Chemical reactivity of group 17 elements decreases down group 17 from top to bottom.Electronegativity decreases down group 17 from top to bottom.Atomic radius increases down Group 17 from top to bottom.All Group 17 (group VIIA or halogen) elements have 7 valence electrons (7 electrons in the valence shell or highest energy level).(ii) covalent (eg, non-metal + halogen → non-metal halide) (i) ionic (eg, metal + halogen → metal halide) Group 17 elements can combine with nearly all the elements in the periodic table.Group 17 elements exist as diatomic molecules (X 2) when not combined with other elements. ![]() Reptiles evolved somewhat later.(a) halogens (2) (name still in common use) They were reptile-like, but not actually reptiles. The earliest amniotes probably looked something like this. The sauropsids gave rise to reptiles, dinosaurs, and birds.Įarly Amniote. Within a few million years, two important amniote groups evolved: synapsids and sauropsids. They may have looked like the animal in Figure below. The earliest amniotes evolved about 350 million years ago. Amniotic eggs were the first eggs that could be laid on land. Therefore, in an amniotic egg, an embryo can breathe without drying out. ![]() The membranes let gases but not water pass through. Amniotes are animals that produce eggs with internal membranes. The first fully terrestrial vertebrates were amniotes. That’s because their eggs lacked a waterproof covering and would dry out on land. However, they still had to return to water to reproduce. \)Īmphibians were the first animals to have true lungs and limbs for life on land.
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