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0 /a 0 = 4.13x10 16 sec-1 Electric field = e/a 0 2 = 5.14x10 9 V/cm Other units: One atomic unit of magnetic field is defined for a Bohr magneton in a B field which has the energy of 13.6 eV. Or P B B 13.6eV where P B e! 2mc 5.788x10 5 eV/ Tesla Thus 1 a.u. Of magnetic field = 2.35x105 Tesla Laser intensity= 2 2 0 1 HcE = 3.51x 1016 W/cm2 for. 0.1 nm (1x 10-10 m). The radius of a nucleus is less than 1/10 000 of that of the atom (a bout 1 x 10-14 m). All atoms of a particular element have the same number of protons. Atoms of different elements have different numbers of protons. The number of protons in an atom is called its atomic number (p roton number). Atoms are arranged in the. 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. Atomic structure refers to the structure of an atom comprising a nucleus (centre) in which the protons (positively charged) and neutrons (neutral) are present. The negatively charged particles called electrons revolve around the centre of the nucleus. The history of atomic structure and quantum mechanics dates back to the times of Democritus, the man who first proposed that matter is composed.

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All matter that we are familiar with, including mineral crystals, is made up of atoms, and all atoms are made up of three main particles: , , and . As summarized in Table 2.1, protons are positively charged, neutrons are uncharged and electrons are negatively charged. The −1 charge of one electron balances the +1 charge of one proton. Both protons and neutrons have a mass of 1, while electrons have almost no mass.

Perian v1 2 3 for mac. 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 , and the number of protons plus neutrons is the . 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.

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Perian v1 2 3 for mac. 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 , and the number of protons plus neutrons is the . 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.

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Media Attributions

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