Nucleus, bare carbon

To provide more accurate descriptions of anions, or neutral molecules with unshared pairs, basis sets may be further augmented with so-called diffnse fnnctions. These are intended to improve the basis set at large distances from the nnclei, thns better describing the barely bound electrons of anions. For example, a p-orbital on carbon may be polarized away from the nucleus by mixing into it a d-orbital. Snch a basis set will be designated with a -I- as in 6-3lH-G(d), adding an additional fonr basis functions to each heavy atom (Figure 9.2). 

Figure 1. Graphs of cross sections of electron capture (B and C) and loss (A) by carbon ions in water as a function of velocity. For capture, a bare nucleus is considered whereas for loss the ion considered has one electron bound to it in the K-shell before the occurrence of the process. Bohrs formulas are used with some modifications (see text). Notice that when the velocity is less than the K-or-bital velocity, capture may only take place at higher orbitals...

The atomic number, the number of protons and hence the positive charge of the nucleus, determines the number of electrons that surround it. An electron has the same magnitude of electric charge as a proton, but opposite in sign. Therefore, for an atom to be electrically neutral the number of electrons outside the nucleus must be the same as the number of protons inside the nucleus. That is, the number of electrons is equal to the atomic number. Thus, hydrogen (atomic number 1) has one electron, carbon (atomic number 6) has six electrons, and so on, up to livermorium with its 116 electrons. Electrons are much lighter than protons and neutrons (by a factor of nearly 2,000), so their presence barely affects the mass of an atom. They have a profound effect on the chemical and physical properties of the element, and almost all chemistry can be traced to their behaviour. 

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