across.  A femtometre (fm) is 10⁻¹⁵ m. In other words, a helium atom’s electron cloud is about 100,000 times bigger than its nucleus.  Stanley Park in Vancouver is about 2 km across.  If Stanley Park was a helium atom, the nucleus would be the size of a walnut.

Electrons orbiting around the nucleus of an atom are arranged in shells—also known as “energy levels.” The first shell can hold only two electrons, while the next shell holds up to eight electrons. Subsequent shells can hold more electrons, but the outermost shell of any atom holds no more than eight electrons. As we’ll see, the electrons in the outermost shell play an important role in bonding between atoms. The electron shell configurations for 29 of the first 36 elements are listed in Table 2.2.

Table 2.2 Electron shell configurations up to element 36. (The inert elements, with filled outer shells, have a *.)

[Skip Table]
Element	Symbol	Atomic No.	Number of Electrons in the 1st Shell	Number of Electrons in the 2nd Shell	Number of Electrons in the 3rd Shell	Number of Electrons in the 4th Shell
Hydrogen	H	1	1	0	0	0
Helium *	He	2	2	0	0	0
Lithium	Li	3	2	1	0	0
Beryllium	Be	4	2	2	0	0
Boron	B	5	2	3	0	0
Carbon	C	6	2	4	0	0
Nitrogen	N	7	2	5	0	0
Oxygen	O	8	2	6	0	0
Fluorine	F	9	2	7	0	0
Neon *	Ne	10	2	8	0	0
Sodium	Na	11	2	8	1	0
Magnesium	Mg	12	2	8	2	0
Aluminum	Al	13	2	8	3	0
Silicon	Si	14	2	8	4	0
Phosphorus	P	15	2	8	5	0
Sulphur	S	16	2	8	6	0
Chlorine	Cl	17	2	8	7	0
Argon *	Ar	18	2	8	8	0
Potassium	K	19	2	8	8	1
Calcium	Ca	20	2	8	8	2
Scandium	Sc	21	2	8	9	2
Titanium	Ti	22	2	8	10	2
Vanadium	V	23	2	8	11	2
Chromium	Cr	24	2	8	13	1
Manganese	Mn	25	2	8	13	2
Iron	Fe	26	2	8	14	2
.	.	.	.	.	.	.
Selenium	Se	34	2	8	18	6
Bromine	Br	35	2	8	18	7
Krypton *	Kr	36	2	8	18	8

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Media Attributions

• Figure 2.1.1: Helium Atom QM. © Yzmo. CC BY-SA.

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