![]() The electronic arrangements of the first 20 elementsĪfter the third level, the pattern is altered due to the transition series. 2 in the first level, 8 in the second, and 7 in the third). The arrangement of the electrons is 2, 8, 7 (i.e.nucleus Figure 3.37 Size distributions of Ag nuclei: (a) reduction. Therefore, there are 17 protons and 17 electrons. Since nucleation should take place over a very short. The periodic table gives an atomic number of 17.This process is demonstrated using chlorine: Arrange the electrons in levels, always filling up an inner level before filling an outer one.This is the number of protons, and hence the number of electrons. Look up the atomic number in the periodic table.Electrons always occupy the lowest possible energy level (nearest the nucleus), provided there is space.ĭetermining the electronic arrangement of an atom These levels can be visualized as getting successively further from the nucleus. The first level (nearest the nucleus) holds two electrons, and the second and third levels each hold eight. Each energy level can only hold a certain number of electrons. Similarly, every chlorine atom (atomic number = 17) has 17 protons every uranium atom (atomic number = 92) has 92 protons.Įlectrons are found at considerable distances from the nucleus, arranged in successive energy levels. If an atom has 12 protons (atomic number = 12), it must be magnesium. If an atom has 8 protons (atomic number = 8), it must be oxygen. The atomic number is tied to the position of the element in the periodic table the number of protons therefore defines the element of interest. If there are 9 protons, there must be 10 neutrons adding up to a total of 19 nucleons in the atom. The atomic number is the number of protons (9) the mass number counts protons + neutrons (19). This information can be expressed in the following form: The mass number is also called the nucleon number. Number of protons + number of neutrons = MASS NUMBER of the atom The atomic number is also given the more descriptive name of proton number. Number of protons = ATOMIC NUMBER of the atom Virtually all the mass of the atom is concentrated in the nucleus because electrons weigh so little in comparison to the nucleons.ĭetermining numbers of protons and neutrons Protons and neutrons are collectively known as nucleons. The nucleus, located at the center of the atom, contains the protons and neutrons. Thus, the columns of the periodic table represent the potential shared state of these elements' outer electron shells that is responsible for their similar chemical characteristics.\) When an atom gains an electron to become a negatively-charged ion this is indicated by a minus sign after the element symbol for example, \(F^-\). Group 17 elements, including fluorine and chlorine, have seven electrons in their outermost shells they tend to fill this shell by gaining an electron from other atoms, making them negatively-charged ions. When an atom loses an electron to become a positively-charged ion, this is indicated by a plus sign after the element symbol for example, Na +. As a result of losing a negatively-charged electron, they become positively-charged ions. This means that they can achieve a stable configuration and a filled outer shell by donating or losing an electron. In comparison, the group 1 elements, including hydrogen (H), lithium (Li), and sodium (Na), all have one electron in their outermost shells. Their non-reactivity has resulted in their being named the inert gases (or noble gases). As shown in, the group 18 atoms helium (He), neon (Ne), and argon (Ar) all have filled outer electron shells, making it unnecessary for them to gain or lose electrons to attain stability they are highly stable as single atoms. The periodic table is arranged in columns and rows based on the number of electrons and where these electrons are located, providing a tool to understand how electrons are distributed in the outer shell of an atom. Elements in other groups have partially-filled valence shells and gain or lose electrons to achieve a stable electron configuration.Īn atom may gain or lose electrons to achieve a full valence shell, the most stable electron configuration. A full valence shell is the most stable electron configuration. Group 18 elements (helium, neon, and argon are shown) have a full outer, or valence, shell. \):īohr diagrams indicate how many electrons fill each principal shell.
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