When an electron is added to a neutral atom, energy is released. To use electron affinities properly, it is essential to keep track of signs. Electron affinities are more difficult to measure than ionization energies.Īn atom of Boron in the gas phase, for example, gives off energy when it gains an electron to form an ion of Boron.ī + e – → B – – ∆H = Affinity = 26.7 kJ/mol Note that ionization energies measure the tendency of a neutral atom to resist the loss of electrons. In other words, it can be expressed as the neutral atom’s likelihood of gaining an electron. The change in energy (in kJ/mole) of a neutral atom or molecule (in the gaseous phase) when an electron is added to the atom to form a negative ion. In chemistry and atomic physics, the electron affinity of an atom or molecule is defined as: The Standard English unit is pounds mass per cubic foot ( lbm/ft 3).Įlectron Affinity and Electronegativity of BoronĮlectron Affinity of Boron is 26.7 kJ/mol. The standard SI unit is kilograms per cubic meter ( kg/m 3). In other words, the density (ρ) of a substance is the total mass (m) of that substance divided by the total volume (V) occupied by that substance. It is an intensive property, which is mathematically defined as mass divided by volume: Typical densities of various substances at atmospheric pressure.ĭensity is defined as the mass per unit volume. For 63Cu, the atomic mass is less than 63, so this must be the dominant factor. A nucleus with greater binding energy has lower total energy, and therefore a lower mass according to Einstein’s mass-energy equivalence relation E = mc 2. The nuclear binding energy varies between nuclei.This increases the mass of nuclei with more neutrons than protons relative to the atomic mass unit scale based on 12C with equal numbers of protons and neutrons. The neutron is slightly heavier than the proton.There are two reasons for the difference between mass number and isotopic mass, known as the mass defect: For example, 63Cu (29 protons and 34 neutrons) has a mass number of 63, and an isotopic mass in its nuclear ground state is 62.91367 u. The isotopic mass usually differs for other isotopes and is usually within 0.1 u of the mass number. One unified atomic mass unit is approximately the mass of one nucleon (either a single proton or neutron) and is numerically equivalent to 1 g/mol.įor 12C, the atomic mass is exactly 12u since the atomic mass unit is defined from it. One atomic mass unit is equal to 1.66 x 10 -24 grams. The unit of measure for mass is the atomic mass unit (amu). Note that, each element may contain more isotopes, therefore this resulting atomic mass is calculated from naturally-occuring isotopes and their abundance. See also: Properties of Boron Atomic Mass of Boron Most of (n,alpha) reactions of thermal neutrons are 10B(n,alpha)7Li reactions accompanied by 0.48 MeV gamma emission.īoron – Properties Element Boron Atomic Number 5 Symbol B Element Category Metalloids Phase at STP Solid Atomic Mass 10.811 Density at STP 2.46 Electron Configuration 2s2 2p1 Possible Oxidation States +3 Electron Affinity 26.7 Electronegativity 2.04 1st Ionization Energy 8.298 Year of Discovery 1808 Discoverer Davy, Sir Humphry & Thénard, Louis-Jaques & Gay-Lussac, Louis-Joseph Thermal properties Melting Point 2079 Boiling Point 2550 Thermal Conductivity 27 Specific Heat 1.02 Heat of Fusion 50.2 Heat of Vaporization 489.7 Isotope 11B has absorption cross-section for thermal neutrons about 0.005 barns (for 0.025 eV neutron). Its (n,alpha) reaction cross-section for thermal neutrons is about 3840 barns (for 0.025 eV neutron). In nuclear industry boron is commonly used as a neutron absorber due to the high neutron cross-section of isotope 10B. Natural boron consists primarily of two stable isotopes, 11B (80.1%) and 10B (19.9%). There are over 100 different borate minerals, but the most common are: borax, kernite, ulexite etc. Significant concentrations of boron occur on the Earth in compounds known as the borate minerals. Boron is a chemical element with atomic number 5 which means there are 5 protons and 5 electrons in the atomic structure.
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